GB2590722A

GB2590722A - Data encoding and decoding

Info

Publication number: GB2590722A
Application number: GB1919468.7A
Authority: GB
Inventors: James Sharman Karl; Mark Keating Stephen; Richard Browne Adrian
Original assignee: Sony Corp
Current assignee: Sony Corp
Priority date: 2019-12-31
Filing date: 2019-12-31
Publication date: 2021-07-07
Also published as: GB201919468D0

Abstract

A decoding method comprises: receiving input data representing groups of encoded data values as data set(s) and escape codes for values not encoded by the data sets, an escape code comprising a first portion (e.g. prefix) and non-unary coded second portion (e.g. suffix) having a length, in bits, dependent upon a second portion size value, groups of encoded values each having an associated encoding order; decoding the data set(s); and when input data provides an escape code applicable to a given encoded data value of a given group of encoded data values, deriving the second portion size value for the given encoded data value other than a first or second encoded data value of the group, depending upon a maximum data value of two or more previously decoded data values of the given group, decoding that escape code depending upon the derived second portion size and generating a decoded data value depending upon the decoded data set(s) and that decoded escape code. Also disclosed is a similar method with the groups of encoded data values each having encoded data values having a bit depth selected from a first predetermined bit depth and a second predetermined bit depth.

Description

DATA ENCODING AND DECODING

BACKGROUND Field

This disclosure relates to data encoding and decoding.

Description of Related Art

The "background" description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description which may not otherwise qualify as prior art at the time of filing, is neither expressly or impliedly admitted as

prior art against the present disclosure.

There are several systems, such as video or image data encoding and decoding systems which involve transforming video data into a frequency domain representation, quantising the frequency domain coefficients and then applying some form of entropy encoding to the quantised coefficients. This can achieve compression of the video data. A corresponding decoding or decompression technique is applied to recover a reconstructed version of the original video data.

In some examples, the entropy encoding process can involve generating one or more "data sets" (such as a significance map, a greater than one map, a greater than two map and/or other data sets) to describe a block of coefficients, with any excess values which cannot be encoded by the significance maps alone being encoded as so-called escape values. The coding of an escape value can On some examples) be performed by generating a first portion (for example, a unary or truncated unary coded portion such as a prefix) and a non-unary coded second portion (such as a suffix) having a length, in bits, dependent upon a second portion size value.

SUMMARY

The present disclosure addresses or mitigates problems arising from this processing. Respective aspects and features of the present disclosure are defined in the appended claims.

It is to be understood that both the foregoing general description and the following detailed description are exemplary, but are not restrictive, of the present technology.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the disclosure and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, 35 wherein: Figure 1 schematically illustrates an audio/video (AN) data transmission and reception system using video data compression and decompression; Figure 2 schematically illustrates a video display system using video data decompression; Figure 3 schematically illustrates an audio/video storage system using video data compression and decompression; Figure 4 schematically illustrates a video camera using video data compression; Figures 5 and 6 schematically illustrate storage media; Figure 7 provides a schematic overview of a video data compression and decompression apparatus; Figure 8 schematically illustrates a predictor; Figure 9 schematically illustrates a transform skip mode; Figures 10 and 11 schematically illustrate respective scanning directions; Figure 12 is a schematic diagram illustrating an encoding apparatus; Figure 13 is a schematic diagram illustrating a decoding apparatus; Figures 14 to 20 schematically illustrate blocks of coefficients; and Figures 21 to 28 are schematic flowcharts illustrating respective methods.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to the drawings, Figures 1-4 are provided to give schematic illustrations of apparatus or systems making use of the compression and/or decompression apparatus to be described below in connection with embodiments of the present technology.

All of the data compression and/or decompression apparatus to be described below may be implemented in hardware, in software running on a general-purpose data processing apparatus such as a general-purpose computer, as programmable hardware such as an application specific integrated circuit (ASIC) or field programmable gate array (FPGA) or as combinations of these. In cases where the embodiments are implemented by software and/or firmware, it will be appreciated that such software and/or firmware, and non-transitory data storage media by which such software and/or firmware are stored or otherwise provided, are considered as embodiments of the present technology.

Figure 1 schematically illustrates an audio/video data transmission and reception system using video data compression and decompression. In this example, the data values to be encoded or decoded represent image data.

An input audio/video signal 10 is supplied to a video data compression apparatus 20 which compresses at least the video component of the audio/video signal 10 for transmission along a transmission route 30 such as a cable, an optical fibre, a wireless link or the like. The compressed signal is processed by a decompression apparatus 40 to provide an output audio/video signal 50. For the return path, a compression apparatus 60 compresses an audio/video signal for transmission along the transmission route 30 to a decompression apparatus 70.

The compression apparatus 20 and decompression apparatus 70 can therefore form one node of a transmission link. The decompression apparatus 40 and decompression apparatus 60 can form another node of the transmission link. Of course, in instances where the transmission link is uni-directional, only one of the nodes would require a compression apparatus and the other node would only require a decompression apparatus.

Figure 2 schematically illustrates a video display system using video data decompression. In particular, a compressed audio/video signal 100 is processed by a decompression apparatus 110 to provide a decompressed signal which can be displayed on a display 120. The decompression apparatus 110 could be implemented as an integral part of the display 120, for example being provided within the same casing as the display device.

Alternatively, the decompression apparatus 110 maybe provided as (for example) a so-called set top box (STB), noting that the expression "set-top" does not imply a requirement for the box to be sited in any particular orientation or position with respect to the display 120; it is simply a term used in the art to indicate a device which is connectable to a display as a peripheral device.

Figure 3 schematically illustrates an audio/video storage system using video data compression and decompression. An input audio/video signal 130 is supplied to a compression apparatus 140 which generates a compressed signal for storing by a store device 150 such as a magnetic disk device, an optical disk device, a magnetic tape device, a solid state storage device such as a semiconductor memory or other storage device. For replay, compressed data is read from the storage device 150 and passed to a decompression apparatus 160 for decompression to provide an output audio/video signal 170.

It will be appreciated that the compressed or encoded signal, and a storage medium such as a machine-readable non-transitory storage medium, storing that signal, are considered as embodiments of the present technology.

Figure 4 schematically illustrates a video camera using video data compression. In Figure 4, an image capture device 180, such as a charge coupled device (CCD) image sensor and associated control and read-out electronics, generates a video signal which is passed to a compression apparatus 190. A microphone (or plural microphones) 200 generates an audio signal to be passed to the compression apparatus 190. The compression apparatus 190 generates a compressed audio/video signal 210 to be stored and/or transmitted (shown generically as a schematic stage 220).

The techniques to be described below relate primarily to video data compression and decompression. It will be appreciated that many existing techniques may be used for audio data compression in conjunction with the video data compression techniques which will be described, to generate a compressed audio/video signal. Accordingly, a separate discussion of audio data compression will not be provided. It will also be appreciated that the data rate associated with video data, in particular broadcast quality video data, is generally very much higher than the data rate associated with audio data (whether compressed or uncompressed). It will therefore be appreciated that uncompressed audio data could accompany compressed video data to form a compressed audio/video signal. It will further be appreciated that although the present examples (shown in Figures 1-4) relate to audio/video data, the techniques to be described below can find use in a system which simply deals with (that is to say, compresses, decompresses, stores, displays and/or transmits) video data. That is to say, the embodiments can apply to video data compression without necessarily having any associated audio data handling at all.

Figure 4 therefore provides an example of a video capture apparatus comprising an image sensor and an encoding apparatus of the type to be discussed below. Figure 2 therefore provides an example of a decoding apparatus of the type to be discussed below and a display to which the decoded images are output.

A combination of Figure 2 and 4 may provide a video capture apparatus comprising an image sensor 180 and encoding apparatus 190, decoding apparatus 110 and a display 120 to which the decoded images are output.

Figures 5 and 6 schematically illustrate storage media, which store (for example) the compressed data generated by the apparatus 20, 60, the compressed data input to the apparatus 110 or the storage media or stages 150, 220. Figure 5 schematically illustrates a disc storage medium such as a magnetic or optical disc, and Figure 6 schematically illustrates a solid state storage medium such as a flash memory. Note that Figures 5 and 6 can also provide examples of non-transitory machine-readable storage media which store computer software which, when executed by a computer, causes the computer to carry out one or more of the methods to be discussed below.

Therefore, the above arrangements provide examples of video storage, capture, transmission or reception apparatuses embodying any of the present techniques.

Figure 7 provides a schematic overview of a video or image data compression and decompression apparatus, for encoding and/or decoding image data representing one or more images.

A controller 343 controls the overall operation of the apparatus and, in particular when referring to a compression mode, controls a trial encoding processes by acting as a selector to select various modes of operation such as block sizes and shapes, and whether the video data is to be encoded losslessly or otherwise. The controller is considered to form part of the image encoder or image decoder (as the case may be). Successive images of an input video signal 300 are supplied to an adder 310 and to an image predictor 320. The image predictor 320 will be described below in more detail with reference to Figure 8. The image encoder or decoder (as the case may be) plus the intra-image predictor of Figure 8 may use features from the apparatus of Figure 7. This does not mean that the image encoder or decoder necessarily requires every feature of Figure 7 however.

The adder 310 in fact performs a subtraction (negative addition) operation, in that it receives the input video signal 300 on a "+" input and the output of the image predictor 320 on a "-" input, so that the predicted image is subtracted from the input image. The result is to generate a so-called residual image signal 330 representing the difference between the actual and predicted images.

One reason why a residual image signal is generated is as follows. The data coding techniques to be described, that is to say the techniques which will be applied to the residual image signal, tend to work more efficiently when there is less "energy" in the image to be encoded. Here, the term "efficiently" refers to the generation of a small amount of encoded data; for a particular image quality level, it is desirable (and considered "efficient") to generate as little data as is practicably possible. The reference to "energy" in the residual image relates to the amount of information contained in the residual image. If the predicted image were to be identical to the real image, the difference between the two (that is to say, the residual image) would contain zero information (zero energy) and would be very easy to encode into a small amount of encoded data. In general, if the prediction process can be made to work reasonably well such that the predicted image content is similar to the image content to be encoded, the expectation is that the residual image data will contain less information (less energy) than the input image and so will be easier to encode into a small amount of encoded data.

Therefore, encoding (using the adder 310) involves predicting an image region for an image to be encoded; and generating a residual image region dependent upon the difference between the predicted image region and a corresponding region of the image to be encoded. In connection with the techniques to be discussed below, the ordered array of data values comprises data values of a representation of the residual image region. Decoding involves predicting an image region for an image to be decoded; generating a residual image region indicative of differences between the predicted image region and a corresponding region of the image to be decoded; in which the ordered array of data values comprises data values of a representation of the residual image region; and combining the predicted image region and the residual image region.

The remainder of the apparatus acting as an encoder (to encode the residual or difference image) will now be described.

The residual image data 330 is supplied to a transform unit or circuitry 340 which generates a discrete cosine transform (DCT) representation of blocks or regions of the residual image data. The DCT technique itself is well known and will not be described in detail here.

Note also that the use of DCT is only illustrative of one example arrangement. Other transforms which might be used include, for example, the discrete sine transform (DST). A transform could also comprise a sequence or cascade of individual transforms, such as an arrangement in which one transform is followed (whether directly or not) by another transform. The choice of transform may be determined explicitly and/or be dependent upon side information used to configure the encoder and decoder. In other examples a so-called "transform skip" mode can selectively be used in which no transform is applied.

Therefore, in examples, an encoding and/or decoding method comprises predicting an image region for an image to be encoded; and generating a residual image region dependent upon the difference between the predicted image region and a corresponding region of the image to be encoded; in which the ordered array of data values (to be discussed below) comprises data values of a representation of the residual image region.

The output of the transform unit 340, which is to say On an example), a set of OCT coefficients for each transformed block of image data, is supplied to a quantiser 350. Various quantisation techniques are known in the field of video data compression, ranging from a simple multiplication by a quantisation scaling factor through to the application of complicated lookup tables under the control of a quantisation parameter. The general aim is twofold. Firstly, the quantisation process reduces the number of possible values of the transformed data. Secondly, the quantisation process can increase the likelihood that values of the transformed data are zero. Both of these can make the entropy encoding process, to be described below, work more efficiently in generating small amounts of compressed video data.

A data scanning process is applied by a scan unit 360. The purpose of the scanning process is to reorder the quantised transformed data so as to gather as many as possible of the non-zero quantised transformed coefficients together, and of course therefore to gather as many as possible of the zero-valued coefficients together. These features can allow so-called run-length coding or similar techniques to be applied efficiently. So, the scanning process involves selecting coefficients from the quantised transformed data, and in particular from a block of coefficients corresponding to a block of image data which has been transformed and quantised, according to a "scanning order" so that (a) all of the coefficients are selected once as part of the scan, and (b) the scan tends to provide the desired reordering. One example scanning order which can tend to give useful results is a diagonal order such as a so-called up-right diagonal scanning order.

The scanning order can be different, as between transform-skip blocks and transform blocks (blocks which have undergone at least one spatial frequency transformation).

The scanned coefficients are then passed to an entropy encoder (EE) 370. Again, various types of entropy encoding may be used. Two examples are variants of the so-called CABAC (Context Adaptive Binary Arithmetic Coding) system and variants of the so-called CAVLC (Context Adaptive Variable-Length Coding) system. In general terms, CABAC is considered to provide a better efficiency, and in some studies has been shown to provide a 10- 20% reduction in the quantity of encoded output data for a comparable image quality compared to CAVLC. However, CAVLC is considered to represent a much lower level of complexity (in terms of its implementation) than CABAC. Note that the scanning process and the entropy encoding process are shown as separate processes, but in fact can be combined or treated together. That is to say, the reading of data into the entropy encoder can take place in the scan order. Corresponding considerations apply to the respective inverse processes to be described below.

The output of the entropy encoder 370, along with additional data (mentioned above and/or discussed below), for example defining the manner in which the predictor 320 generated the predicted image, whether the compressed data was transformed or transform-skipped or the like, provides a compressed output video signal 380.

However, a return path 390 is also provided because the operation of the predictor 320 itself depends upon a decompressed version of the compressed output data.

The reason for this feature is as follows. At the appropriate stage in the decompression process (to be described below) a decompressed version of the residual data is generated. This decompressed residual data has to be added to a predicted image to generate an output image (because the original residual data was the difference between the input image and a predicted image). In order that this process is comparable, as between the compression side and the decompression side, the predicted images generated by the predictor 320 should be the same during the compression process and during the decompression process. Of course, at decompression, the apparatus does not have access to the original input images, but only to the decompressed images. Therefore, at compression, the predictor 320 bases its prediction (at least, for inter-image encoding) on decompressed versions of the compressed images.

The entropy encoding process carried out by the entropy encoder 370 is considered (in at least some examples) to be "lossless", which is to say that it can be reversed to arrive at exactly the same data which was first supplied to the entropy encoder 370. So, in such examples the return path can be implemented before the entropy encoding stage. Indeed, the scanning process carried out by the scan unit 360 is also considered lossless, so in the present embodiment the return path 390 is from the output of the quantiser 350 to the input of a complimentary inverse quantiser 420. In instances where loss or potential loss is introduced by a stage, that stage (and its inverse) may be included in the feedback loop formed by the return path. For example, the entropy encoding stage can at least in principle be made lossy, for example by techniques in which bits are encoded within parity information. In such an instance, the entropy encoding and decoding should form part of the feedback loop.

In general terms, an entropy decoder 410, the reverse scan unit 400, an inverse quantiser 420 and an inverse transform unit or circuitry 430 provide the respective inverse functions of the entropy encoder 370, the scan unit 360, the quantiser 350 and the transform unit 340. For now, the discussion will continue through the compression process; the process to decompress an input compressed video signal will be discussed separately below.

In the compression process, the scanned coefficients are passed by the return path 390 from the quantiser 350 to the inverse quantiser 420 which carries out the inverse operation of the scan unit 360. An inverse quanfisation and inverse transformation process are carried out by the units 420, 430 to generate a compressed-decompressed residual image signal 440. The image signal 440 is added, at an adder 450, to the output of the predictor 320 to generate a reconstructed output image 460 (although this may be subject to so-called loop filtering and/or other filtering before being output -see below). This forms one input to the image predictor 320, as will be described below.

Turning now to the decoding process applied to decompress a received compressed video signal 470, the signal is supplied to the entropy decoder 410 and from there to the chain of the reverse scan unit 400, the inverse quantiser 420 and the inverse transform unit 430 before being added to the output of the image predictor 320 by the adder 450. So, at the decoder side, the decoder reconstructs a version of the residual image and then applies this (by the adder 450) to the predicted version of the image (on a block by block basis) so as to decode each block. In straightforward terms, the output 460 of the adder 450 forms the output decompressed video signal 480 (subject to the filtering processes discussed below). In practice, further filtering may optionally be applied (for example, by a loop filter 565 shown in Figure 8 but omitted from Figure 7 for clarity of the higher level diagram of Figure 7) before the signal is output.

The apparatus of Figures 7 and 8 can act as a compression (encoding) apparatus or a decompression (decoding) apparatus. The functions of the two types of apparatus substantially overlap. The scan unit 360 and entropy encoder 370 are not used in a decompression mode, and the operation of the predictor 320 (which will be described in detail below) and other units follow mode and parameter information contained in the received compressed bit-stream rather than generating such information themselves.

Figure 8 schematically illustrates the generation of predicted images, and in particular the operation of the image predictor 320.

There are two basic modes of prediction carried out by the image predictor 320: so-called intra-image prediction and so-called inter-image, or motion-compensated (MC), prediction. At the encoder side, each involves detecting a prediction direction in respect of a current block to be predicted, and generating a predicted block of samples according to other samples (in the same (intra) or another (inter) image). By virtue of the units 310 or 450, the difference between the predicted block and the actual block is encoded or applied so as to encode or decode the block respectively.

(At the decoder, or at the reverse decoding side of the encoder, the detection of a prediction direction may be in response to data associated with the encoded data by the encoder, indicating which direction was used at the encoder. Or the detection may be in response to the same factors as those on which the decision was made at the encoder).

Intra-image prediction bases a prediction of the content of a block or region of the image on data from within the same image. This corresponds to so-called l-frame encoding in other video compression techniques. In contrast to I-frame encoding, however, which involves encoding the whole image by intra-encoding, in the present embodiments the choice between intra-and inter-encoding can be made on a block-by-block basis, though in other embodiments the choice is still made on an image-by-image basis.

Motion-compensated prediction is an example of inter-image prediction and makes use of motion information which attempts to define the source, in another adjacent or nearby image, of image detail to be encoded in the current image. Accordingly, in an ideal example, the contents of a block of image data in the predicted image can be encoded very simply as a reference (a motion vector) pointing to a corresponding block at the same or a slightly different position in an adjacent image.

A technique known as "block copy" prediction is in some respects a hybrid of the two, as it uses a vector to indicate a block of samples at a position displaced from the currently predicted block within the same image, which should be copied to form the currently predicted 20 block.

Returning to Figure 8, two image prediction arrangements (corresponding to intra-and inter-image prediction) are shown, the results of which are selected by a multiplexer 500 under the control of a mode signal 510 (for example, from the controller 343) so as to provide blocks of the predicted image for supply to the adders 310 and 450. The choice is made in dependence upon which selection gives the lowest "energy" (which, as discussed above, may be considered as information content requiring encoding), and the choice is signalled to the decoder within the encoded output data-stream. Image energy, in this context, can be detected, for example, by carrying out a trial subtraction of an area of the two versions of the predicted image from the input image, squaring each pixel value of the difference image, summing the squared values, and identifying which of the two versions gives rise to the lower mean squared value of the difference image relating to that image area. In other examples, a trial encoding can be carried out for each selection or potential selection, with a choice then being made according to the cost of each potential selection in terms of one or both of the number of bits required for encoding and distortion to the picture.

The actual prediction, in the intra-encoding system, is made on the basis of image blocks received as part of the signal 460 (as filtered by loop filtering; see below), which is to say, the prediction is based upon encoded-decoded image blocks in order that exactly the same prediction can be made at a decompression apparatus. However, data can be derived from the input video signal 300 by an intra-mode selector 520 to control the operation of the intra-image predictor 530.

For inter-image prediction, a motion compensated (MC) predictor 540 uses motion information such as motion vectors derived by a motion estimator 550 from the input video signal 300. Those motion vectors are applied to a processed version of the reconstructed image 460 by the motion compensated predictor 540 to generate blocks of the inter-image prediction. Accordingly, the units 530 and 540 (operating with the estimator 550) each act as detectors to detect a prediction direction in respect of a current block to be predicted, and as a generator to generate a predicted block of samples (forming part of the prediction passed to the units 310 and 450) according to other samples defined by the prediction direction.

The processing applied to the signal 460 will now be described.

Firstly, the signal may be filtered by a so-called loop filter 565. Various types of loop filters may be used. One technique involves applying a "deblocking" filter to remove or at least tend to reduce the effects of the block-based processing carried out by the transform unit 340 and subsequent operations. A further technique involving applying a so-called sample adaptive offset (SAO) filter may also be used. In general terms, in a sample adaptive offset filter, filter parameter data (derived at the encoder and communicated to the decoder) defines one or more offset amounts to be selectively combined with a given intermediate video sample (a sample of the signal 460) by the sample adaptive offset filter in dependence upon a value of:(i) the given intermediate video sample; or 00 one or more intermediate video samples having a predetermined spatial relationship to the given intermediate video sample.

Also, an adaptive loop filter is optionally applied using coefficients derived by processing the reconstructed signal 460 and the input video signal 300. The adaptive loop filter is a type of filter which, using known techniques, applies adaptive filter coefficients to the data to be filtered.

That is to say, the filter coefficients can vary in dependence upon various factors. Data defining which filter coefficients to use is included as part of the encoded output data-stream.

The filtered output from the loop filter unit 565 in fact forms the output video signal 480 when the apparatus is operating as a decompression apparatus. It is also buffered in one or more image or frame stores 570; the storage of successive images is a requirement of motion compensated prediction processing, and in particular the generation of motion vectors. To save on storage requirements, the stored images in the image stores 570 may be held in a compressed form and then decompressed for use in generating motion vectors. For this particular purpose, any known compression / decompression system may be used. The stored images may be passed to an interpolation filter 580 which generates a higher resolution version of the stored images; in this example, intermediate samples (sub-samples) are generated such that the resolution of the interpolated image is output by the interpolation filter 580 is 4 times (in each dimension) that of the images stored in the image stores 570 for the luminance channel of 4:2:0 and 8 times On each dimension) that of the images stored in the image stores 570 for the chrominance channels of 4:2:0. The interpolated images are passed as an input to the motion estimator 550 and also to the motion compensated predictor 540.

The way in which an image is partitioned for compression processing will now be described. At a basic level, an image to be compressed is considered as an array of blocks or regions of samples. The splitting of an image into such blocks or regions can be carried out by a decision tree, such as that described in SERIES H: AUDIOVISUAL AND MULTIMEDIA SYSTEMS Infrastructure of audio-visual services -Coding of moving video High efficiency video coding Recommendation ITU-T H.265 12/2016. Also: High Efficiency Video Coding (HEVC) algorithms and Architectures, Editors: Madhukar Budagavi, Gary J. Sullivan, Vivienne Sze; chapter 3; ISBN 978-3-319-06894-7; 2014 which are incorporated herein in their respective entireties by reference.

In some examples, the resulting blocks or regions have sizes and, in some cases, shapes which, by virtue of the decision tree, can generally follow the disposition of image features within the image. This in itself can allow for an improved encoding efficiency because samples representing or following similar image features would tend to be grouped together by such an arrangement. In some examples, square blocks or regions of different sizes (such as 4x4 samples up to, say, 64x64 or larger blocks) are available for selection. In other example arrangements, blocks or regions of different shapes such as rectangular blocks or arrays (for example, vertically or horizontally oriented) can be used. Other non-square and non-rectangular blocks are envisaged. The result of the division of the image into such blocks or regions is On at least the present examples) that each sample of an image is allocated to one, and only one, such block or region.

Transform skip mode Figure 9 schematically illustrates a so-called transform skip mode. In this mode, blocks of samples, for example rectangular encoding blocks or arrays of samples such as so-called transform units (TUs) are assigned a 'transform skip' mode indicator, for example by a part of the functionality of the controller 343. When the transform skip indicator is set, as shown by the schematic bypass path 900 in Figure 9, the transform unit 340 On the encoding path) and the inverse transform unit 430 (in the decoding path of the encoding side or in a decoder) is bypassed so that no spatial frequency transform is applied to the samples in that particular block.

The transform skip mode is selectable by the controller 343, alongside a possible selection, of, DCT, DST or another transform mode, in dependence upon properties of the block in question, properties of nearby blocks, trial (full or partial) encodings or the like. Generally, the aim of the selection algorithm executed by the controller 343 is to improve the efficiency of the encoding of the block in question.

In some previously proposed example arrangements, transform skip mode was restricted to 4x4 block sizes or smaller. In more recent examples, this restriction has been relaxed and the transform skip mode can be selectively applied to larger blocks.

Figures 10 and 11 schematically illustrate respective scanning directions, with Figure 10 providing an example applicable to a 4x4 transform skip block and Figure 11 providing an example applicable to a so-called transform block, which is to say a block for which transform skip mode was not enabled and so the block has undergone a spatial frequency transform by the transform unit 340 during encoding.

Referring to Figure 10, in the case of transform skip blocks, the scan order is in this example a diagonal order from the top left ("1) to lower right ("16"). In contrast, as shown in Figure 11, in the case of a transform block (as an example of an encoding block) the scan order is a diagonal order from the lower right to top left. Note that the scan order in use makes little substantive difference to the techniques to be discussed below other than in terms of which coefficients or samples are available "already encoded" or "already decoded" for use in the derivation of encoding parameters for subsequent samples or coefficients.

In the case of larger blocks, a similar scan order can be used, or sub-blocks of (for example) 4x4 coefficients can be scanned as shown, with a predetermined pattern being used to scan each sub-block in order.

In general terms, the blocks of samples of coefficients may be considered as groups of data values (or, once encoded, groups of encoded data values), each having an associated encoding order On other words, the scan order as illustrated by the examples of Figures 10 and 11).

Data sets and escape codes In example arrangements, the entry encoding stage (for example, performed by the entropy encoder 370, with the inverse process being performed by the entropy decoder 410) involves encoding the scanned quantized transform coefficients (with the scan applied by the scan unit 360 being accorded to the examples shown in Figures 10 and 11 for transform skip and transform blocks respectively).

The entropy encoding is arranged to encode the values as one or more so-called data sets along with escape codes for remaining values not encoded by the data sets.

To generate the data sets, the data values to be encoded are handled in the encoding order (for example the scan order). The data sets generated in respect of a block of samples such as a 4x4 block or a 4x4 (or other) sub-portion of a larger block may include: * Significance map (Sig) which indicates the position of so-called "significant" coefficients or samples, which is to say non-zero coefficients or samples. A significance flag indicating a non-zero value is coded for each coefficient position in the block.

* Level greater than 1 flag (GT1) which indicates whether the level is greater than 1 for each significant coefficient. In some examples for a 4x4 block, the flag is sent only for the first 8 significant coefficients in the encoding order; in other examples, it can be sent for each significant coefficient.

* "Value & 1" flag is effectively the least significant bit (LSB) at this stage (where & signifies a logical AND operation).

* Level greater than 2 flag which indicates if the coefficient level is greater than 2 up to and including the first coefficient in the scanning order with this property. Note that the flag is sent only for coefficients larger than 1 as indicated by the Gil flag.

In some examples, after the occurrence of the first coefficient which is greater than 2 in the scanning order, the GT2 flag is not further sent. However, in the present examples being discussed, the GT2 flag is sent at each calculated occurrence.

* Coefficient sign which is provided for the significant coefficients. The absolute coefficient value (ABS(COEFF)) is modified during each coding pass of the above arrangement and the modified value is used in the next pass. The modification is: * At the generation of the significance map, subtract 1; * At the generation of the GT1 map, subtract 1; * At the generation of the value & 1 flag, divide by 2.

In other words, a coefficient for which the GT2 flag has been generated in fact has a minimum value of 4, as shown in the following example in which each data set is shown in turn, with the following column indicating a remaining value (Val) to be encoded after the modification mentioned above: Val (i/p) Sig Val Gt1 Val Val&1 Val Gt2 0 0 1 1 0 0 2 1 1 1 0 0 0 0 3 1 2 1 1 1 0 0 4 1 3 1 2 0 1 1 1 4 1 3 1 1 1 6 1 5 1 4 0 2 1 Escape codes Escape codes are used to encode the remaining absolute level, which is to say level information which has not been encoded by the data sets outlined above. Because of the effective subtraction of 4 discussed above, in an arrangement in which the GT2 flag is always sent when it is applicable, the remaining absolute level needs to be encoded only for "coeff -4". The remaining absolute level is encoded by an escape code, for example, comprising a first portion and a non-unary coded second portion. The second portion may have a length, in bits, dependent on a second portion size value.

Such an arrangement may be referred to as a Golomb-Rice code in which a value to be encoded is considered as two portions (the first and second portions mentioned above). The first portion is the result of the division of the value to be encoded by M, where M = 2b, and the second portion is the remainder for example b least significant bits of the value to be encoded.

In the discussion provided here, the parameter b is referred to as the second portion size value.

In examples, the quotient or first portion is encoded using unary coding and is followed by the remainder encoded using, for example, truncated binary encoding. Note that if M = 1, then this coding is equivalent to unary coding.

In example embodiments the first portion is a prefix and the second portion is a suffix.

For example, the first portion may comprise a unary encoded value. For example, the first portion may comprise a truncated unary value. Note however that the terms "first" and "second" are simply identifiers and do not necessarily imply any requirement for the first portion to precede the second portion in an encoding or transmission order.

Derivation of the second portion size value on the encoding side Figure 12 schematically illustrates an example encoding apparatus. The apparatus will be described in detail below, but a significant feature from the point of view of the present discussion is that the apparatus adaptively generates the second portion size value for use in encoding a particular data value, on the basis of previously-encoded data values in the encoding order (except for the first data value which will be discussed further below).

Referring to Figure 12, data values 1200 are received in the scanning order from the scan unit 360. A generator 1210 generates the data sets described above, namely the significance map, the Gil flag, the LSB (val & 1) flag and the GT2 flag. These are provided to an output unit 1220 for output to the encoded data stream.

An encoder 1230 encodes the escape codes. The encoder 1230 is responsive to a detector 1240 which detects whether there are any remaining absolute values to be encoded, and also to a generator 1250 which generates the second portion size value. The generator 1250 receives as inputs previously encoded data values from the data values 1200 and optionally a parameter defining the bit depth 1255 of the data values 1200, and may refer to a lookup table (LUT) 1260.

The generator 1250 can generate a second portion size value 1265 in respect of each data value 1200, whether or not that data value requires an escape code. Alternatively, a second portion size value may be generated only in respect of data values which do in fact need an escape code.

The encoder 1230 then performs the escape code encoding as discussed above and provides the escape codes to the output unit 1220 for output to the data stream.

In the case that a remaining value is too large to be entirely represented by an escape code, a further code (an escape-escape code) may be provided.

By using the previously encoded data values as the source information (or at least as part of the source information) to derive the second portion size values, the generator 1250 is using information which will also be available at the decoder side, given that the entropy encoding and decoding process is lossless. So, at the decoder side, a similar derivation can be performed with respect to previously decoded data values. Of course, if the particular entropy encoder and decoding process used was not lossless, then encoded and subsequently decoded data values could be used as the source information at the encoder side, which would be the equivalent of providing the return path 390 in Figure 7 after the entropy encoding stage.

However, given the lossless nature of encoding used in this example, equivalent information can be obtained from the data values to be encoded, at the encoder side and the already-decoded data values at the decoder side.

Referring to Figure 13, a decoding apparatus comprises a data set decoder 1300 configured to decode the data sets (up to the GT2 flag) discussed above. A detector 1310 detects whether any escape codes or values are provided and, if so, passes these to an escape code decoder 1320 which applies an inverse operation to the escape code encoding, namely a Golomb-Rice decoding. In order to do this, the decoder 1320 makes use of second portion size value information 1330 provided by a generator 1340 which corresponds in function to the generator 1250 in that it is potentially responsive to the bit depth 1345 and to a lookup table 1350 (identical to the lookup table 1260) and also to the decoded data values 1360. The decoder 1320 decodes the escape code(s) using the second portion size value information 1330 and outputs the combination of the decoded data sets with any decoded escape codes as decoded data 1370 which forms the output of the process and which also forms the input 1360 to the generator 1340.

Examples of source data for derivation of second portion size values Example arrangements for the derivation of second portion size values will be discussed with respect to Figures 14 to 20, which schematically illustrate blocks of coefficients. Here, the blocks could refer to data values provided by the scan unit 360 for entropy encoding or to already-decoded data values output by the arrangement of Figure 13 and provided as inputs 1360 to the generated 1340. The process is symmetrical and equivalent as between these two arrangements in order that an equivalent process for representing data values can be performed at the decoder and the encoder.

As mentioned above, the generation of a second portion size value can be performed only when required or can be performed at other instances, for example in respect of all data values in the group of data values to be encoded or decoded. In either event, it is performed at least when required, which is to say at least when the input data to decoder provides an escape code applicable to a given encoded data value of a given group of data values or at the encoding side, when the encoding of a given data value of a given group of data values require an escape code.

Previously proposed examples Figure 14 schematically illustrates a first previously proposed example, in which a second portion size value for a data value 1400 is obtained as follows: * Sum the two neighbouring previously encoded/previously decoded values 1410, 1420; (note that this example relates to transform skip operation using the scanning order shown in Figure 10) * Apply the result to the lookup table 1260/1350; * This generates a second portion size value between zero and 5.

The lookup table may contain second portion size values according to a pattern along the following lines (where the order from the left as shown may be a mapping to successive sum values or successive ranges of sum values): In other words, this provides a predetermined and non-linear relationship between the sum of the two neighbouring values and the resulting second portion size value. Second portion size values of 4 or 5 are obtained only for very large coefficient values.

This previously proposed arrangement of Figure 14 uses a vertical neighbour 1410 and a horizontal neighbour 1420 from amongst the previously encoded/previously decoded data values in a rectangular array of such values. In some instances, however, such as those shown in Figure 15, the data value 1500, 1520 has only one such neighbour 1510, 1530 available by virtue of the position within the block of the data value 1500, 1520.

Example embodiments using at least two source values In these examples, at the decoded side, when the input data provides an escape code applicable to a given encoded data value (or at the encoding side when the encoding other data value of a given group of data values requires an escape code), the second portion size is derived for a given data value of a given group of data values, other than a first or second data value of the given group in dependence upon data values of a set of two or more previously decoded data values of the given group.

In other words, a constraint is applied so that even if the horizontal and vertical neighbours (for example in a rectangular array) preceding the current sample in the encoding or decoding order are not available, at least two data values will still be used.

In an example of Figure 16, for a data value 1600, the source data values which are already encoded/decoded are the data values 1610, 1620. Alternatively, data values 1610, 1630 could be used. Similarly, for a data value 1640, an example of the source data values would be the data values 1650, 1660. In this way, at least two data values are used.

The constraint described here, namely a minimum of two source data values for each data value position other than the first or second in the encoding order, can be applied to any one of the examples given here, including the previously proposed example of Figures 14 and 15.

Where two or more other source values are used, those two or more previously encoded or decoded data values may have respective predetermined positions, within the given group of encoded data values (for example, a rectangular array), relative to the data value to be encoded or decoded. Such relative positions may for example be positions spatially closest to the data value to be encoded or decoded.

Example embodiments using max values In the previously proposed examples, a sum or (equivalently) a mean of the source data values is used. In the present examples, when an escape code is used (or at least when an escape code is used), the second portion size is derived for the given data value other than a first or a second data value of the given group, in dependence upon a maximum data value of a set of two or more previously encoded or decoded data values of the given group.

The dependence may be such that the lookup table is addressed by the maximum value of any one of the data values, or the maximum value may be doubled in order to provide a comparable entry into the lookup table of the sum of the two data values. In other words, in at least some examples, the look up table may be the same, but the value used as an index (for example, under the previously proposed indexing arrangements) may be differently derived.

This represents an example of comprises detecting a second portion size value from a look-up table addressed by a table address dependent upon the maximum data value of the set of two or more previously decoded data values of the given group.

It has been found empirically that this arrangement can give more efficient data encoding than the previously proposed arrangements using the sum of the source data values. 30 Example embodiments relating to the first data value in the encoding order In the case of the first data value in the encoding order, there are no available source data values to use in the derivation of a second portion size value. Therefore, a default of predetermined value may be used.

In previously proposed examples, the default or predetermined value is set to 0 irrespective of the bit depth of the data to be encoded or decoded data.

In examples of the present arrangement, a different predetermined second portion size value can be used in dependence upon the bit depth of the underlying data.

This can be handled by the generator 1250 being responsive to the bit depth parameter 1255 and by the generator 1340 being responsive to the bit depth parameter 1345.

In these examples, when an escape code is required (or at least when an escape code is required) the second portion size value is derived by the respective generator for the first data value of the given group as a predetermined size value which is different for encoded data values of a first predetermined bit depth and a second predetermined bit depth. In some examples, the following relationship is used Bit depth Default value 8 0 1 Other predetermined second portion size values could be used for higher or different bit depths, for example 2 for a bit depth of 12.

Note that the other example embodiments can operate using this technique or using a single predetermined size value such as 0.

Figure 17 schematically illustrates the first data value 1700 in the encoding order in respect of a transform skip block.

Example embodiments relating to transform blocks Note that as discussed above, the scan order for transform blocks may be the reserve of that applicable to transform skip blocks, namely a scan order shown in Figure 11. This can be applied to the whole transform block or to 4x4 sub-portions of the transform block.

In a transform block, a previously proposed arrangement shown in Figure 18 involves deriving the second portion size value from 5 previously encoded or decoded coefficients, for example a second portion size value for use in respect of data value 1800 is derived from previously encoded or decoded data values 1810.

Again, in the previously proposed arrangements, a sum of these is used as an input to a lookup table such as: The sum requires scaling in the event that, because of the location of a given value (relative to the encoding / decoding order) in the block of the data values 1800, not all five adjacent values are available.

In this example, if the maximum function is used rather than the sum, this can achieve the beneficial results discussed above and also does not require scaling (or at least does not necessarily require scaling dependent upon the number of available values) in the case that not all source values are available.

In some examples, in order to achieve a comparable entry into the lookup table, 5 x the maximum of whichever coefficient data values are considered may be used. This allows, for example, groups of two, three, five or other numbers of data values to be used as source values 1910 for a data value 1900 as shown in the examples of Figure 19 with no other scaling to be required.

Example embodiments using weighted combinations In these examples, such as that shown schematically in Figure 20 in respect of a transform block, when an escape code is required (or at least when an escape code is required) other than a first or second data value in the encoding order, the second portion size value is derived in dependence upon a weighted combination of data values of a set of two or more previously encoded or decoded data values of the given group having respective predetermined positions within the given group relative to the current data value. The weighted combination may be such that a lower weighting is applied to data values which are further away (with respect to the rectangular array arrangement) from the data value to be encoded or decoded.

In the sample of Figure 20, for a data value 2000, the lookup table entry could be obtained by a weighted sum of three data values 2010 plus 50% of two further away data values 2020. Alternatively, the weighting could be applied before a maximum function is applied.

In embodiments, the weighting of each of the 3 closest values need not be equal to one other, neither does the weighting of the two further away values need to be equal to one another. There may be advantages to weighting equidistant values differently for example when coding vertical or near vertical features or horizontal or near horizontal features. The weighting could be derived from other parameters such as a prediction mode.

Obtaining the second data value's second portion size value In the case of the second data value in the encoding order, only one source value is available, namely the data value of the data value of the first encoded or first decoded data value. This means that the choice of whether to use the sum of that single value or the maximum of that single value is equivalent in that particular specific instance. Therefore, the second data value in the encoding order, the second portion size value can be obtained simply in dependence upon the first encoded or first decoded data value.

Example methods

Figure 21 is a schematic flowchart illustrating an example decoding method comprising: receiving (at a step 2100) input data representing groups of encoded data values as one or more data sets and escape codes for values not encoded by the data sets, an escape code comprising a first portion and a non-unary coded second portion having a length, in bits, dependent upon a second portion size value, the groups of encoded data values each having an associated encoding order; decoding (at a step 2110) the one or more data sets; and when the input data provides an escape code applicable to a given encoded data value of a given group of encoded data values (at a step 2120), deriving the second portion size value for the given encoded data value other than a first or a second encoded data value of the given group, in dependence upon a maximum data value of a set of two or more previously decoded data values of the given group, decoding that escape code in dependence upon the derived second portion size and generating a decoded data value for the given encoded data value in dependence upon the one or more decoded data sets and that decoded escape code.

Figure 22 is a schematic flowchart illustrating an example encoding method comprising: encoding (at a step 2200) groups of data values as one or more data sets and escape codes for values not encoded by the data sets, an escape code comprising a first portion and a non-unary coded second portion having a length, in bits, dependent upon a second portion size value, the groups of data values each having an associated encoding order; and when the encoding of a given data value of a given group of data values requires an escape code (at a step 2210), deriving the second portion size value for the given data value, other than a first or a second data value of the given group, in dependence upon a maximum data value of a set of two or more previously encoded data values of the given group, and encoding that escape code in dependence upon the derived second portion size.

Figure 23 is a schematic flowchart illustrating an example decoding method comprising: receiving (at a step 2300) input data representing groups of encoded data values as one or more data sets and escape codes for values not encoded by the data sets, an escape code comprising a first portion and a non-unary coded second portion having a length, in bits, dependent upon a second portion size value, the groups of encoded data values each having an associated encoding order and the encoded data values having a bit depth selected from a first predetermined bit depth and a second predetermined bit depth; decoding (at a step 2310) the one or more data sets; and when the input data provides an escape code applicable to a given encoded data value of a given group of encoded data values (at a step 2320), deriving the second portion size value for the given encoded data value, other than a first encoded data value of the given group, in dependence upon data values of a set of one or more previously decoded data values of the given group, and for the first encoded data value of the given group, as a predetermined size value which is different for encoded data values of the first predetermined bit depth and the second predetermined bit depth, decoding that escape code in dependence upon the derived second portion size and generating a decoded data value for the given encoded data value in dependence upon the one or more decoded data sets and that decoded escape code.

Figure 24 is a schematic flowchart illustrating an example encoding method comprising: encoding (at a step 2400) groups of data values as one or more data sets and escape codes for values not encoded by the data sets, an escape code comprising a first portion and a non-unary coded second portion having a length, in bits, dependent upon a second portion size value, the groups of data values each having an associated encoding order and the data values having a bit depth selected from a first predetermined bit depth and a second predetermined bit depth; and when the encoding of a given data value of a given group of data values requires an escape code (at a step 2410), deriving the second portion size value for the given data value, other than a first data value of the given group, in dependence upon data values of a set of one or more previously encoded data values of the given group, and for the first encoded data value of the given group, as a predetermined size value which is different for encoded data values of the first predetermined bit depth and the second predetermined bit depth, and encoding that escape code in dependence upon the derived second portion size.

Figure 25 is a schematic flowchart illustrating an example decoding method comprising: receiving (at a step 2500) input data representing groups of encoded data values as one or more data sets and escape codes for values not encoded by the data sets, an escape code comprising a first portion and a non-unary coded second portion having a length, in bits, dependent upon a second portion size value, the groups of encoded data values each having an associated encoding order; decoding (at a step 2510) the one or more data sets; and when the input data provides an escape code applicable to a given encoded data value of a given group of encoded data values (at a step 2520), deriving the second portion size value for the given encoded data value, other than a first or a second encoded data value of the given group, in dependence upon data values of a set of two or more previously decoded data values of the given group, decoding that escape code in dependence upon the derived second portion size and generating a decoded data value for the given encoded data value in dependence upon the one or more decoded data sets and that decoded escape code.

Figure 26 is a schematic flowchart illustrating an example encoding method comprising: encoding (at a step 2600) groups of data values as one or more data sets and escape codes for values not encoded by the data sets, an escape code comprising a first portion and a non-unary coded second portion having a length, in bits, dependent upon a second portion size value, the groups of data values each having an associated encoding order; and when the encoding of a given data value of a given group of data values requires an escape code (at a step 2610), deriving the second portion size value for the given data value, other than a first or a second encoded data value of the given group, in dependence upon data values of a set of two or more previously decoded data values of the given group, and encoding that escape code in dependence upon the derived second portion size.

Figure 27 is a schematic flowchart illustrating an example decoding method comprising: receiving (at a step 2700) input data representing groups of encoded data values as one or more data sets and escape codes for values not encoded by the data sets, an escape code comprising a first portion and a non-unary coded second portion having a length, in bits, dependent upon a second portion size value, the groups of encoded data values each having an associated encoding order and representing respective rectangular arrays of encoded data values; decoding (at a step 2710) the one or more data sets; and when the input data provides an escape code applicable to a given encoded data value of a given group of encoded data values (at a step 2720), deriving the second portion size value for the given encoded data value, other than a first or a second encoded data value of the given group, in dependence upon a weighted combination of data values of a set of two or more previously decoded data values of the given group having respective predetermined positions within the given group of encoded data values, relative to the given encoded data value, the weighted combination being such that a lower weighting is applied to decoded data values which are further away in the rectangular array from the given encoded data value, decoding that escape code in dependence upon the derived second portion size and generating a decoded data value for the given encoded data value in dependence upon the one or more decoded data sets and that decoded escape code.

Figure 28 is a schematic flowchart illustrating an example encoding method comprising: encoding (at a step 2800) groups of data values as one or more data sets and escape codes for values not encoded by the data sets, an escape code comprising a first portion and a non-unary coded second portion having a length, in bits, dependent upon a second portion size value, the groups of encoded data values each having an associated encoding order and representing respective rectangular arrays of encoded data values; and when the encoding of a given data value of a given group of data values requires an escape code (at a step 2810), deriving the second portion size value for the given data value, other than a first or a second data value of the given group, in dependence upon a weighted combination of data values of a set of two or more previously encoded data values of the given group having respective predetermined positions within the given group of data values, relative to the given data value, the weighted combination being such that a lower weighting is applied to data values which are further away in the rectangular array from the given data value, and encoding that escape code in dependence upon the derived second portion size.

Any one or more of the above encoding methods may be implemented by the apparatus of Figures 7 and/or 8 and/or 12.

Any one or more of the above decoding methods may be implemented by the apparatus of Figures 7 and/or Band/or 13.

In so far as embodiments of the disclosure have been described as being implemented, at least in part, by software-controlled data processing apparatus, it will be appreciated that a non-transitory machine-readable medium carrying such software, such as an optical disk, a magnetic disk, semiconductor memory or the like, is also considered to represent an embodiment of the present disclosure. Similarly, a data signal comprising coded data generated according to the methods discussed above (whether or not embodied on a non-transitory machine-readable medium) is also considered to represent an embodiment of the present disclosure.

It will be apparent that numerous modifications and variations of the present disclosure are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended clauses, the technology may be practised otherwise than as specifically described herein.

It will be appreciated that the above description for clarity has described embodiments with reference to different functional units, circuitry and/or processors. However, it will be apparent that any suitable distribution of functionality between different functional units, circuitry and/or processors may be used without detracting from the embodiments.

Described embodiments may be implemented in any suitable form including hardware, software, firmware or any combination of these. Described embodiments may optionally be implemented at least partly as computer software running on one or more data processors and/or digital signal processors. The elements and components of any embodiment may be physically, functionally and logically implemented in any suitable way. Indeed the functionality may be implemented in a single unit, in a plurality of units or as part of other functional units. As such, the disclosed embodiments may be implemented in a single unit or may be physically and functionally distributed between different units, circuitry and/or processors.

Although the present disclosure has been described in connection with some embodiments, it is not intended to be limited to the specific form set forth herein. Additionally, although a feature may appear to be described in connection with particular embodiments, one skilled in the art would recognize that various features of the described embodiments may be combined in any manner suitable to implement the technique.

Respective aspects and features are defined by the following numbered clauses: 1. A decoding method comprising: receiving input data representing groups of encoded data values as one or more data sets and escape codes for values not encoded by the data sets, an escape code comprising a first portion and a non-unary coded second portion having a length, in bits, dependent upon a second portion size value, the groups of encoded data values each having an associated encoding order; decoding the one or more data sets; and when the input data provides an escape code applicable to a given encoded data value of a given group of encoded data values, deriving the second portion size value for the given encoded data value other than a first or a second encoded data value of the given group, in dependence upon a maximum data value of a set of two or more previously decoded data values of the given group, decoding that escape code in dependence upon the derived second portion size and generating a decoded data value for the given encoded data value in dependence upon the one or more decoded data sets and that decoded escape code.

2. The method of clause 1, in which the first portion is a prefix and the second portion is a suffix 3. The method of clause 1 or clause 2, in which the first portion comprises a unary encoded value.

4. The method of clause 3, in which the first portion comprises a truncated unary value.

5. The method of any one of the preceding clauses, in which the set of two or more previously decoded data values for the given encoded data value have respective predetermined positions, within the given group of encoded data values, relative to the given encoded data value.

6. The method of clause 5, in which each group of encoded data values represents a rectangular array of encoded data values.

7. The method of clause 6, in which the predetermined positions comprise predetermined positions spatially closest, in the rectangular array, to the position of the given encoded data value.

8. The method of clause 6 or clause 7, in which the encoding order is a diagonal scanning order.

9. The method of any one of the preceding clauses, in which: the input data represents encoded video data; the groups of encoded data values represent respective encoding blocks; and the encoding blocks are one of transform blocks and transform-skip blocks.

10. The method of any one of the preceding clauses, in which the deriving step comprises detecting a second portion size value from a look-up table addressed by a table address 30 dependent upon the maximum data value of the set of two or more previously decoded data values of the given group.

11. The method of any one of the preceding clauses, comprising when the input data provides an escape code applicable to a second encoded data value of the given group of encoded data values, deriving the second portion size value for the second encoded data value in dependence upon the previously decoded first data value of the given group.

12. Computer software which, when executed by a computer, causes the computer to perform the method of any one of the preceding clauses.

13. A non-transitory machine-readable medium which stores the computer software of clause 12.

14. An encoding method comprising: encoding groups of data values as one or more data sets and escape codes for values not encoded by the data sets, an escape code comprising a first portion and a non-unary coded second portion having a length, in bits, dependent upon a second portion size value, the groups of data values each having an associated encoding order; and when the encoding of a given data value of a given group of data values requires an escape code, deriving the second portion size value for the given data value, other than a first or a second data value of the given group, in dependence upon a maximum data value of a set of two or more previously encoded data values of the given group, and encoding that escape code in dependence upon the derived second portion size.

15. The method of clause 14, in which the first portion is a prefix and the second portion is a suffix 16. The method of clause 14 or clause 15, in which the first portion comprises a unary encoded value.

17. The method of clause 16, in which the first portion comprises a truncated unary value.

18. The method of any one of clauses 14 to 17, in which the set of two or more previously encoded data values for the given data value have respective predetermined positions, within the given group of encoded data values, relative to the given encoded data value.

19. The method of clause 18, in which each group of data values represents a rectangular array of data values.

20. The method of clause 19, in which the predetermined positions comprise predetermined positions spatially closest, in the rectangular array, to the position of the given data value.

21. The method of clause 19 or clause 20, in which the encoding order is a diagonal scanning order.

22. The method of any one of clauses 14 to 21, in which: the data values represent video data; the groups of data values represent respective encoding blocks; and the encoding blocks are one of transform blocks and transform-skip blocks.

23. The method of any one of clauses 14 to 22, in which the deriving step comprises detecting a second portion size value from a look-up table addressed by a table address dependent upon the maximum data value of the set of two or more previously encoded data values of the given group.

24. The method of any one of clauses 14 to 23, comprising when the input data provides an escape code applicable to a second data value of the given group of data values, deriving the second portion size value for the second data value in dependence upon the previously encoded first data value of the given group.

25. Computer software which, when executed by a computer, causes the computer to perform the method of any one of clauses 14 to 24 26. A non-transitory machine-readable medium which stores the computer software of clause 25.

27. A decoding apparatus comprising: a decoder configured to receive input data representing groups of encoded data values as one or more data sets and escape codes for values not encoded by the data sets, an escape code comprising a first portion and a non-unary coded second portion having a length, in bits, dependent upon a second portion size value, the groups of encoded data values each having an associated encoding order; the decoder being configured to decode the one or more data sets; and the decoder being configured, when the input data provides an escape code applicable to a given encoded data value of a given group of encoded data values, to derive the second portion size value for the given encoded data value other than a first or a second encoded data value of the given group, in dependence upon a maximum data value of a set of two or more previously decoded data values of the given group, to decode that escape code in dependence upon the derived second portion size and to generate a decoded data value for the given encoded data value in dependence upon the one or more decoded data sets and that decoded escape code.

28 The apparatus of clause 27, in which the first portion is a prefix and the second portion is a suffix.

29. The apparatus of clause 27 or clause 28, in which the first portion comprises a unary encoded value.

30. The apparatus of clause 29, in which the first portion comprises a truncated unary value.

31. The apparatus of any one of clauses 27 to 30, in which the set of two or more previously decoded data values for the given encoded data value have respective predetermined positions, within the given group of encoded data values, relative to the given encoded data value.

32. The apparatus of clause 31, in which each group of encoded data values represents a rectangular array of encoded data values.

33. The apparatus of clause 32, in which the predetermined positions comprise predetermined positions spatially closest, in the rectangular array, to the position of the given encoded data value.

34. The apparatus of clause 32 or clause 33, in which the encoding order is a diagonal scanning order.

35. The apparatus of any one of clauses 27 to 34, in which: the input data represents encoded video data; the groups of encoded data values represent respective encoding blocks; and the encoding blocks are one of transform blocks and transform-skip blocks.

36. The apparatus of any one of clauses 27 to 35, in which the decoder is configured to detect a second portion size value from a look-up table addressed by a table address dependent upon the maximum data value of the set of two or more previously decoded data values of the given group.

37. The apparatus of any one of clauses 27 to 36, in which the decoder is configured, when the input data provides an escape code applicable to a second encoded data value of the given group of encoded data values, to derive the second portion size value for the second encoded data value in dependence upon the previously decoded first data value of the given group.

38. Video data capture, transmission, display and/or storage apparatus comprising the apparatus of any one of clauses 27 to 37.

39. An encoding apparatus comprising: an encoder configured to encode groups of data values as one or more data sets and escape codes for values not encoded by the data sets, an escape code comprising a first portion and a non-unary coded second portion having a length, in bits, dependent upon a second portion size value, the groups of data values each having an associated encoding order; the encoder being configured, when the encoding of a given data value of a given group of data values requires an escape code, to derive the second portion size value for the given data value, other than a first or a second data value of the given group, in dependence upon a maximum data value of a set of two or more previously encoded data values of the given group, and to encode that escape code in dependence upon the derived second portion size.

40. The apparatus of clause 39, in which the first portion is a prefix and the second portion is a suffix.

41. The apparatus of clause 39 or clause 40, in which the first portion comprises a unary encoded value.

42. The apparatus of clause 41, in which the first portion comprises a truncated unary value.

43. The apparatus of any one of clauses 39 to 42, in which the set of two or more previously encoded data values for the given data value have respective predetermined positions, within the given group of encoded data values, relative to the given encoded data value.

44. The apparatus of clause 43, in which each group of data values represents a rectangular array of data values 45. The apparatus of clause 44, in which the predetermined positions comprise predetermined positions spatially closest, in the rectangular array, to the position of the given data value.

46. The apparatus of clause 44 or clause 45, in which the encoding order is a diagonal scanning order.

47. The apparatus of any one of clauses 39 to 46, in which: the data values represent video data; the groups of data values represent respective encoding blocks; and the encoding blocks are one of transform blocks and transform-skip blocks.

48. The apparatus of any one of clauses 39 to 47, in which the encoder is configured to detect a second portion size value from a look-up table addressed by a table address dependent upon the maximum data value of the set of two or more previously encoded data values of the given group.

49. The apparatus of any one of clauses 39 to 48, in which the encoder is configured, when the input data provides an escape code applicable to a second data value of the given group of data values, to derive the second portion size value for the second data value in dependence upon the previously encoded first data value of the given group.

50. Video data capture, transmission, display and/or storage apparatus comprising the apparatus of any one of clauses 39 to 49.

51. A decoding method comprising: receiving input data representing groups of encoded data values as one or more data sets and escape codes for values not encoded by the data sets, an escape code comprising a first portion and a non-unary coded second portion having a length, in bits, dependent upon a second portion size value, the groups of encoded data values each having an associated encoding order and the encoded data values having a bit depth selected from a first predetermined bit depth and a second predetermined bit depth; decoding the one or more data sets; and when the input data provides an escape code applicable to a given encoded data value of a given group of encoded data values, deriving the second portion size value for the given encoded data value, other than a first encoded data value of the given group, in dependence upon data values of a set of one or more previously decoded data values of the given group, and for the first encoded data value of the given group, as a predetermined size value which is different for encoded data values of the first predetermined bit depth and the second predetermined bit depth, decoding that escape code in dependence upon the derived second portion size and generating a decoded data value for the given encoded data value in dependence upon the one or more decoded data sets and that decoded escape code.

52. The method of clause 51, in which the first predetermined bit depth is a bit depth of 8 bits and the second predetermined bit depth is a bit depth of 10 bits.

53. The method of clause 51 or clause 52, in which the predetermined size value is 0 for encoded data values of the first predetermined bit depth and 1 for encoded data values of the second predetermined bit depth.

54. Computer software which, when executed by a computer, causes the computer to perform the method of any one of clauses 51 to 53 55. A non-transitory machine-readable medium which stores the computer software of clause 54.

56. An encoding method comprising: encoding groups of data values as one or more data sets and escape codes for values not encoded by the data sets, an escape code comprising a first portion and a non-unary coded second portion having a length, in bits, dependent upon a second portion size value, the groups of data values each having an associated encoding order and the data values having a bit depth selected from a first predetermined bit depth and a second predetermined bit depth; and when the encoding of a given data value of a given group of data values requires an escape code, deriving the second portion size value for the given data value, other than a first data value of the given group, in dependence upon data values of a set of one or more previously encoded data values of the given group, and for the first encoded data value of the given group, as a predetermined size value which is different for encoded data values of the first predetermined bit depth and the second predetermined bit depth, and encoding that escape code in dependence upon the derived second portion size.

57. The method of clause 56, in which the first predetermined bit depth is a bit depth of 8 bits and the second predetermined bit depth is a bit depth of 10 bits.

58. The method of clause 56 or clause 57, in which the predetermined size value is 0 for encoded data values of the first predetermined bit depth and 1 for encoded data values of the second predetermined bit depth.

59. Computer software which, when executed by a computer, causes the computer to perform the method of any one of clauses 56 to 58 60. A non-transitory machine-readable medium which stores the computer software of clause 59.

61. A decoding apparatus comprising: a decoder configured to receive input data representing groups of encoded data values as one or more data sets and escape codes for values not encoded by the data sets, an escape code comprising a first portion and a non-unary coded second portion having a length, in bits, dependent upon a second portion size value, the groups of encoded data values each having an associated encoding order and the encoded data values having a bit depth selected from a first predetermined bit depth and a second predetermined bit depth; the decoder being configured to decode the one or more data sets; and the decoder being configured, when the input data provides an escape code applicable to a given encoded data value of a given group of encoded data values, to derive the second portion size value for the given encoded data value, other than a first encoded data value of the given group, in dependence upon data values of a set of one or more previously decoded data values of the given group, and for the first encoded data value of the given group, as a predetermined size value which is different for encoded data values of the first predetermined bit depth and the second predetermined bit depth, to decode that escape code in dependence upon the derived second portion size and to generate a decoded data value for the given encoded data value in dependence upon the one or more decoded data sets and that decoded escape code.

62. The apparatus of clause 61, in which the first predetermined bit depth is a bit depth of 8 bits and the second predetermined bit depth is a bit depth of 10 bits.

63. The apparatus of clause 61 or clause 62, in which the predetermined size value is 0 for encoded data values of the first predetermined bit depth and 1 for encoded data values of the second predetermined bit depth.

64. Video data capture, transmission, display and/or storage apparatus comprising the apparatus of any one of clauses 61 to 63.

65. An encoding apparatus comprising: an encoder configured to encode groups of data values as one or more data sets and escape codes for values not encoded by the data sets, an escape code comprising a first portion and a non-unary coded second portion having a length, in bits, dependent upon a second portion size value, the groups of data values each having an associated encoding order and the data values having a bit depth selected from a first predetermined bit depth and a second predetermined bit depth; and the encoder being configured, when the encoding of a given data value of a given group of data values requires an escape code, to derive the second portion size value for the given data value, other than a first data value of the given group, in dependence upon data values of a set of one or more previously encoded data values of the given group, and for the first encoded data value of the given group, as a predetermined size value which is different for encoded data values of the first predetermined bit depth and the second predetermined bit depth, and to encode that escape code in dependence upon the derived second portion size.

66. The apparatus of clause 65, in which the first predetermined bit depth is a bit depth of 8 bits and the second predetermined bit depth is a bit depth of 10 bits.

67. The apparatus of clause 65 or clause 66, in which the predetermined size value is 0 for encoded data values of the first predetermined bit depth and 1 for encoded data values of the second predetermined bit depth.

68. Video data capture, transmission, display and/or storage apparatus comprising the apparatus of any one of clauses 65 to 67 69. A decoding method comprising: receiving input data representing groups of encoded data values as one or more data sets and escape codes for values not encoded by the data sets, an escape code comprising a first portion and a non-unary coded second portion having a length, in bits, dependent upon a second portion size value, the groups of encoded data values each having an associated encoding order; decoding the one or more data sets; and when the input data provides an escape code applicable to a given encoded data value of a given group of encoded data values, deriving the second portion size value for the given encoded data value, other than a first or a second encoded data value of the given group, in dependence upon data values of a set of two or more previously decoded data values of the given group, decoding that escape code in dependence upon the derived second portion size and generating a decoded data value for the given encoded data value in dependence upon the one or more decoded data sets and that decoded escape code.

70. Computer software which, when executed by a computer, causes the computer to perform the method of clause 69.

71. A non-transitory machine-readable medium which stores the computer software of clause 70.

72. An encoding method comprising encoding groups of data values as one or more data sets and escape codes for values not encoded by the data sets, an escape code comprising a first portion and a non-unary coded second portion having a length, in bits, dependent upon a second portion size value, the groups of data values each having an associated encoding order; and when the encoding of a given data value of a given group of data values requires an escape code, deriving the second portion size value for the given data value, other than a first or a second encoded data value of the given group, in dependence upon data values of a set of two or more previously decoded data values of the given group, and encoding that escape code in dependence upon the derived second portion size.

73. Computer software which, when executed by a computer, causes the computer to perform the method of clause 72.

74. A non-transitory machine-readable medium which stores the computer software of clause 73.

75. A decoding apparatus comprising: a decoder configured to receive input data representing groups of encoded data values as one or more data sets and escape codes for values not encoded by the data sets, an escape code comprising a first portion and a non-unary coded second portion having a length, in bits, dependent upon a second portion size value, the groups of encoded data values each having an associated encoding order; the decoder being configured to decode the one or more data sets; and the decoder being configured, when the input data provides an escape code applicable to a given encoded data value of a given group of encoded data values, to derive the second portion size value for the given encoded data value, other than a first or a second encoded data value of the given group, in dependence upon data values of a set of two or more previously decoded data values of the given group, to decode that escape code in dependence upon the derived second portion size and to generate a decoded data value for the given encoded data value in dependence upon the one or more decoded data sets and that decoded escape code.

76. Video data capture, transmission, display and/or storage apparatus comprising the apparatus of clause 75.

77. An encoding apparatus comprising: an encoder configured to encode groups of data values as one or more data sets and escape codes for values not encoded by the data sets, an escape code comprising a first portion and a non-unary coded second portion having a length, in bits, dependent upon a second portion size value, the groups of data values each having an associated encoding order; the encoder being configured, when the encoding of a given data value of a given group of data values requires an escape code, to derive the second portion size value for the given data value, other than a first or a second encoded data value of the given group, in dependence upon data values of a set of two or more previously decoded data values of the given group, and to encode that escape code in dependence upon the derived second portion size.

78. Video data capture, transmission, display and/or storage apparatus comprising the apparatus of clause 77.

79. A decoding method comprising: receiving input data representing groups of encoded data values as one or more data sets and escape codes for values not encoded by the data sets, an escape code comprising a first portion and a non-unary coded second portion having a length, in bits, dependent upon a second portion size value, the groups of encoded data values each having an associated encoding order and representing respective rectangular arrays of encoded data values; decoding the one or more data sets; and when the input data provides an escape code applicable to a given encoded data value of a given group of encoded data values, deriving the second portion size value for the given encoded data value, other than a first or a second encoded data value of the given group, in dependence upon a weighted combination of data values of a set of two or more previously decoded data values of the given group having respective predetermined positions within the given group of encoded data values, relative to the given encoded data value, the weighted combination being such that a lower weighting is applied to decoded data values which are further away in the rectangular array from the given encoded data value, decoding that escape code in dependence upon the derived second portion size and generating a decoded data value for the given encoded data value in dependence upon the one or more decoded data sets and that decoded escape code.

80. The method of clause 79, in which the encoding order is a diagonal scanning order.

81. Computer software which, when executed by a computer, causes the computer to perform the method of clause 79 or clause 80.

82. A non-transitory machine-readable medium which stores the computer software of clause 81.

83. An encoding method comprising: encoding groups of data values as one or more data sets and escape codes for values not encoded by the data sets, an escape code comprising a first portion and a non-unary coded second portion having a length, in bits, dependent upon a second portion size value, the groups of encoded data values each having an associated encoding order and representing respective rectangular arrays of encoded data values; and when the encoding of a given data value of a given group of data values requires an escape code, deriving the second portion size value for the given data value, other than a first or a second data value of the given group, in dependence upon a weighted combination of data values of a set of two or more previously encoded data values of the given group having respective predetermined positions within the given group of data values, relative to the given data value, the weighted combination being such that a lower weighting is applied to data values which are further away in the rectangular array from the given data value, and encoding that escape code in dependence upon the derived second portion size.

84. The method of clause 83, in which the encoding order is a diagonal scanning order.

85. Computer software which, when executed by a computer, causes the computer to perform the method of clause 83 or clause 84.

86. A non-transitory machine-readable medium which stores the computer software of clause 85.

87. A decoding apparatus comprising: a decoder configured to receive input data representing groups of encoded data values as one or more data sets and escape codes for values not encoded by the data sets, an escape code comprising a first portion and a non-unary coded second portion having a length, in bits, dependent upon a second portion size value, the groups of encoded data values each having an associated encoding order and representing respective rectangular arrays of encoded data values; the decoder being configured to decode the one or more data sets; and the decoder being configured, when the input data provides an escape code applicable to a given encoded data value of a given group of encoded data values, to derive the second portion size value for the given encoded data value, other than a first or a second encoded data value of the given group, in dependence upon a weighted combination of data values of a set of two or more previously decoded data values of the given group having respective predetermined positions within the given group of encoded data values, relative to the given encoded data value, the weighted combination being such that a lower weighting is applied to decoded data values which are further away in the rectangular array from the given encoded data value, to decode that escape code in dependence upon the derived second portion size and to generate a decoded data value for the given encoded data value in dependence upon the one or more decoded data sets and that decoded escape code.

88. The apparatus of clause 87, in which the encoding order is a diagonal scanning order.

89. Video data capture, transmission, display and/or storage apparatus comprising the apparatus of clause 87 or clause 88.

90. An encoding apparatus comprising: an encoder configured to encode groups of data values as one or more data sets and escape codes for values not encoded by the data sets, an escape code comprising a first portion and a non-unary coded second portion having a length, in bits, dependent upon a second portion size value, the groups of encoded data values each having an associated encoding order and representing respective rectangular arrays of encoded data values; the encoder being configured, when the encoding of a given data value of a given group of data values requires an escape code, to derive the second portion size value for the given data value, other than a first or a second data value of the given group, in dependence upon a weighted combination of data values of a set of two or more previously encoded data values of the given group having respective predetermined positions within the given group of data values, relative to the given data value, the weighted combination being such that a lower weighting is applied to data values which are further away in the rectangular array from the given data value, and to encode that escape code in dependence upon the derived second portion size.

91. The apparatus of clause 90, in which the encoding order is a diagonal scanning order.

92. Video data capture, transmission, display and/or storage apparatus comprising the apparatus of clause 90 or clause 91.

Claims

CLAIMS1. A decoding method comprising: receiving input data representing groups of encoded data values as one or more data sets and escape codes for values not encoded by the data sets, an escape code comprising a first portion and a non-unary coded second portion having a length, in bits, dependent upon a second portion size value, the groups of encoded data values each having an associated encoding order; decoding the one or more data sets; and when the input data provides an escape code applicable to a given encoded data value of a given group of encoded data values, deriving the second portion size value for the given encoded data value other than a first or a second encoded data value of the given group, in dependence upon a maximum data value of a set of two or more previously decoded data values of the given group, decoding that escape code in dependence upon the derived second portion size and generating a decoded data value for the given encoded data value in dependence upon the one or more decoded data sets and that decoded escape code.
2. The method of claim 1, in which the first portion is a prefix and the second portion is a suffix.
3. The method of claim 1, in which the first portion comprises a unary encoded value.
The method of claim 3, in which the first portion comprises a truncated unary value.
5. The method of claim 1, in which the set of two or more previously decoded data values for the given encoded data value have respective predetermined positions, within the given group of encoded data values, relative to the given encoded data value.
6. The method of claim 5, in which each group of encoded data values represents a rectangular array of encoded data values.
7. The method of claim 6, in which the predetermined positions comprise predetermined positions spatially closest, in the rectangular array, to the position of the given encoded data 35 value.
8. The method of claim 6, in which the encoding order is a diagonal scanning order.
The method of claim 1, in which: the input data represents encoded video data; the groups of encoded data values represent respective encoding blocks; and the encoding blocks are one of transform blocks and transform-skip blocks.
10. The method of claim 1, in which the deriving step comprises detecting a second portion size value from a look-up table addressed by a table address dependent upon the maximum data value of the set of two or more previously decoded data values of the given group.
11. The method of claim 1, comprising when the input data provides an escape code applicable to a second encoded data value of the given group of encoded data values, deriving the second portion size value for the second encoded data value in dependence upon the previously decoded first data value of the given group.
12. Computer software which, when executed by a computer, causes the computer to perform the method of claim 1.
13. A non-transitory machine-readable medium which stores the computer software of claim 12.
14. An encoding method comprising: encoding groups of data values as one or more data sets and escape codes for values not encoded by the data sets, an escape code comprising a first portion and a non-unary coded second portion having a length, in bits, dependent upon a second portion size value, the groups of data values each having an associated encoding order; and when the encoding of a given data value of a given group of data values requires an escape code, deriving the second portion size value for the given data value, other than a first or a second data value of the given group, in dependence upon a maximum data value of a set of two or more previously encoded data values of the given group, and encoding that escape code in dependence upon the derived second portion size.
15. The method of claim 14, in which the first portion is a prefix and the second portion is a suffix.
16. The method of claim 14, in which the first portion comprises a unary encoded value.
17. The method of claim 16, in which the first portion comprises a truncated unary value.
18. The method of claim 14, in which the set of two or more previously encoded data values for the given data value have respective predetermined positions, within the given group of encoded data values, relative to the given encoded data value.
19. The method of claim 18, in which each group of data values represents a rectangular array of data values.
20. The method of claim 19, in which the predetermined positions comprise predetermined positions spatially closest, in the rectangular array, to the position of the given data value.
21. The method of claim 19, in which the encoding order is a diagonal scanning order.
22. The method of claim 14, in which: the data values represent video data; the groups of data values represent respective encoding blocks; and the encoding blocks are one of transform blocks and transform-skip blocks.
23. The method of claim 14, in which the deriving step comprises detecting a second portion size value from a look-up table addressed by a table address dependent upon the maximum data value of the set of two or more previously encoded data values of the given group.
24. The method of claim 14, comprising when the input data provides an escape code applicable to a second data value of the given group of data values, deriving the second portion size value for the second data value in dependence upon the previously encoded first data value of the given group.
25. Computer software which, when executed by a computer, causes the computer to perform the method of claim 14.
26. A non-transitory machine-readable medium which stores the computer software of claim 25.
27. A decoding apparatus comprising: a decoder configured to receive input data representing groups of encoded data values as one or more data sets and escape codes for values not encoded by the data sets, an escape code comprising a first portion and a non-unary coded second portion having a length, in bits, dependent upon a second portion size value, the groups of encoded data values each having an associated encoding order; the decoder being configured to decode the one or more data sets; and the decoder being configured, when the input data provides an escape code applicable to a given encoded data value of a given group of encoded data values, to derive the second portion size value for the given encoded data value other than a first or a second encoded data 10 value of the given group, in dependence upon a maximum data value of a set of two or more previously decoded data values of the given group, to decode that escape code in dependence upon the derived second portion size and to generate a decoded data value for the given encoded data value in dependence upon the one or more decoded data sets and that decoded escape code.
28 The apparatus of claim 27, in which the first portion is a prefix and the second portion is a suffix.
29. The apparatus of claim 27, in which the first portion comprises a unary encoded value. 20
30. The apparatus of claim 29, in which the first portion comprises a truncated unary value.
31. The apparatus of claim 27, in which the set of two or more previously decoded data values for the given encoded data value have respective predetermined positions, within the given group of encoded data values, relative to the given encoded data value.
32. The apparatus of claim 31, in which each group of encoded data values represents a rectangular array of encoded data values.
33. The apparatus of claim 32, in which the predetermined positions comprise predetermined positions spatially closest, in the rectangular array, to the position of the given encoded data value.
34. The apparatus of claim 32, in which the encoding order is a diagonal scanning order.
35. The apparatus of claim 27, in which: the input data represents encoded video data; the groups of encoded data values represent respective encoding blocks; and the encoding blocks are one of transform blocks and transform-skip blocks.
36. The apparatus of claim 27, in which the decoder is configured to detect a second portion size value from a look-up table addressed by a table address dependent upon the maximum data value of the set of two or more previously decoded data values of the given group.
37. The apparatus of claim 27, in which the decoder is configured, when the input data provides an escape code applicable to a second encoded data value of the given group of encoded data values, to derive the second portion size value for the second encoded data value in dependence upon the previously decoded first data value of the given group.
38. Video data capture, transmission, display and/or storage apparatus comprising the apparatus of claim 27.
39. An encoding apparatus comprising: an encoder configured to encode groups of data values as one or more data sets and escape codes for values not encoded by the data sets, an escape code comprising a first portion and a non-unary coded second portion having a length, in bits, dependent upon a second portion size value, the groups of data values each having an associated encoding order; the encoder being configured, when the encoding of a given data value of a given group of data values requires an escape code, to derive the second portion size value for the given data value, other than a first or a second data value of the given group, in dependence upon a maximum data value of a set of two or more previously encoded data values of the given group, and to encode that escape code in dependence upon the derived second portion size.
40. The apparatus of claim 39, in which the first portion is a prefix and the second portion is a suffix.
41. The apparatus of claim 39, in which the first portion comprises a unary encoded value. 30
42. The apparatus of claim 41, in which the first portion comprises a truncated unary value.
43. The apparatus of claim 39, in which the set of two or more previously encoded data values for the given data value have respective predetermined positions, within the given group of encoded data values, relative to the given encoded data value.
44. The apparatus of claim 43, in which each group of data values represents a rectangular array of data values.
45. The apparatus of claim 44, in which the predetermined positions comprise predetermined positions spatially closest, in the rectangular array, to the position of the given data value.
46. The apparatus of claim 44, in which the encoding order is a diagonal scanning order.
47. The apparatus of claim 39, in which: the data values represent video data; the groups of data values represent respective encoding blocks; and the encoding blocks are one of transform blocks and transform-skip blocks.
48. The apparatus of claim 39, in which the encoder is configured to detect a second portion size value from a look-up table addressed by a table address dependent upon the maximum data value of the set of two or more previously encoded data values of the given group.
49. The apparatus of claim 39, in which the encoder is configured, when the input data provides an escape code applicable to a second data value of the given group of data values, to derive the second portion size value for the second data value in dependence upon the previously encoded first data value of the given group.
50. Video data capture, transmission, display and/or storage apparatus comprising the apparatus of claim 39.
51. A decoding method comprising: receiving input data representing groups of encoded data values as one or more data sets and escape codes for values not encoded by the data sets, an escape code comprising a first portion and a non-unary coded second portion having a length, in bits, dependent upon a second portion size value, the groups of encoded data values each having an associated encoding order and the encoded data values having a bit depth selected from a first predetermined bit depth and a second predetermined bit depth; decoding the one or more data sets; and when the input data provides an escape code applicable to a given encoded data value of a given group of encoded data values, deriving the second portion size value for the given encoded data value, other than a first encoded data value of the given group, in dependence upon data values of a set of one or more previously decoded data values of the given group, and for the first encoded data value of the given group, as a predetermined size value which is different for encoded data values of the first predetermined bit depth and the second predetermined bit depth, decoding that escape code in dependence upon the derived second portion size and generating a decoded data value for the given encoded data value in dependence upon the one or more decoded data sets and that decoded escape code.
52. The method of claim 51, in which the first predetermined bit depth is a bit depth of 8 bits and the second predetermined bit depth is a bit depth of 10 bits.
53. The method of claim 52, in which the predetermined size value is 0 for encoded data values of the first predetermined bit depth and 1 for encoded data values of the second predetermined bit depth.
54. Computer software which, when executed by a computer, causes the computer to perform the method of claim 51.
55. A non-transitory machine-readable medium which stores the computer software of claim 54.
56. An encoding method comprising: encoding groups of data values as one or more data sets and escape codes for values not encoded by the data sets, an escape code comprising a first portion and a non-unary coded second portion having a length, in bits, dependent upon a second portion size value, the groups of data values each having an associated encoding order and the data values having a bit depth selected from a first predetermined bit depth and a second predetermined bit depth; and when the encoding of a given data value of a given group of data values requires an escape code, deriving the second portion size value for the given data value, other than a first data value of the given group, in dependence upon data values of a set of one or more previously encoded data values of the given group, and for the first encoded data value of the given group, as a predetermined size value which is different for encoded data values of the first predetermined bit depth and the second predetermined bit depth, and encoding that escape code in dependence upon the derived second portion size.
57. The method of claim 56, in which the first predetermined bit depth is a bit depth of 8 bits and the second predetermined bit depth is a bit depth of 10 bits.
58. The method of claim 57, in which the predetermined size value is 0 for encoded data values of the first predetermined bit depth and 1 for encoded data values of the second predetermined bit depth.
59. Computer software which, when executed by a computer, causes the computer to perform the method of claim 56.
60. A non-transitory machine-readable medium which stores the computer software of claim 59.
61. A decoding apparatus comprising: a decoder configured to receive input data representing groups of encoded data values as one or more data sets and escape codes for values not encoded by the data sets, an escape code comprising a first portion and a non-unary coded second portion having a length, in bits, dependent upon a second portion size value, the groups of encoded data values each having an associated encoding order and the encoded data values having a bit depth selected from a first predetermined bit depth and a second predetermined bit depth; the decoder being configured to decode the one or more data sets; and the decoder being configured, when the input data provides an escape code applicable to a given encoded data value of a given group of encoded data values, to derive the second portion size value for the given encoded data value, other than a first encoded data value of the given group, in dependence upon data values of a set of one or more previously decoded data values of the given group, and for the first encoded data value of the given group, as a predetermined size value which is different for encoded data values of the first predetermined bit depth and the second predetermined bit depth, to decode that escape code in dependence upon the derived second portion size and to generate a decoded data value for the given encoded data value in dependence upon the one or more decoded data sets and that decoded escape code.
62. The apparatus of claim 61, in which the first predetermined bit depth is a bit depth of 8 bits and the second predetermined bit depth is a bit depth of 10 bits.
63. The apparatus of claim 62, in which the predetermined size value is 0 for encoded data values of the first predetermined bit depth and 1 for encoded data values of the second predetermined bit depth.
64. Video data capture, transmission, display and/or storage apparatus comprising the apparatus of claim 60.
65. An encoding apparatus comprising: an encoder configured to encode groups of data values as one or more data sets and escape codes for values not encoded by the data sets, an escape code comprising a first portion and a non-unary coded second portion having a length, in bits, dependent upon a second portion size value, the groups of data values each having an associated encoding order and the data values having a bit depth selected from a first predetermined bit depth and a second predetermined bit depth; and the encoder being configured, when the encoding of a given data value of a given group of data values requires an escape code, to derive the second portion size value for the given data value, other than a first data value of the given group, in dependence upon data values of a set of one or more previously encoded data values of the given group, and for the first encoded data value of the given group, as a predetermined size value which is different for encoded data values of the first predetermined bit depth and the second predetermined bit depth, and to encode that escape code in dependence upon the derived second portion size.
66. The apparatus of claim 65, in which the first predetermined bit depth is a bit depth of 8 bits and the second predetermined bit depth is a bit depth of 10 bits.
67. The apparatus of claim 66, in which the predetermined size value is 0 for encoded data values of the first predetermined bit depth and 1 for encoded data values of the second predetermined bit depth.
68. Video data capture, transmission, display and/or storage apparatus comprising the apparatus of claim 65.
69. A decoding method comprising: receiving input data representing groups of encoded data values as one or more data sets and escape codes for values not encoded by the data sets, an escape code comprising a first portion and a non-unary coded second portion having a length, in bits, dependent upon a second portion size value, the groups of encoded data values each having an associated encoding order; decoding the one or more data sets; and when the input data provides an escape code applicable to a given encoded data value of a given group of encoded data values, deriving the second portion size value for the given encoded data value, other than a first or a second encoded data value of the given group, in dependence upon data values of a set of two or more previously decoded data values of the given group, decoding that escape code in dependence upon the derived second portion size and generating a decoded data value for the given encoded data value in dependence upon the one or more decoded data sets and that decoded escape code.
70. Computer software which, when executed by a computer, causes the computer to perform the method of claim 69.
71. A non-transitory machine-readable medium which stores the computer software of claim 10 70.
72. An encoding method comprising: encoding groups of data values as one or more data sets and escape codes for values not encoded by the data sets, an escape code comprising a first portion and a non-unary coded second portion having a length, in bits, dependent upon a second portion size value, the groups of data values each having an associated encoding order; and when the encoding of a given data value of a given group of data values requires an escape code, deriving the second portion size value for the given data value, other than a first or a second encoded data value of the given group, in dependence upon data values of a set of two or more previously decoded data values of the given group, and encoding that escape code in dependence upon the derived second portion size.
73. Computer software which, when executed by a computer, causes the computer to perform the method of claim 72.
74. A non-transitory machine-readable medium which stores the computer software of claim 73.
75. A decoding apparatus comprising: a decoder configured to receive input data representing groups of encoded data values as one or more data sets and escape codes for values not encoded by the data sets, an escape code comprising a first portion and a non-unary coded second portion having a length, in bits, dependent upon a second portion size value, the groups of encoded data values each having an associated encoding order; the decoder being configured to decode the one or more data sets; and the decoder being configured, when the input data provides an escape code applicable to a given encoded data value of a given group of encoded data values, to derive the second portion size value for the given encoded data value, other than a first or a second encoded data value of the given group, in dependence upon data values of a set of two or more previously decoded data values of the given group, to decode that escape code in dependence upon the derived second portion size and to generate a decoded data value for the given encoded data value in dependence upon the one or more decoded data sets and that decoded escape code.
76. Video data capture, transmission, display and/or storage apparatus comprising the apparatus of claim 75.
77. An encoding apparatus comprising: an encoder configured to encode groups of data values as one or more data sets and escape codes for values not encoded by the data sets, an escape code comprising a first portion and a non-unary coded second portion having a length, in bits, dependent upon a second portion size value, the groups of data values each having an associated encoding order; the encoder being configured, when the encoding of a given data value of a given group of data values requires an escape code, to derive the second portion size value for the given data value, other than a first or a second encoded data value of the given group, in dependence upon data values of a set of two or more previously decoded data values of the given group, and to encode that escape code in dependence upon the derived second portion size.
78. Video data capture, transmission, display and/or storage apparatus comprising the apparatus of claim 77.
79. A decoding method comprising: receiving input data representing groups of encoded data values as one or more data sets and escape codes for values not encoded by the data sets, an escape code comprising a first portion and a non-unary coded second portion having a length, in bits, dependent upon a second portion size value, the groups of encoded data values each having an associated encoding order and representing respective rectangular arrays of encoded data values; decoding the one or more data sets; and when the input data provides an escape code applicable to a given encoded data value of a given group of encoded data values, deriving the second portion size value for the given encoded data value, other than a first or a second encoded data value of the given group, in dependence upon a weighted combination of data values of a set of two or more previously decoded data values of the given group having respective predetermined positions within the given group of encoded data values, relative to the given encoded data value, the weighted combination being such that a lower weighting is applied to decoded data values which are further away in the rectangular array from the given encoded data value, decoding that escape code in dependence upon the derived second portion size and generating a decoded data value for the given encoded data value in dependence upon the one or more decoded data sets and that decoded escape code.
80. The method of claim 79, in which the encoding order is a diagonal scanning order.
81. Computer software which, when executed by a computer, causes the computer to perform the method of claim 79.
82. A non-transitory machine-readable medium which stores the computer software of claim 81.
83. An encoding method comprising: encoding groups of data values as one or more data sets and escape codes for values not encoded by the data sets, an escape code comprising a first portion and a non-unary coded second portion having a length, in bits, dependent upon a second portion size value, the groups of encoded data values each having an associated encoding order and representing respective rectangular arrays of encoded data values; and when the encoding of a given data value of a given group of data values requires an escape code, deriving the second portion size value for the given data value, other than a first or a second data value of the given group, in dependence upon a weighted combination of data values of a set of two or more previously encoded data values of the given group having respective predetermined positions within the given group of data values, relative to the given data value, the weighted combination being such that a lower weighting is applied to data values which are further away in the rectangular array from the given data value, and encoding that escape code in dependence upon the derived second portion size.
84. The method of claim 83, in which the encoding order is a diagonal scanning order.
85. Computer software which, when executed by a computer, causes the computer to perform the method of claim 83.
86. A non-transitory machine-readable medium which stores the computer software of claim 85.
87. A decoding apparatus comprising: a decoder configured to receive input data representing groups of encoded data values as one or more data sets and escape codes for values not encoded by the data sets, an escape code comprising a first portion and a non-unary coded second portion having a length, in bits, dependent upon a second portion size value, the groups of encoded data values each having an associated encoding order and representing respective rectangular arrays of encoded data values; the decoder being configured to decode the one or more data sets; and the decoder being configured, when the input data provides an escape code applicable to a given encoded data value of a given group of encoded data values, to derive the second portion size value for the given encoded data value, other than a first or a second encoded data value of the given group, in dependence upon a weighted combination of data values of a set of two or more previously decoded data values of the given group having respective predetermined positions within the given group of encoded data values, relative to the given encoded data value, the weighted combination being such that a lower weighting is applied to decoded data values which are further away in the rectangular array from the given encoded data value, to decode that escape code in dependence upon the derived second portion size and to generate a decoded data value for the given encoded data value in dependence upon the one or more decoded data sets and that decoded escape code.
88. The apparatus of claim 87, in which the encoding order is a diagonal scanning order.
89. Video data capture, transmission, display and/or storage apparatus comprising the apparatus of claim 87.
90. An encoding apparatus comprising: an encoder configured to encode groups of data values as one or more data sets and escape codes for values not encoded by the data sets, an escape code comprising a first portion and a non-unary coded second portion having a length, in bits, dependent upon a second portion size value, the groups of encoded data values each having an associated encoding order and representing respective rectangular arrays of encoded data values; the encoder being configured, when the encoding of a given data value of a given group of data values requires an escape code, to derive the second portion size value for the given data value, other than a first or a second data value of the given group, in dependence upon a weighted combination of data values of a set of two or more previously encoded data values of the given group having respective predetermined positions within the given group of data values, relative to the given data value, the weighted combination being such that a lower weighting is applied to data values which are further away in the rectangular array from the given data value, and to encode that escape code in dependence upon the derived second portion size.
91. The apparatus of claim 90, in which the encoding order is a diagonal scanning order.
92. Video data capture, transmission, display and/or storage apparatus comprising the apparatus of claim 90.