WO2018010492A1 - 视频编码中帧内预测模式的快速决策方法 - Google Patents

视频编码中帧内预测模式的快速决策方法 Download PDF

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WO2018010492A1
WO2018010492A1 PCT/CN2017/085605 CN2017085605W WO2018010492A1 WO 2018010492 A1 WO2018010492 A1 WO 2018010492A1 CN 2017085605 W CN2017085605 W CN 2017085605W WO 2018010492 A1 WO2018010492 A1 WO 2018010492A1
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mode
prediction mode
pml
prediction
rate distortion
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French (fr)
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董胜富
王萌
王文敏
王振宇
王荣刚
李英
赵辉
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北京大学深圳研究生院
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    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
    • H04N19/00—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals
    • H04N19/10—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals using adaptive coding
    • H04N19/102—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals using adaptive coding characterised by the element, parameter or selection affected or controlled by the adaptive coding
    • H04N19/103—Selection of coding mode or of prediction mode
    • H04N19/11—Selection of coding mode or of prediction mode among a plurality of spatial predictive coding modes
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
    • H04N19/00—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals
    • H04N19/10—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals using adaptive coding
    • H04N19/134—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals using adaptive coding characterised by the element, parameter or criterion affecting or controlling the adaptive coding
    • H04N19/136—Incoming video signal characteristics or properties
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
    • H04N19/00—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals
    • H04N19/10—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals using adaptive coding
    • H04N19/134—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals using adaptive coding characterised by the element, parameter or criterion affecting or controlling the adaptive coding
    • H04N19/146—Data rate or code amount at the encoder output
    • H04N19/147—Data rate or code amount at the encoder output according to rate distortion criteria
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
    • H04N19/00—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals
    • H04N19/10—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals using adaptive coding
    • H04N19/169—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals using adaptive coding characterised by the coding unit, i.e. the structural portion or semantic portion of the video signal being the object or the subject of the adaptive coding
    • H04N19/17—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals using adaptive coding characterised by the coding unit, i.e. the structural portion or semantic portion of the video signal being the object or the subject of the adaptive coding the unit being an image region, e.g. an object
    • H04N19/176—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals using adaptive coding characterised by the coding unit, i.e. the structural portion or semantic portion of the video signal being the object or the subject of the adaptive coding the unit being an image region, e.g. an object the region being a block, e.g. a macroblock
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
    • H04N19/00—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals
    • H04N19/50—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals using predictive coding
    • H04N19/593—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals using predictive coding involving spatial prediction techniques

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  • the present invention relates to a video coding method, and more particularly to a method for fast decision making in an intra prediction mode in video coding.
  • Predictive coding is one of the core technologies in video coding.
  • the coding algorithm uses the spatial correlation between adjacent pixels of the video signal and the temporal correlation between adjacent images to predictively encode, eliminate redundancy in the video signal, and achieve efficient compression.
  • the new generation of video coding technology uses two methods of intra prediction and inter prediction. Among them, intra prediction uses spatial correlation to predict uncoded pixels using coded pixels, eliminating spatial redundancy in the video signal. By analyzing the information of the current coded block and the adjacent coded block, the best prediction of the current block is obtained by using the angle mode and the plane mode.
  • the New Generation Video Coding adopts a recursive coding tree structure and proposes the concepts of Coding Unit (CU), Prediction Unit (PU) and Transform Unit.
  • the complex block partitioning mechanism allows the encoder to adaptively select the encoding mode based on the video content characteristics.
  • the prediction of the luminance block uses up to 35 prediction modes, including 33 angle modes and 2 kinds. Non-angle mode.
  • the improvement of coding performance brought by the above two aspects is based on increasing the complexity of the algorithm, and the rate-distortion optimization (RDO) of all prediction modes is performed for each block-sized unit. huge.
  • the proposed JCTVC-D283 standard complements the Rough Mode Design (RMD) algorithm for intra prediction and the fast mode decision algorithm for selecting the Most Possible Mode (MPM).
  • RMD Rough Mode Design
  • MPM Most Possible Mode
  • the present invention provides a new method for intra prediction fast mode decision making, which fully utilizes the main feature information of the coding block, reduces the computational complexity of intra prediction, and improves on the premise of ensuring coding performance.
  • the speed of video encoding is a new method for intra prediction fast mode decision making, which fully utilizes the main feature information of the coding block, reduces the computational complexity of intra prediction, and improves on the premise of ensuring coding performance.
  • the principle of the present invention is that in HEVC intra prediction, the coefficients in the 33 angular modes are ⁇ 2, 6, 10, 14, 18, 22, 26,
  • the set of direction patterns of 30, 34 ⁇ can representatively represent the texture information of the current block, and the present invention uses the above-mentioned set of direction patterns as a principal prediction orientation set (Principal Prediction Orientation); two predictions that define an absolute value of the coefficient difference of 4 The orientations are adjacent; the three orientations with the least cost are selected and recorded as M0, M1, and M2, respectively.
  • a prediction mode candidate list (PML) is constructed, and if the least cost is two adjacent prediction directions ⁇ M0, M1 ⁇ (assuming that M0 is a mode with a small cost and a large mode coefficient), then further The three modes separated by 1 between the two modes, the two modes respectively outward, and the two non-angle modes of Planar and DC are added to the candidate list ⁇ M0, M1, 0, 1, M0-1 , M0-2, M0-3, M0+1, M0+2, M1-1, M1-2 ⁇ ; if the least cost is two non-adjacent prediction directions, then ⁇ M0, M1 ⁇ are adjacent A mode with an interval of 1, 2, a mode with a ⁇ M2 ⁇ interval of 2, and two non-angle modes of Planar and DC are added to the candidate list ⁇ M0, M1, 0, 1, M0-1, M0-2, M0+1, M0+2, M1-1, M1-2, M1+1, M1+2, M2, M2+2, M2-2 ⁇ .
  • the number of prediction modes is determined according to the size of the prediction unit PU, that is, the candidate prediction mode in which the rate distortion optimization is finally performed.
  • the candidate prediction mode in which the rate distortion optimization is finally performed For PU blocks larger than 8 ⁇ 8, only the two modes with the least cost in PML are selected for rate-distortion optimization, and the other cases are selected with the least cost-optimized selection.
  • a fast decision method for intra prediction mode in video coding which determines the prediction orientation, uses the correlation of the angle mode and the size of the prediction unit PU to construct a prediction mode candidate list, thereby reducing the number of prediction modes for rate distortion optimization, thereby Reduce the computational complexity of intra prediction and improve the video encoding speed; including the following steps:
  • Step 1 For the current PU block of any size, predict the nine directions of the mode coefficients of the main predicted azimuth set as ⁇ 2, 6, 10, 14, 18, 22, 26, 30, 34 ⁇ , and calculate the rough rate. Distortion cost; sorting the patterns of the main predicted azimuth set according to the rate distortion cost, selecting the first three modes M0, M1 and M2 with the best rate distortion cost, and the rate distortion cost of the M0, M1 and M2 satisfies J(M0) ⁇ J(M1) ⁇ J(M2); the orientation relationship of M0, M1 and M2 is adjacent or not adjacent;
  • Step 2 When M0 is adjacent to M1, the optimal direction of prediction is near M0 and M1. At this time, three modes (M0-1, M0-2, M0-3) and M0 and M1 between M0 and M1 are calculated. The adjacent four modes (M0+1, M0+2, M1-1, M1-2) are added, and the DC mode and the Planar mode are added to the prediction mode candidate list PML; when M0 is not adjacent to M1 , respectively, the ⁇ M0, M1 ⁇ mode with adjacent intervals of 1 and 2, the mode of ⁇ M2 ⁇ interval 2, and the two non-angle modes of Planar and DC are added to the prediction mode candidate list PML;
  • Step 3 Calculate the rate distortion cost in the prediction mode candidate list PML.
  • the block size of the PU block is 4 ⁇ 4 or 8 ⁇ 8, only the two modes with the lowest rate distortion cost in the prediction mode candidate list PML are retained. Optimization; when the block size of the PU block is 16 ⁇ 16, 32 ⁇ 32 or 64 ⁇ 64, three modes with the lowest rate distortion cost in the prediction mode candidate list PML are selected for rate distortion optimization;
  • Step 4 adding the pattern in the most probable prediction mode candidate list MPM to the prediction mode candidate list PML without repetition;
  • Step 5 Perform last rate-distortion optimization on the mode in the prediction mode candidate list PML obtained in step 4; use the mode with the lowest rate-distortion cost as the optimal mode of intra prediction of the current PU block.
  • the main predicted azimuth set in step 1 is the HEVC intra prediction angle mode coefficient is ⁇ 2, 6, 10, 14, 18, 22, 26, 30 , 34 ⁇ the set of direction modes, including nine orientations.
  • the two predicted azimuths are adjacent.
  • step 1 specifically calculates the coarse rate distortion cost by using a Hada code conversion method.
  • step 2 when M0 is adjacent to M1, M0 is set to a larger mode, and the prediction mode candidate list PML is updated to ⁇ M0, M1, 0. , 1M0-1, M0-2, M0-3, M0+1, M0+2, M1-1, M1-2 ⁇ ; when M0 is not adjacent to M1, the prediction mode candidate list PML is updated to ⁇ M0, M1 , 0, 1, M0-1, M0-2, M0+1, M0+2, M1-1, M1-2, M1+1, M1+2, M2, M2+2, M2-2 ⁇ .
  • the coefficients in the range of the angle mode are acquired by loop, and then added to Prediction mode candidate list PML.
  • the rate distortion cost is calculated by using the Hada code conversion method.
  • the final rate distortion optimization performed in step 5 includes the following steps:
  • the invention provides a new intra prediction fast mode decision method, fully utilizes the main feature information of the coding block, reduces the computational complexity of the intra prediction, and improves the video coding speed under the premise of ensuring the coding performance.
  • the invention has the following advantages:
  • the present invention makes full use of the correlation between the angle modes by judging the predicted orientation, and reduces the number of modes for performing the Hadamard transform, thereby reducing the computational complexity of the prediction process.
  • the present invention constructs a prediction mode candidate list, which can make a faster and more accurate judgment on the direction mode, and reduces the number of modes for performing rate distortion optimization for the size of the PU.
  • FIG. 1 is a schematic diagram of prediction point coordinates and reference point selection of HEVC intra prediction
  • each square represents one pixel; Rx, y represents a predicted reference pixel; Px, y represents a predicted pixel; x and y represent the position of the pixel, and the upper left corner is a (0, 0) origin.
  • FIG. 2 is a flow chart of a method for quickly determining an intra prediction mode provided by the present invention.
  • FIG. 3 is a schematic diagram of a direction prediction mode of HEVC and a main predicted orientation set of the present invention
  • the black solid line indicates the main predicted orientation set of the present invention, specifically, the nine prediction mode sets whose coefficients are ⁇ 2, 6, 10, 14, 18, 22, 26, 30, 34 ⁇ .
  • FIG. 4 is a schematic diagram of candidate prediction modes when primary prediction azimuths are adjacent in an embodiment of the present invention
  • M0 is a larger mode
  • the candidate prediction mode list is ⁇ M0, M1, 0, 1M0-1, M0-2, M0-3, M0+1, M0+2, M1-1, M1-2 ⁇ .
  • FIG. 5 is a schematic diagram of candidate prediction modes when primary prediction azimuths are not adjacent in an embodiment of the present invention.
  • ⁇ M0, M1 ⁇ when M0 and M1 are not adjacent, respectively, a mode in which ⁇ M0, M1 ⁇ is adjacent to interval 1 or 2, a mode in which ⁇ M2 ⁇ interval is 2, and two non-angle modes in Planar and DC are added to the candidate prediction mode list.
  • the invention provides a fast decision method for intra prediction mode in video coding, which determines a prediction mode by using a correlation of an angle mode and a size of a prediction unit PU to reduce a prediction mode for performing rate distortion optimization.
  • the number thereby reducing the computational complexity of intra prediction and improving the video encoding speed.
  • the following embodiment describes an intra prediction implementation process that encodes a current frame luma block, constructing a prediction block that differs the least from the original luma block.
  • CU Coding Unit
  • LCU maximum coding unit
  • SCU minimum coding unit
  • Intra prediction performs mode decision and block partitioning on the current CU, and multiple recursive selection of the best partitioning method and optimal mode.
  • Prediction Unit The same as the size of the current CU. When the current CU is an LCU, it can continue to be divided into four smaller prediction blocks.
  • HEVC Luminance Component Intra Prediction supports five sizes of PUs: 4x4, 8x8, 16x16, 32x32, and 64x64.
  • HEVC intra prediction includes 35 prediction modes, which are different predictions of the current block according to certain rules by the coded pixel values of the left adjacent column and the upper adjacent row of the current block. the way. These include 33 angle prediction modes and 2 non-angle prediction modes.
  • Non-angle prediction includes direct current (DC) prediction and plane (Planar) prediction. The coordinates of the predicted points and the selection of the reference points are shown in Fig. 1.
  • the angle prediction mode has a coefficient of 2 to 34, and the DC mode and the Planar mode have coefficients of 0 and 1, respectively.
  • MCM Probable Prediction Mode
  • Rate Distortion Optimization HEVC uses the rate-distortion optimization process to measure the rate and distortion performance of different prediction modes and select the optimal prediction mode.
  • Intra prediction is to obtain the optimal mode and prediction mode of different divided CUs by mode decision, and complete the prediction process.
  • the flow of the present invention is shown in Figure 2.
  • the core method of intra mode decision includes the following steps:
  • Step 1 predicting 9 kinds of orientations of mode coefficients of the main predicted azimuth set of ⁇ 2, 6, 10, 14, 18, 22, 26, 30, 34 ⁇ for the current PU block of any size, in order to improve the coding speed,
  • the Hada code transform is used instead of the discrete cosine transform to calculate the coarse rate distortion cost after the Hada code transform.
  • the cost function is shown in Equation 1, where J is the rate distortion cost, MODE is the coefficient (0-35) of all modes of HEVC intra prediction, ⁇ is the Lagrangian daily quantity, and SATD (Sum of Absolute Transform Difference) is The sum of the absolute values of the block prediction mode and the original block pixel value after the Hada code is transformed.
  • R MODE indicates the current mode residual
  • B MODE indicates the number of bits that the mode needs to encode.
  • the rate distortion cost calculated according to Equation 1 is ordered by the mode of the main predicted azimuth set, and the rate distortion cost is selected therefrom.
  • the HEVC intra prediction all mode and the main predicted azimuth set proposed by the present invention are as shown in FIG. 3, and the main predicted azimuth set is 9 coefficients having coefficients of ⁇ 2, 6, 10, 14, 18, 22, 26, 30, 34 ⁇ .
  • the set of prediction modes is indicated by a solid black line in FIG.
  • Step 2 If M0 and M1 are adjacent, that is, the absolute value of the difference is 4, indicating that the predicted optimal direction is near M0 and M1, then the three modes between M0 and M1 are further calculated (M0-1, M0-2, M0-3) and four adjacent modes of M0 and M1 (M0+1, M0+2, M1-1, M1-2), as shown in Figure 4, and DC mode and The Planar mode is added to the prediction mode candidate list PML. Assuming M0 is a larger mode, the list is updated to ⁇ M0, M1, 0, 1M0-1, M0-2, M0-3, M0+1, M0+2, M1-1, M1-2 ⁇ . If M0 and M1 are not adjacent, that is, the absolute value of the difference is greater than 4, as shown in FIG.
  • Step 3 Calculate the rate distortion penalty in PML according to Equation 1. For PU blocks less than or equal to 8 ⁇ 8, that is, the block size is 4 ⁇ 4 or 8 ⁇ 8, only the two modes with the lowest rate distortion cost in the list are retained, otherwise, the block size is 16 ⁇ 16, 32 ⁇ 32 or At 64 ⁇ 64, the choice of the three least-rate-rate-optimized optimizations is selected.
  • Step 4 Using the MPM, add the patterns in its candidate list to the PML list without duplication.
  • step 5 the last rate-distortion optimization is performed on the mode in the PML list, and the mode with the least cost is the optimal mode of intra prediction of the current block, which specifically includes:
  • This step includes the process of transforming, quantifying, and reconstructing
  • Equation 2 Calculate the rate distortion cost of each predicted angle mode in the PML list by Equation 2:
  • Equation 2 J is the rate distortion cost, and D(Mode) and B(Mode) respectively represent the distortion and the number of bits when using different angle modes;
  • the mode in which the rate distortion cost is the smallest among the rate distortion costs obtained in step 52) is taken as the optimal intra prediction direction mode.
  • the specific implementation of the present invention is to implement and test the HEVC general test sequence including Class A, Class B, Class C, Class D and Class E on the HEVC reference software HM16.0, configured as a full I frame, and the HEVC standard specifies 52 quantization steps. Long, corresponding to 52 quantization parameters (QP). According to the general condition, the quantization parameter QP is set to 22, 27, 32, 37, respectively.
  • QP quantization parameters
  • Table 1 represents the luminance component
  • U and V represent the chrominance components, wherein the average BD-rate of the luminance portion is slightly increased by 0.65%, and the overall time saving (TS) is 26.76%.

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Abstract

本发明公布了一种视频编码中帧内预测模式的快速决策方法，通过判断预测方位，利用角度模式的相关性和PU尺寸构造PML，减少进行率失真优化的预测模式数目；包括：对主要预测方位集中的方位进行预测，选取率失真代价最优的前三个模式；将其相邻方向及Planar、DC模式加入PML中；计算PML中模式的率失真代价；将MPM中的模式无重复地添加入PML中；进行最后的率失真优化；将率失真代价最小的模式作为当前PU块的帧内预测的最优模式。本发明能够降低帧内预测的运算复杂度，提高视频编码速度。

Description

视频编码中帧内预测模式的快速决策方法 技术领域
本发明涉及视频编码方法，尤其涉及一种视频编码中针对帧内预测模式快速决策的方法。
背景技术
视频编码技术解决的重点问题是数字视频海量数据的编码压缩问题。预测编码是视频编码中的核心技术之一。编码算法利用视频信号相邻像素间的空间相关性和相邻图像之间的时间相关性进行预测编码，消除视频信号中的冗余，实现高效压缩。新一代视频编码技术采用帧内预测和帧间预测两种方法。其中，帧内预测利用空间相关性，使用已编码像素预测未编码像素，消除视频信号中的空间冗余。通过分析当前编码块与相邻已编码块的信息，利用角度模式以及平面模式获取当前块的最佳预测。
新一代视频编码标准(High Efficiency Video Coding,HEVC)采用了递归编码树结构，并提出了编码单元(Coding Unit,CU)、预测单元(Prediction Unit,PU)和变换单元(Transform Unit)的概念,复杂的块划分机制使得编码器根据视频内容特性自适应选择编码模式。同时，为了更好地匹配视频中的复杂纹理，更好地去除空间冗余，在HEVC的帧内预测中，亮度块的预测采用了多达35种预测模式，包括33种角度模式和2种非角度模式。以上两方面所带来的编码性能的提升，是以增加算法复杂度为代价，为每一个块大小的单元都进行全部预测模式的率失真最优选择(Rate-Distortion optimization，RDO)，计算量巨大。
该标准已采纳的提案JCTVC-D283，为帧内预测补充了粗略模式决策(Rough Mode Design，RMD)算法以及选取最可能模式(Most Possible Mode，MPM)的快速模式决策算法。首先通过Hadamard(哈达玛)变换及码率估计，根据PU尺寸选择3或8个模式，另外加入相邻块预测模式的方法，来减少率失真优化的模式个数，达到了一定的加速效果。但是，这些方法不能充分利用角度模式之间的相关性，HEVC的帧内预测过程仍然有很大的提速空间。
发明内容
为了克服上述现有技术的不足，本发明提供一种新的帧内预测快速模式决策的方法，充分利用编码块主要特征信息，降低帧内预测的运算复杂度，在保证编码性能的前提下提高视频编码的速度。
本发明的原理是：在HEVC帧内预测中，33种角度模式中系数为{2,6,10,14,18,22，26, 30,34}的方向模式集合能够有代表性地表征当前块的纹理信息，本发明将上述方向模式集合作为主要预测方位集(Principal Prediction Orientation)；定义系数差的绝对值为4的两个预测方位是相邻的；选取代价最小的3个方位，分别记为M0、M1、M2。然后，构建预测模式候选列表(Prediction Mode List，PML)，如果代价最小的是两个相邻的预测方位{M0,M1}(假设M0为代价较小、模式系数较大的模式)，则进一步将该两种模式之间的相隔为1的三种模式、其分别向外的两种模式、和Planar、DC两个非角度模式加入候选列表中{M0,M1,0,1,M0-1,M0-2,M0-3,M0+1,M0+2,M1-1,M1-2}；如果代价最小的是两个不相邻的预测方位，则分别将{M0,M1}相邻间隔为1、2的模式、{M2}间隔为2的模式和Planar、DC两个非角度模式加入候选列表中{M0,M1,0,1,M0-1,M0-2,M0+1,M0+2,M1-1,M1-2,M1+1,M1+2,M2,M2+2,M2-2}。之后，根据预测单元PU的尺寸确定预测模式个数，即最终进行率失真优化的候选预测模式。对于大于8×8的PU块，仅选取PML中的代价最小的2种模式进行率失真优化选择，其余情况选取代价最小的3个进行率失真优化选择。
本发明提供的技术方案是：
一种视频编码中帧内预测模式的快速决策方法，该方法通过判断预测方位，利用角度模式的相关性和预测单元PU的尺寸构造预测模式候选列表，减少进行率失真优化的预测模式数目，从而降低帧内预测的运算复杂度，提高视频编码速度；包括如下步骤：
步骤1，对当前任意尺寸的PU块，对主要预测方位集中的模式系数为{2,6,10,14,18,22，26,30,34}的九种方位进行预测，计算得到粗略率失真代价；根据率失真代价对主要预测方位集中的模式排序，从中选取率失真代价最优的前三个模式M0、M1和M2，所述M0、M1和M2的率失真代价满足J(M0)<J(M1)<J(M2)；所述M0、M1和M2的方位关系为相邻或不相邻；
步骤2，当M0与M1相邻时，预测最优方向在M0和M1附近，此时计算M0与M1之间的三个模式(M0-1,M0-2,M0-3)和M0与M1之外的相邻的四个模式(M0+1，M0+2，M1-1，M1-2)，并将DC模式和Planar模式加入预测模式候选列表PML中；当M0与M1不相邻时，分别将{M0,M1}相邻间隔为1和2的模式、{M2}间隔为2的模式和Planar和DC两个非角度模式加入预测模式候选列表PML中；
步骤3，计算预测模式候选列表PML中的率失真代价，当PU块的块大小为4×4或8×8时，仅保留预测模式候选列表PML中率失真代价最小的两个模式进行率失真优化；当PU块的块大小为16×16、32×32或64×64时，选择预测模式候选列表PML中率失真代价最小的三个模式进行率失真优化；
步骤4，将最可能预测模式候选列表MPM中的模式无重复地添加入预测模式候选列表PML中；
步骤5，对步骤4得到的预测模式候选列表PML中的模式进行最后的率失真优化；将率失真代价最小的模式作为当前PU块的帧内预测的最优模式。
针对上述视频编码中帧内预测模式的快速决策方法，进一步地，步骤1所述主要预测方位集是HEVC帧内预测角度模式系数为{2,6,10,14,18,22,26,30,34}的方向模式集合，具体包括九种方位。
针对上述视频编码中帧内预测模式的快速决策方法，进一步地，在主要预测方位集中，当两个预测方位的模式系数的差的绝对值为4时，所述两个预测方位为相邻。
针对上述视频编码中帧内预测模式的快速决策方法，进一步地，步骤1具体采用哈达码变换方法计算得到所述粗略率失真代价。
针对上述视频编码中帧内预测模式的快速决策方法，进一步地，步骤2中，当M0与M1相邻时，设定M0为较大模式，预测模式候选列表PML更新为{M0,M1,0,1M0-1,M0-2,M0-3,M0+1,M0+2,M1-1,M1-2}；当M0与M1不相邻时，预测模式候选列表PML更新为{M0,M1,0,1,M0-1,M0-2,M0+1,M0+2,M1-1,M1-2,M1+1,M1+2,M2,M2+2,M2-2}。
针对上述视频编码中帧内预测模式的快速决策方法，进一步地，当加入预测模式候选列表PML的模式超出[2,34]范围时，通过循环的方式获取角度模式范围内的系数，再加入到预测模式候选列表PML中。
针对上述视频编码中帧内预测模式的快速决策方法，进一步地，步骤3具体采用哈达码变换方法计算得到所述率失真代价。
针对上述视频编码中帧内预测模式的快速决策方法，进一步地，步骤5所述进行最后的率失真优化包括如下步骤：
51)将预测模式候选列表PML中的每一种预测角度模式作为当前PU的预测模式；
52)计算得到PML列表中每一种预测角度模式的率失真代价；
53)从步骤52)得到的率失真代价中选取率失真代价最小的模式，作为最优的帧内预测方向模式。
与现有技术相比，本发明的有益效果是：
本发明提供一种新的帧内预测快速模式决策的方法，充分利用编码块主要特征信息，降低帧内预测的运算复杂度，在保证编码性能的前提下提高视频编码的速度。本发明具有以下优点：
(一)本发明通过判断预测方位，更加充分地利用了角度模式之间的相关性，减少了进行哈达玛变换的模式数目，进而降低了预测过程的计算复杂度。
(二)本发明构造一个预测模式候选列表，能够对方向模式做出更快速准确的判断，针对PU的尺寸，减少了进行率失真优化的模式个数。
附图说明
图1是HEVC帧内预测的预测点坐标和参考点选取的示意图；
其中，每个方格代表一个像素点；Rx,y表示预测参考像素点；Px,y表示预测像素点；x和y表示像素点的位置，左上角为(0，0)原点。
图2是本发明提供的帧内预测模式快速决策方法的流程框图。
图3是HEVC的方向预测模式和本发明的主要预测方位集的示意图；
其中，黑实线标示本发明的主要预测方位集，具体是系数为{2,6,10,14,18,22，26,30,34}的9个预测模式集合。
图4是本发明实施例中当主要预测方位相邻时的候选预测模式的示意图；
其中，假设M0为较大模式，候选预测模式列表为{M0,M1,0,1M0-1,M0-2,M0-3,M0+1,M0+2,M1-1,M1-2}。
图5是本发明实施例中当主要预测方位不相邻时的候选预测模式的示意图；
其中，当M0与M1不相邻时，分别将{M0,M1}相邻间隔为1或2的模式、{M2}间隔为2的模式和Planar、DC两个非角度模式加入候选预测模式列表中{M0,M1,0,1,M0-1,M0-2,M0+1,M0+2,M1-1,M1-2,M1+1,M1+2,M2,M2+2,M2-2}。
具体实施方式
下面结合附图，通过实施例进一步描述本发明，但不以任何方式限制本发明的范围。
本发明提供一种视频编码中帧内预测模式的快速决策方法，该方法通过判断预测方位，利用角度模式的相关性和预测单元PU的尺寸构造预测模式候选列表，减少进行率失真优化的预测模式数目，从而降低帧内预测的运算复杂度，提高视频编码速度。
以下实施例描述了编码当前帧亮度块的帧内预测实现过程，构造与原亮度块差别最小的预测块。为了方便理解本发明实施例，首先在此介绍本发明实施例描述中引入的几个要素。
编码单元(Coding Unit，CU)：CU可向下划分进行编码，帧内预测允许最大编码单元(Largest Coding Unit，LCU)大小64×64，最小编码单元(Smallest Coding Unit，SCU)大小为8×8。帧内预测时会对当前CU进行模式决策以及块划分，多次递归选择最佳的划分方法以及最优模式。
预测单元(Prediction Unit，PU)：与当前CU的大小相同，当前CU为LCU时，可以继续划分为四个更小预测块。HEVC亮度分量帧内预测支持5种大小PU：4×4、8×8、16×16、32×32和64×64。
预测模式(Prediction Mode)：HEVC帧内预测包含35种预测模式，是通过当前块的左侧相邻列与上侧相邻行的已编码像素值按照一定的规则对当前块做出的不同预测方式。其中包括33种角度预测模式和2种非角度预测模式，非角度预测包括直流(DC)预测和平面(Planar)预测。预测点的坐标以及参考点的选取如图1所示。角度预测模式的系数为2～34，DC模式和Planar模式的系数分别为0和1。
最可能预测模式(MPM)：HEVC标准利用相邻块之间的较强相关性建立的存储相邻上方及左侧PU预测模式的候选列表。
率失真优化(Rate Distortion Optimization，RDO)：HEVC通过率失真优化过程，衡量不同预测模式的码率与失真性能，选择最优的预测模式。
帧内预测是通过模式决策获得不同划分的CU的最优模式以及预测模式，完成预测过程。本发明的流程如图2所示，帧内模式决策的核心方法包括以下几个步骤：
步骤1，对当前任意尺寸的PU块进行主要预测方位集中的模式系数为{2,6,10,14,18,22，26,30,34}的9种方位进行预测，为了提高编码速度，用哈达码变换代替离散余弦变换，计算哈达码变换之后的粗略率失真代价。代价函数如式1所示，其中，J为率失真代价，MODE为HEVC帧内预测所有模式的系数(0-35)，λ为拉格朗日常量，SATD(Sum of Absolute Transform Difference)是经过哈达码变换后该块预测模式与原始块像素值差值的绝对值之和。RMODE表示当前模式残差，BMODE表示该模式需要编码的比特数。
J＝SATD(RMODE)+λ·BMODE    (式1)
根据公式1计算出的率失真代价对主要预测方位集中的模式排序，从中选取率失真代价 最优的前三个模式M0、M1和M2，其中M0、M1和M2的率失真代价关系为J(M0)<J(M1)<J(M2)。HEVC帧内预测全部模式及本发明提出的主要预测方位集如图3所示，主要预测方位集是系数为{2,6,10,14,18,22，26,30,34}的9个预测模式集合，在图3中以黑实线标示。
步骤2，如果M0与M1是相邻的，即其差的绝对值为4，则表示预测最优方向在M0和M1附近，则进一步计算M0与M1之间的三个模式(M0-1,M0-2,M0-3)和M0与M1之外的相邻的四个模式(M0+1,M0+2，M1-1，M1-2)，如图4所示，并将DC模式和Planar模式加入预测模式候选列表PML中。假设M0为较大模式，列表更新为{M0,M1,0,1M0-1,M0-2,M0-3,M0+1,M0+2,M1-1,M1-2}。如果M0与M1不是相邻的，即其差的绝对值大于4，如图5所示，则分别将{M0,M1}相邻间隔为1、2的模式、{M2}间隔为2的模式和Planar、DC两个非角度模式加入候选列表中{M0,M1,0,1,M0-1,M0-2,M0+1,M0+2,M1-1,M1-2,M1+1,M1+2,M2,M2+2,M2-2}。另外，若加入候选列表的模式超出了[2,34]范围，则用循环的方式取角度模式范围内的系数。例如M0＝2，则M0-1取34，M0-2取33。
步骤3，根据公式1计算PML中的率失真代价。对于小于等于8×8的PU块，即块大小为4×4或8×8，则仅保留列表中率失真代价最小的2个模式，否则，即块大小为16×16、32×32或64×64时，则选择代价最小的3个进行率失真优化的选择。
步骤4，使用MPM，将其候选列表中的模式无重复地添加入PML列表。
步骤5，对PML列表中的模式进行最后的率失真优化，代价最小的模式即为当前块的帧内预测的最优模式，具体包括：
51)尝试预测模式候选列表PML中的每一种预测角度模式作为当前PU的预测模式；
此步骤包括进行变换、量化、重构的过程；
52)通过式2计算PML列表中每一种预测角度模式的率失真代价：
J＝D(Mode)+λ·B(Mode)    (式2)
式2中，J为率失真代价，D(Mode)和B(Mode)分别表示采用不同角度模式时的失真和比特数；
53)从步骤52)得到的率失真代价中选取率失真代价最小的模式作为最优的帧内预测方向模式。
至此，该实例的所有步骤完成。
本发明具体实施是在HEVC参考软件HM16.0上实现并测试HEVC通测序列包括Class A、Class B、Class C、Class D和Class E，配置为全I帧，HEVC标准规定了52个量化步长，对应于52个量化参数(Quantization Parameter，QP)。根据通测条件，量化参数QP分别设置为22、27、32、37。测试结果如表1所示，Y表示亮度分量，U和V表示色度分量，其中亮度部分的平均BD-rate轻微增加0.65％，整体节省编码时间(Time Save,TS)为26.76％。
表1
Figure PCTCN2017085605-appb-000001
需要注意的是，公布实施例的目的在于帮助进一步理解本发明，但是本领域的技术人员可以理解：在不脱离本发明及所附权利要求的精神和范围内，各种替换和修改都是可能的。因此，本发明不应局限于实施例所公开的内容，本发明要求保护的范围以权利要求书界定的范围为准。

Claims (9)

  1. 一种视频编码中帧内预测模式的快速决策方法，其特征是，通过判断预测方位，利用角度模式的相关性和预测单元PU的尺寸构造预测模式候选列表，减少进行率失真优化的预测模式数目，从而降低帧内预测的运算复杂度，提高视频编码速度；包括如下步骤：
    步骤1，对当前任意尺寸的PU块，对主要预测方位集中的方位进行预测，计算得到粗略率失真代价；根据率失真代价对主要预测方位集中的模式排序，从中选取率失真代价最优的前三个模式M0、M1和M2，所述M0、M1和M2的率失真代价满足J(M0)<J(M1)<J(M2)；所述M0、M1和M2的方位关系为相邻或不相邻；
    步骤2，当M0与M1相邻时，预测最优方向在M0和M1附近，计算M0与M1之间的三个模式和M0与M1之外的相邻的四个模式，并将DC模式和Planar模式加入预测模式候选列表PML中；当M0与M1不相邻时，分别将{M0,M1}相邻间隔为1和2的模式、{M2}间隔为2的模式、Planar模式和DC模式加入预测模式候选列表PML中；
    步骤3，计算预测模式候选列表PML中的率失真代价，当PU块的块大小为4×4或8×8时，仅保留预测模式候选列表PML中率失真代价最小的两个模式进行率失真优化；当PU块的块大小为16×16、32×32或64×64时，选择预测模式候选列表PML中率失真代价最小的三个模式进行率失真优化；
    步骤4，将最可能预测模式候选列表MPM中的模式无重复地添加入预测模式候选列表PML中；
    步骤5，对步骤4得到的预测模式候选列表PML中的模式进行最后的率失真优化；将率失真代价最小的模式作为当前PU块的帧内预测的最优模式。
  2. 如权利要求1所述视频编码中帧内预测模式的快速决策方法，其特征是，步骤1所述主要预测方位集是HEVC帧内预测角度模式系数为{2,6,10,14,18,22，26,30,34}的方向模式集合，具体包括九种方位。
  3. 如权利要求1所述视频编码中帧内预测模式的快速决策方法，其特征是，步骤1具体采用哈达码变换方法计算得到所述粗略率失真代价。
  4. 如权利要求1所述视频编码中帧内预测模式的快速决策方法，其特征是，在主要预测方位集中，当两个预测方位的模式系数的差的绝对值为4时，所述两个预测方位为相邻。
  5. 如权利要求1所述视频编码中帧内预测模式的快速决策方法，其特征是，步骤2中， 当M0与M1相邻时，设定M0为系数较大模式，预测模式候选列表PML更新为{M0,M1,0,1,M0-1,M0-2,M0-3,M0+1,M0+2,M1-1,M1-2}；当M0与M1不相邻时，预测模式候选列表PML更新为{M0,M1,0,1,M0-1,M0-2,M0+1,M0+2,M1-1,M1-2,M1+1,M1+2,M2,M2+2,M2-2}。
  6. 如权利要求1所述视频编码中帧内预测模式的快速决策方法，其特征是，当加入预测模式候选列表PML的模式超出[2,34]范围时，通过循环的方式获取角度模式范围内的系数，再加入到预测模式候选列表PML中。
  7. 如权利要求1所述视频编码中帧内预测模式的快速决策方法，其特征是，步骤3具体采用哈达码变化方法计算得到率失真代价。
  8. 如权利要求1所述视频编码中帧内预测模式的快速决策方法，其特征是，步骤5所述进行最后的率失真优化包括如下步骤：
    51)将预测模式候选列表PML中的每一种预测角度模式作为当前PU的预测模式；
    52)计算得到PML列表中每一种预测角度模式的率失真代价；
    53)从步骤52)得到的率失真代价中选取率失真代价最小的模式，作为最优的帧内预测方向模式。
  9. 如权利要求1所述视频编码中帧内预测模式的快速决策方法，其特征是，步骤52)具体通过式2计算PML列表中每一种预测角度模式的率失真代价：
    J＝D(Mode)+λ·B(Mode)    (式2)
    式2中，J为率失真代价，D(Mode)和B(Mode)分别表示采用不同角度模式时的失真和比特数。
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