GB2186765A - Data display apparatus - Google Patents

Abstract

A data display apparatus which provides bit-map text displays on CRT monitor may have a memory in which is stored a number of different character fonts. For large character sizes, the amount of storage capacity which is required can become excessive when character data is stored in a bit-map form which corresponds to the image and size of the character shape as displayed. In order to reduce the amount of storage required, apart from reducing character size and number it is known to use scaling algorithms to display characters from a single font in different heights and widths. However, this can result in crude character shapes of low resolutions. The present invention provides for the storage of character data in run-length coded bytes, of which certain bits pertain to character colour and the remainder of the bits pertain to run-length. This enables a large number of character fonts to be stored economically on a floppy disc or the like which can be accessed directly to provide the display in 'real-time'. The single Figure exemplifies a data display apparatus to which the invention can be applied. <IMAGE>

Description

SPECIFICATION Data display apparatus This invention relates to digitally operable data display apparatus of a type for displaying as an entity on the screen of a CRT (cathode ray tube) or other display device a quantity of data which is represented by digital codes stored in a display memory, the displayed data being in the form of discrete pixels or dots, each ofwhich has its colour and/or luminance defined by a respective digital code in the display memory at a location corresponding to the position of the pixel in the display, the apparatus including a processor for controlling digitally the storage, selection and display of data.

The display produced by apparatus of the above type is termed a bit-map display and is, for example, a 300 x 250 dqt resolution matrix colour display. In the case of a raster scan display device the digital codes stored in the displayed memory are accessed repe atedly by the processorto update the display in a recurrent cycle of scanning lines which may be produced with our without interlaced field scanning.

Where a data display apparatus of the above type is to produce comprehensive and visually aesthetic text displays, it is usual to provide data for a large number of differentcharacterfonts in a background memory, and the data for individual characters selected for display is read from the background memory and written into the display memory.

It is known to store the data for each character of each character font in a bit-map form which corresponds to the image and size of the character shape.

This character data, when selected, can then be transferred directly to the display memory with a minimum ofprocessor logic. However, where the total numberofcharacter shapes is large and the character shapes are also of different sizes, the amount of background memory needed to store all the character data can be excessive. With a view to reducing the amount of character data that has to be stored at the expense of increasing the complexity ofthe processor logic, it is also known to provide software algorithms which can be used to scale a characterto produce displayed characters of different height and/or different width from the same character data.However, this technique, in addition to increasing the complex- ity of the processor logic, has the shortcoming thatthe resolution of some displayed characters suffers and other characters become crude in shape and no longer visually aesthetic.

It is an object of the present invention to provide for a data display apparatus ofthe above type the character data for a large number of character fonts, but without the above-mentioned disadvantages of excessive background memory or complex processor logic accompanied by possible degrading of display ed character shapes.

According to the present invention, there is provided a method of storing character data in the background memory of a data display apparatus of the type set forth above, which consists in using run-length coding such thatthe data for a character is stored as a sequence of bytes, certain bits of each byte pertaining to the colour ofthe run and the remaining bits performing to the length of the run.

In a particular embodiment of the invention which is contemplated, 8-bit bytes are used for storing the character data, the first two bits in each byte pertain ingtotherun colour and the remaining six bits pertaining to the run length.

This permits character fonts to contain characters for display in up to three foreground colours, the fourth 2-bit combinations specifying background colour.

The invention also extends to a memory device containing character data in run-length coded form, as aforesaid. Such a memory device may be a floppy disk orthe like, or a read-only memory, in which a large number of different character fonts can be provided in a simple and relatively inexpensive way.

Bywayofexample,thesingleFigureofthe accompanying drawing shows a block diag ram of a data display apparatus in which the present invention can be embodied.

Referring to the drawing, the data display apparatus there shown comprises a display device 1, a display generator 2, a processor 3, a background memory 4, a display memory 5 and user interface apparatus 6 and 7. The display device is suitably a colourtelevision monitorwhich is connected to receive R, G, B, video signals from the display generator 2. These R, G, B, video signals are produced in the display generator 2 bythree digital-to-analogue converters 8,9 and 10 respectively.The display generator 2 also includes a colour look-up table 11 which is suitably a read/write memory and is responsive to dot information received from the display memory 5 over a bus 12 to produce digital signalsfordriving the converters 8,9 and 10.A displaytimer 13 inthedisplaygenerator2 provides line and field synchronisation signals LS and FSforthe television monitor 1 over a connection 14. The timer 13 also provides over a connection 15 timing signals for controlling the transfer of dot information from the display memoryStothe colour look-up table 11.

The display memory 5 is a random-access memory which had a capacity for storing dot information for at least one display frame. The dot information comprises digital codes composed of one or more bits per dot to be displayed, depending on the range of colours affordedbythecolourlook-uptable 11.Acombined address/data bus 16 interconnects the display generator 2 and the display memory 5 with the processor 3.

The background memory 4, which is also at least partially a random-access memory, is also connected to the address/data bus 16. The background memory 4 may also have a read-only memory part of which contains permanent program data for controlling the "house-keeping" operations ofthe processor 3. The user interface apparatus comprises a keyboard data entry device 6 and a writing tablet 7. Such interface apparatus is well-known in the art and specific details thereof are unnecessary for an understanding ofthe present invention. The processor3 can be a commer cially available microprocessor, for instance the Signetics S68000 Iup.

Consider now in detail the nature of the invention for storing the character data for different character fonts in the background memory 4. It is proposed that any character which can be displayed may be typed on the keyboard device 6. Assuming that the keyboard device 6 uses the conventiona! ASCII codes, data for all possible 95 ASCII characters may need to be stored in respect of a number of different character fonts.

Characters may be "multi-coloured" or "outlined" which require a similaramountoflogicforimplementation. An "outlined" character is one with a (thin) blackorcontracting colourlinearound its edges. Each pixel of each characterthen requires a minimum of three items of stored data for identifying background, foreground and outline/colour. These items can be stored for each pixel as a 2-bit quantity. This effectively allows selection of an extra item, which increases display choices in that with one 2-bit code combination used to identify background colour, three further 2-bitcodecombinationsareavailableto identify up to three foreground coloursfor shading, 3-D shadowed characters, logos, etc.

Text using charactersfrom a particular character font can be displayed in different sizes simply by providing stored data for multiple fonts. This is preferred using single size characterfonts and then scaling characters as displayed using software: this latter technique requires considerable user 'assist- ante'for implementing the appropriate 'scale' commands and it may only permit relatively crude scaling oftextoncethetext has been displayed.

Character data is therefore stored at the full display resolution. The biggest characters that are required to be displayed could be as large assay, one quarter of the available height and one fifth ofthe available width ofthe display screen ofthetelevision monitor. In practice, this could mean that such a character required 144 (rows) x 144 (colums) pixels for its display.Therefore, the totaling storage capacity required in the background memory 4for storing in-bit mapform onlyasinglefontofcharactersofthissize would be 95x 144x 144x2 ---------- - 492, 480 (489 K.bytes) 8 Where 95 = the number of characters in the font 144 = thenumberofpixelsperrow 144 = thenumberofpixelspercolumn 2 = thenumberofbitsperpixel 8 = the numberofbits per byte This is a considerable storage requirement which could dictate:- (i) storing the differentcharacterfonts on a back-up memory, (ii) having less characters perfont, (iii) having smallercharactersizes, (iv) using a lower character resolution, (v) using fewer colour choices.

However, none ofthese options is particularly desirable.

More specifically: (a) storing the characterfonts on a disk would result in the allowable typing speed for a user on the keyboard device 6 to be less than 0.5 characters per second.

(b) having less characters perfont is a possibility, but even storing the data forA-Z capital letters would only reduce the storage capacity required to around 132 K.bytes.

(c) having smal ler character size is also a possibil ity, butthis would reduce the display facilities.

(d) as regards lower character resolution, assuming for the purposes of illustration that 40 K.bytes are allowedforstoring a single characterfont, then the numberofpixelsavailablefordefiningeach character ofthe font would be: 40 K.bytes = 95x No. of RowsXno. of Colsx2/8 .-.No.ofRowsxNo.ofCols~211.

assuming that characters are going to be approximately square, then No. of Rows and No. of Cols. = square root (211) = 15 pixels.

characters of thins small size and low resolution would not be suitable forscaling.

(e) using fewer colour choices would only reduce the storage capacity required bya factor of 2 and would result in loss of the ability to produce outlined characters.

In accordance with the present invention it is proposed to obviate the disadvantages incurred by the above options by using run-length coding to store the character data. Whilst this form of coding affords little or no saving on storage capacityforsmall character fonts, it affords a considerable saving for large characterfonts.

It is proposed to constrain run-lengthsto be, at most, in respect of 64 pixels, so that the colour of a run can be signified by the two highest-order bits of an 8-bit byte, and the length of a run can be signified by the remaining six bits.

The stored data for each character ofthe character font will consist of two parts and each corresponding partofallthe characters will be stored in a respective one oftwo memory areas. The first memory area will contain the character data forgenerating the actual threeforeground colours for each character, the size ofthe character and a pointerto the run-length information forthat character. All the run-codes will be stored with one byte per run in the second memory area. Ifthe total number of runs per characterfont it restricted to 40,000 then 40 K.bytes will be needed for run-coding, together with a further 384 bytes to store data for character size and pointers.

The data rowfor approximately 100 such character fonts could therefore be stored on a single floppy disk, as the data for each fontwould occupy at most 11 granules ofdiskspace.

The structure in the first memory area forthefirst partofthestored data for each ofthecharacters ch can be as follows:- 1. carrier font header-- identifies a particular carrier font.

2. character ch red (4) character ch green (4) character ch blue (4) - gives the R,G,B, values of 4 possible colours for the character.

3. character ch n rows (96) - number of pixel rows for the character.

4. character ch m cols (96) - number of pixel columns for the character 5. character ch offset (96) -- vertical offset for the character 6. short ch start (96) - start of run-length informa tion Each run-byte is constructed as follows:- Bit 7 6 : 5 4 3 2 1 0 colour : run-length Assuming the largest allowable font size to be (128 x 128) pixels, then for what is probably the worst case character, a capital M, with 3-D shadowing and an outline around the outside of the letter and between the shadowing and the letter itself, the stored data would require 1800 bytes. (This compares favourably with 8192 bytes that would be required using simple cell pattern storage of the character shape.) For simplercharacters in thesamefonta farless number of bytes is required. For instance, a capital I requires only740 bytes.

By using run-length coding in accordance with the invention it becomes possible to store a large number of different character fonts on a floppy disk orthe like, which can then serve as the background memory(4) to be accessed directly by the processor in response to user operation of the keyboard device 6to read character data directly from the disk and write it into the display memory 5. When character data is stored on a disk using conventional cell pattern storage of the character shapes, the read/write operation performed by the processor may be unacceptably long sothatthe disk can then serve only as a back-up memory from which data has to be transferred to a random access background memory before it can be accessed sufficiently quickly to be displayed in real-time operation ofthe processor.

Claims (7)

1. Adata displayapparatusfordisplaying asan entity on the screen of a display device a quantity of data which is represented by digital codes stored in a display memory, the displayed data being in the form of discrete pixels or dots each of which has its colour and/or luminance defined by a respective digital code in the display memory at a location corresponding to the position of the pixel in the display, the apparatus including a background memory from which selected data stored therein can be transferred to the display memory, and a processor for controlling digitally the storage, selection and display of data, characterised in that there is provided a method ofstoring character data in the background memorywhich consists in using run-length coding such that the data for a character is stored as a sequence of bytes, certain bits pertaining ofthe length ofthe run.

2. A data display apparatus as claimed inClaim 1, characterised in that 8-bit bytes are used for storing the character data, the first two bits in each byte pertaining to the run colour and the remaining six bits pertaining to the run-length.

3. A data display apparatus as claimed in Claim 1 or Claim 2, characterised in thatthe character data for a font of characters is stored in two memory areas one of which contains the run-bytesforthe characters and the other contains information as to character size, R, G, B, values forthe coded character colours, and pointers identifying the vertical offset of charcters and the start oftheir run-length information.

4. A data display apparatus as claimed in any preceding Claim, characterised in that the background memory is a floppy disk, or the like.

5. A data display apparatus as claimed in any one of Claims 1 to 4, characterised in that the background memory is a read-only memory.

6. A data display apparatus having a background memory in which character data is stored using a method of run-length coding substantially as hereinbefore described.

7. A data display apparatus substantially as herein before described with reference to the accompanying drawing.