Ian Ashdown is a consulting software engineer specializing in C/C++ programming and embedded systems design. He has a B.App.Sc. in electrical engineering from the University of British Columbia He may be contacted at byHeart Software, 620 Ballantree Road, West Vancouver, B.C., Canada V7S 1W3. (604) 922-6148; Fax (604) 987-7621.
Looking to add monochrome or full-color bit-mapped graphics to your application programs? If so, you might consider the PCX graphics file format. Originally developed in 1982 by ZSoft Corporation for their PC Paintbrush" products, it has become a de facto industry standard for storing and transferring bit-mapped images on MS-DOS machines. It can support displays of any resolution using palettes of up to 256 simultaneous colors, and is very simple to implement. Furthermore, it's not limited to MS-DOS and OS/2-based machines; the PCX format is applicable to any environment supporting bit-mapped graphics.
Today, more commercial programs support ZSoft's PCX format than any other, including Aldus-Microsoft's Tag Image File Format (TIFF). However, unlike TIFF with its publicly-available technical specifications, the PCX file format has never been completely documented. When ZSoft first created PC Paintbrush, the only video displays they had to contend with were two monochrome adapters (Hercules and Tecmar) and the IBM Color Graphics Adapter (CGA). They have since quietly modified and extended their format on several occasions to support EGA, VGA and SuperVGA displays. Documentation is scarce, incomplete and sometimes contradictory. There are a few magazine articles, chapters in a few books, a small booklet available from ZSoft that describes the current version (with several omissions), and a sample Pascal program from their CompuServe forum (GO WINAPB) (see the references at the end of this article).
Personally, I think it's about time to remedy this situation. The following is a complete set of technical specifications for the current version of PCX. All of the information has been derived from printed information provided by ZSoft and conversations with their Technical Services department. This is it, folks! We now have formal (if not exactly official) specifications to work with when including the PCX graphics file format in our application programs.
If you don't want to develop your own PCX file handling routines from scratch, PCX_LIB is available through the CUG Library as CUG350. It includes fully commented C source code for reading and writing PCX image files, complete with software drivers for Hercules, CGA, EGA, and VGA display adapters.
PCX Specifications
The PCX graphics file format (Version 5) is designed to store monochrome and color bit-mapped images of any resolution with palettes of up to 256 simultaneous colors. It was originally designed for MS-DOS microcomputers, but is adaptable to other bit-mapped graphic environments. A simple but effective byte-oriented, run-length encoding scheme is used to compress the image data.
There are two or three sections to a PCX graphics file — a 128-byte header, the encoded image data (which can be of any length) and an optional 256-color palette (see Table 1) . This palette is appended to the file only if the image contains more than 16 colors.
PCX File Header
The file header describes the graphical environment in which the image is to be displayed. The information contained in the header is somewhat device-dependent in that the PCX file format implicitly assumes the presence of a standard IBM PC-compatible display adapter. Furthermore, the specific type and video mode of the adapter needed to display the image correctly cannot be uniquely determined from the file header information. It is in general the user's responsibility to ensure that the correct video mode has been selected before displaying a PCX image.
The file header structure is shown in Table 2. A complete description of each structure member is as follows:
PCX Flag
A constant value (0x0a) that signifies a PCX image file.
Version
Indicates the PCX file format version. It can be one of five values:
0 — Version 2.5 of PC Paintbrush.
2 — Version 2.8 (with palette information).
3 — Version 2.8 (without palette information).
4 — PC Paintbrush for Windows (not 3.0).
5 — Version 3.0 and greater of PC Paintbrush and Paintbrush Plus, including Publisher's Paintbrush.
Most commercial programs supporting the PCX file format conform to Version 5. See the section entitled Color Palettes for further information.
Encoding
A constant value (0x01) that indicates run-length encoding was used to encode and compress the image data.1
Bits per Pixel
The number of bits per pixel per color plane (typically 1, 2, 4 or 8).
Window
A structure with the members in Table 3. These members describe the position and size of the image within the display, and are measured in pixels (starting with zero).
HDPI
"Horizontal dots per inch." The value represents the horizontal resolution of the device used to create the image.2
VDPI (2)
"Vertical dots per inch." The value represents the vertical resolution of the device used to create the image.2
Color Map
The color palette to be used when displaying an image with 16 or fewer colors. See the section entitled Color Palettes for further information.
Reserved
This member was used to indicate the appropriate MS-DOS video mode in previous PCX file format versions. It is ignored in Version 5, but should be set to zero.
NPlanes
The number of color planes used to display the image (typically 1 or 4). See the section entitled Color Palettes for further information.
Bytes per Line
The number of bytes required for a buffer when decoding one color plane scan line. This value should be an even number (for compatibility with some existing commercial programs). See the section entitled Image Decoding and Encoding for further information.
Palette Info
A bit-mapped variable indicating how to interpret the color palette. Only the lowest two bits are significant; the others are ignored. It can have one of two possible values:
0x01 — color or black & white
0x02 — grayscale
If the variable is set to 0x02 (grayscale), the color palette must be set to shades of gray.3
HScreen Size
Horizontal screen size in pixels.4
VScreen Size
Vertical screen size in pixels.4
Filler
Blank space to fill out 128-byte header. All bytes within this member should be set to zero.
Notes
1. ZSoft has reserved the right to change the encoding scheme for better image compression performance in future versions.
2. Horizontal and vertical resolution for video display adapters is defined as the total number of displayed pixels for the current video mode. For scanners, it is defined in terms of dots per inch.(These values are provided for information only. They are not required for encoding or decoding PCX image files.)
3. The "palette info" member of the file header was used in previous versions of the PCX file format to indicate whether the palette represented a color or grayscale palette. If it was set to 0x02 (as a bitmap — the upper six bits could be set at random), the file decoding functions could assume a default grayscale palette if necessary. However, the palette already had a true (and possibly nonlinear) grayscale, so the "palette info" member was never really used by ZSoft. The current PC Paintbrush IV and IV Plus products simply ignore it.
4. The "HScreen Size" and "VScreen Size" members were added to Version 5 of the PCX format to support PC Paintbrush IV Version 1.02 and IV Plus Version 1.0. Since earlier Version 5 PCX files may contain uninitialized data in place of these members, ZSoft specifically recommends that they not be used to determine the appropriate video mode for displaying the files.
Image Decoding And Encoding
The PCX graphics file format considers an image to be a contiguous sequence (block) of eight-bit bytes representing a bit-mapped raster display. A simple byte-oriented, run-length encoding (RLE) scheme is used to compress the display data. When the display is represented by more than one color plane (such as color images on EGA displays), each scan line is stored sequentially by color plane.
The run-length encoding scheme uses a byte pair consisting of a "count" byte and a following "data" byte to represent sequences of display bytes with the same value. A count byte is uniquely identified by having its two most significant bits set; its six least significant bits are used to represent the count value (one to 63). The following data byte value is repeated in the display data the number of times indicated by the count value.
Any display data byte which is not part of a sequence of bytes or the same value and which does not have its two most significant bits set is stored as itself in the encoded image data. Single display data bytes with a value of 0xc0 or greater are encoded with a count value of 1.
Decoding
Decoding display data from encoded image data is done on a line-by-line basis. The pixel dimensions of the displayed image are calculated as
horz_size = Window.xlr - Window.xul + 1and
vert_size = Window.ylr - Window.yul + 1The number of bytes reqired to buffer one complete scan line for all color planes in sequence is
buffer_size = NPlanes * Bytes per LineSince there are always an integral number of bytes in the buffer, there may be unused data at the end of each color plane scan line if the number of bits per pixel is other than eight. This unused data should be masked off when transferring the line buffer contents to the video display adapter memory.
In theory, each color plane scan line may contain an even or odd number of bytes. However, some application programs expect an even number of bytes. ZSoft ensures that their products create PCX files with an even number of bytes per color plane scan line, and recommends that other programs do the same for compatibility. Of course, decoding functions should be able to read files with either an even or odd number of bytes per color plane scan line.
Decoding begins with the first scan line and proceeds by examining each byte of the encoded image data. If the two most significant bits of the byte are set, the lower six bits indicate how many times the next byte is to be duplicated in the line buffer. If these two bits are not set, the byte itself is copied (once) to the line buffer. A count is kept of the number of bytes in the line buffer. The current scan line is complete when its value equals buffer_size.
If the display contains more than one color plane, each plane is decoded in sequence. The order in which they are decoded is device-dependent. For instance, the Enhanced Graphics Adapter has four color planes ordered as blue, green, red and intensity. The beginning of each color plane scan line within the line buffer is given by
offset = plane_number * Bytes per Linewhere plane_number is a number between 0 and NPlanes 1.
A decoding break occurs at the end of each scan line. That is, an encoding byte pair can only represent a contiguous sequence of bytes within the current scan line. However, this is not necessarily true for color planes. An encoding byte pair may represent a contiguous sequence of identical bytes that extends across two color planes for one display image scan line.
Decoding continues until all scan lines (as indicated by vert_size) have been decoded. Some older versions of PC Paintbrush padded the image with extra (uninitialized) scan lines so that all blocks of scan lines (eight or 16 lines) read from the file were the same size. The image data was read until end-of-file was returned. ZSoft no longer uses this technique, since it conflicts with the appended color palette (see the section entitled VGA 256-Color Palettes). The extra data read could also overrun the user's image buffer.
A sample C function to decode a complete image scan line (all color planes) from a PCX file is shown in Listing 1.
Encoding
Encoding display image data is also done on a line-by-line basis, following the order of scan lines stored in the display adapter's memory buffer. The current scan line is encoded for each color plane on a per-byte basis. As noted above, ZSoft recommends that all color plane scan lines be padded if necessary to ensure they contain an even number of bytes.
ZSoft also recommends that the data used to pad the last one or two bytes of a scan line represent white data. Apparently, some application programs display this data when printing or faxing the files.
A sample C function to encode a single monochrome or color image scan line for a PCX file is shown in Listing 2.
Color Palettes
The PCX file format supports color palettes of up to 16 colors in the file header. Larger palettes (up to a maximum of 256 colors) are stored in an optional color palette that is appended to the encoded image data portion of the file.
The file header color palette has two different formats, both designed for IBM PC-compatible machines. A device-specific palette is used for Color Graphics Adapters (CGA), and a standard R-G-B palette is used for Enhanced Graphics Adapters (EGA), Multicolor Graphics Adapters (MCGA), Video Graphics Adapters (VGA) and extended Video Graphics Adapters (SuperVGA).
ZSoft's PC Paintbrush products no longer support the CGA color palette. The following information is provided only for compatibility with older PCX files.
CGA Color Palettes
The PCX format supports eight possible CGA color palettes for video modes 4 (320x200 4-color graphics) and 5 (320x200 monochrome graphics, color burst off). Each palette consists of a background color and three foreground colors (or shades of grey). The background can be one of 16 colors, the value for which is stored in the first byte of the PCX file header Color Map member. Only the upper four bits are significant; the value must be right-shifted by four bits (or divided by 16) to determine the appropriate CGA hardware palette register value.
The foreground color palette is specified by the fourth byte of the Color Map, which has the structure shown in Table 4. The lower five bits are ignored.
Most published descriptions of the ROM BIOS call Set CGA Palette (Interrupt 16, Function 11) document only two palettes, obtainable by setting register BL to 0x00 or 0x01. This is equivalent to the Palette bit above. However, the palette intensity (equivalent to the Intensity bit above) can be selected using bit 4 of the BL register (0 = dim, 1 = bright).
The original CGA display adapter was designed for use with NTSC composite video monitors and color televisions. The color burst is a periodic burst of a 3.58 MHz signal superimposed on the composite video signal to synchronize the phase of the monitor's internal 3.58 MHz oscillator. (Without synchronization, the picture has drifting color bars.) The presence or absence of the burst determines whether the image is displayed in color or monochrome.
The Color Burst Enable bit in Table 4 (page 91) actually indicates whether the MS-DOS video mode is to be 4 (color) or 5 (monochrome). However, the color burst signal has no meaning for RGB monitors. Video mode 5 will produce only two distinct color palettes on CGA displays adapters with RGB monitors, and three distinct palettes on EGA, VGA and SuperVGA display adapters emulating a CGA display.
Under video mode 6 (640 x 200 2-color graphics), the first byte of the CGA color palette specifies the foreground color (i.e., the color of the displayed pixels).
EGA/VGA 16-Color Palettes
The 16-color palette for EGA, MCGA, VGA and SuperVGA displays is an array of 16 elements, each a structure with the members shown in Table 5.
All color map entries are stored as unsigned bytes with values ranging between 0 and 255. Where display adapters support fewer intensity levels, the value of each color map entry is interpreted by dividing its value by 256/n, where n is the number of allowable intensity levels (typically 2, 4 or 16).
VGA 256-Color Palettes
The 256-color palette for MCGA, VGA and SuperVGA displays is an array of 256 elements, each a structure with the same members as the EGA/VGA 16-color palette, which is appended to the encoded image data portion of the file (see Table 1) . It is always preceded by a constant byte flag with the value 0x0c (12 decimal).
Only Version 5 PCX-format files support 256-color palettes, and then only when the image has more than 16 colors. ZSoft recommends the following technique to determine if a 256-color palette is present: first verify that the file version number is 5, then count back 769 bytes from the end of the file. If the value of this byte is not 0x0c, the optional 256- color palette is not present and the EGA/VGA 16-color file header palette should be used.
It is possible that a Version 5 PCX-format file with a valid file header palette can have the value 0x0c in the 769th byte from the end of the encoded image data. The above technique would then falsely indicate the presence of an appended 256-color palette. To avoid this problem, it is necessary to first decode the image and note the file position where the encoded image data section ends before counting back 769 bytes from the end of the file. If the supposed 256-color palette flag is located in the image data section, then the file header palette should be used instead.
24-Bit Color
Future versions of ZSoft's Publisher's Paintbrush will support up to 16.7 million simultaneous colors. The PCX file format will be based on three color planes (red, green and blue), with eight bits per pixel per plane. There will be no color palette, since the color of each pixel will be fully specified by the encoded image data.
Disabling The Palette
It is occasionally necessary to disable the color palette of a PCX file in order to correctly display the image. In other words, the current (default) setting of the display adapter's palette registers are used. This can be done by changing the PCX file version number from 5 to 3 (i.e., PC Paintbrush Version 2.8 without palette information). The file decoding functions must then check the file version number and ignore the color palette if it is set to 3.
Other Environments
While the PCX file format was designed for MS-DOS machines, it is nevertheless possible to display PCX images on other machines. It will generally be necessary to map the image representation (i.e., window dimensions, number of color planes, bits per pixel per plane, and the color palette) to the capabilities of the display hardware.
It is also necessary to remember that all information in a PCX-format file is stored as either eight-bit bytes or 16-bit words. Words are stored in the big-endian format characteristic of 80x86-based machines. That is, the eight least significant bits (lower byte) are stored first, followed by the eight most significant bits (upper byte). If PCX-format files are transferred to little-endian machines (such as those based on 680x0 and Z8000 processors), the order of bytes within each word will have to be reversed before they can be interpreted. (This applies to the file header only, since the encoded image data and optional 256-color palette are stored as bytes.)
References
Azer, S. [1988]. "Working With PCX Files", Microcornucopia, No. 42 (July-August), p. 42.
Lindley, C.A. [1990]. Practical Image Processing in C, John Wiley & Sons Inc., New York, N.Y.
Luze, M. [1991]. "Printing PCX Files," C Gazette, Vol. 5:2 (Winter 1990 - 1991), pp. 11-22.
Phoenix Technologies Ltd. [1989]. System BIOS for IBM PC/XT/AT Computers and Compatibles, Addison-Wesley, Reading, MA.
Quirk, K. [1989]. "Translating PCX Files", Dr. Dobb's Journal, Vol. 14:8 (August), pp. 30-36, 105-108.
Rimmer, S. [1990]. Bit-Mapped Graphics, Windcrest Books, Blue Ridge Summit, PA.
ZSoft Corporation [1988]. PCX Technical Reference Manual Revision 4, ZSoft Corporation, Marietta, GA.
