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Ian Ashdown
byHeart Software
620 Ballantree Road
West Vancouver, B.C.
Canada V7S 1W3
Issue 1: 91/02/12
Issue 2: 91/03/27
Issue 3: 91/08/08
Issue 4: 91/08/11
PCX Graphics
by
Ian Ashdown
byHeart Software
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 (R) 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's a small booklet available from ZSoft that
describes the current version (with several omissions), a sample Pascal
program from their CompuServe forum (GO WINAPB), a few magazine articles,
and chapters in a few books (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.
My original plan was to include sample C source code for reading and
writing PCX image files. However, both this article and the source code
grew to the extent that one had to go. If you don't want to develop your
own PCX file handling routines from scratch, PCX_LIB is available through
the CUG Library as CUG Volume #???. 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.
p. 1
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 Figure 1). This palette is appended to
the file only if the image contains more than 16 colors.
p. 2
1. 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 Figure 2. A complete description
of each structure member is as follows:
1.1 PCX Flag
A constant value (0x0a) that signifies a PCX image file.
1.2 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 Section 3, "Color Palettes", for further information.
1.3 Encoding (1)
A constant value (0x01) that indicates run-length encoding was used to
encode and compress the image data.
1.4 Bits per Pixel
The number of bits per pixel per color plane (typically 1, 2, 4 or 8).
1.5 Window
A structure with the following members:
Name Bytes Description
xul 2 Upper left corner horizontal position
yul 2 Upper left corner vertical position
xlr 2 Lower right corner horizontal position
ylr 2 Lower right corner vertical position
These members describe the position and size of the image within the
display, and are measured in pixels (starting with zero).
p. 3
1.6 HDPI (2)
"Horizontal dots per inch". The value represents the horizontal
resolution of the device used to create the image.
1.7 VDPI (2)
"Vertical dots per inch". The value represents the vertical resolution
of the device used to create the image.
1.8 Color Map
The color palette to be used when displaying an image with 16 or fewer
colors. See Section 3, "Color Palettes", for further information.
1.9 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.
1.10 NPlanes
The number of color planes used to display the image (typically 1 or 4).
See Section 3, "Color Palettes", for further information.
1.11 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 Section 2, "Image Encoding and
Decoding", for further information.
1.12 Palette Info (3)
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.
1.13 HScreen Size (4)
Horizontal screen size in pixels.
1.14 VScreen Size (4)
Vertical screen size in pixels.
1.15 Filler
Blank space to fill out 128-byte header. All bytes within this member
should be set to zero.
p. 4
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 6 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.
p. 5
2. Image Encoding and Decoding
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 (1 to 63). The following data byte is the
value that 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 of 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 one.
2.1. 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 + 1
and
vert_size = Window.ylr - Window.yul + 1
The number of bytes required to buffer one complete scan line for all
color planes in sequence is:
buffer_size = NPlanes * Bytes per Line
Since 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
p. 6
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 Line
where "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 (8
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
Subsection 3.3, "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 Figure 3.
2.2. 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 Figure 4.
p. 7
3. 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.
3.1. 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 following structure:
Name Bit Description
Color Burst Enable 7 0 - color
1 - monochrome
Palette 6 0 - yellow
1 - white
Intensity 5 0 - dim
1 - bright
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.
p. 8
The "Color Burst Enable" bit above 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).
3.2. 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 following members:
Name Bytes Description
Red 1 Red intensity
Green 1 Green intensity
Blue 1 Blue intensity
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).
3.3. 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 Figure 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.
p. 9
3.4. 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 8 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.
3.5. 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'.
p. 10
4. 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 8-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.)
p. 11
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.
p. 12
Figures
+--------------------------------------------+
| File Header (128 bytes) |
+--------------------------------------------+
| Encoded Image Data (variable length) |
+--------------------------------------------+
| Optional Color Palette (769 bytes) |
+--------------------------------------------+
Figure 1 - Basic PCX File Format
Name Bytes Description
PCX Flag 1 Constant flag
Version 1 PCX version number
Encoding 1 Run-length encoding flag
Bits per Pixel 1 Number of bits per pixel per plane
Window 8 Window dimensions
HDPI 2 Horizontal image resolution
VDPI 2 Vertical image resolution
Color Map 48 Hardware R-G-B color palette
Reserved 1 (Used to contain video mode)
NPlanes 1 Number of color planes
Bytes per Line 2 Number of bytes per scan line
Palette Info 2 Palette interpretation
HScreen Size 2 Horizontal screen size
VScreen Size 2 Vertical screen size
Filler 54 Initialized filler bytes
Figure 2 - PCX File Header Structure
/* Read an encoded scan line (all color planes) from a */
/* PCX-format image file and write the decoded data to a scan */
/* line buffer */
void pcx_read_line
(
unsigned char *linep, /* PCX scan line buffer pointer */
FILE *fp, /* PCX image file pointer */
int bpline /* # bytes per line (all color planes) */
)
{
int data; /* Image data byte */
int count; /* Image data byte repeat count */
int offset = 0; /* Scan line buffer offset */
while (offset < bpline) /* Decode current scan line */
{
data = getc(fp); /* Get next byte */
/* If top two bits of byte are set, lower six bits show how */
/* many times to duplicate next byte */
p. 13
if ((data & 0xc0) == 0xc0)
{
count = data & 0x3f; /* Mask off repeat count */
data = getc(fp); /* Get next byte */
memset(linep, data, count); /* Duplicate byte */
linep += count;
offset += count;
}
else
{
*linep++ = (unsigned char) data; /* Copy byte */
offset++;
}
}
}
Figure 3 - Decode PCX Image File Scan Line Function
/* Encode a scan line and write it to a PCX file (the line is */
/* assumed to contain the color plane scan lines in sequence, */
/* with padding for an even number of bytes and trailing white */
/* data for each line as appropriate) */
void pcx_write_line
(
unsigned char *linep, /* Scan line buffer pointer */
int length, /* Scan line buffer length (in bytes) */
FILE *fp /* PCX file pointer */
)
{
int curr_data; /* Current data byte */
int prev_data; /* Previous data byte */
int data_count; /* Data repeat count */
int line_count; /* Scan line byte count */
prev_data = *linep++; /* Initialize the previous data byte */
data_count = 1;
line_count = 1;
while (line_count < length) /* Encode scan line */
{
curr_data = *linep++; /* Get the current data byte */
line_count++; /* Increment line byte count */
if (curr_data == prev_data) /* Repeating data bytes ? */
{
data_count++; /* Increment data repeat count */
if (data_count == 0x3f) /* Max allowable repeat count ? */
{
pcx_encode(prev_data, data_count, fp); /* Encode data */
data_count = 0;
}
}
p. 14
else /* End of repeating data bytes */
{
if (data_count > 0)
pcx_encode(prev_data, data_count, fp); /* Encode data */
prev_data = curr_data; /* Current data byte now prev */
data_count = 1;
}
}
if (data_count > 0) /* Any remaining data ? */
{
pcx_encode(prev_data, data_count, fp); /* Encode data */
}
}
/* Write an encoded byte pair (or single byte) to a file */
void pcx_encode
(
int data, /* Data byte */
int count, /* Data byte repeat count */
FILE *fp /* PCX file pointer */
)
{
if (((data & 0xc0) == 0xc0) || count > 1)
{
putc(0xc0 | count, fp); /* Write count byte */
}
putc(data, fp); /* Write data byte */
}
Figure 4 - Encode Image Scan Line Functions
p. 15
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