Image files used by PC Paintbrush product family and FRIEZE (those with a
.PCX extension) begin with a 128 byte header. Usually you can ignore this
header, since your images will probably all have the same resolution. If
you want to process different resolutions or colors, you will need to
interpret the header correctly. The remainder of the image file consists
of encoded graphic data. The encoding method is a simple byte oriented
run-length technique. We reserve the right to change this method to
improve space efficiency. When more than one color plane is stored in
the file, each line of the image is stored by color plane (generally ordered
red, green, blue, intensity), As shown below.
Scan line 0: RRR... (Plane 0)
GGG... (Plane 1)
BBB... (Plane 2)
III... (Plane 3)
Scan line 1: RRR...
GGG...
BBB...
III... (etc.)
The encoding method is:
FOR each byte, X, read from the file
IF the top two bits of X are 1's then
count = 6 lowest bits of X
data = next byte following X
ELSE
count = 1
data = X
Since the overhead this technique requires is, on average, 25% of
the non-repeating data and is at least offset whenever bytes are repeated,
the file storage savings are usually considerable.
ZSoft .PCX FILE HEADER FORMAT
| Byte | Item | Size | Description/Comments |
| 0 | Manufacturer | 1 | Constant Flag, 10 = ZSoft .pcx |
| 1 | Version | 1 | Version information 0 = Version 2.5 of PC Paintbrush 2 = Version 2.8 w/palette information 3 = Version 2.8 w/o palette information 4 = PC Paintbrush for Windows(Plus for Windows uses Ver 5) 5 = Version 3.0 and > of PC Paintbrush and PC Paintbrush +, includes Publisher's Paintbrush . Includes 24-bit .PCX files |
| 2 | Encoding | 1 | 1 = .PCX run length encoding |
| 3 | BitsPerPixel | 1 | Number of bits to represent a pixel (per Plane) : 1, 2, 4, or 8 |
| 4 | Window | 8 | Image Dimensions: Xmin,Ymin,Xmax,Ymax |
| 12 | HDpi | 2 | Horizontal Resolution of image in DPI* |
| 14 | VDpi | 2 | Vertical Resolution of image in DPI* |
| 16 | Colormap | 48 | Color palette setting, see text |
| 64 | Reserved | 1 | Should be set to 0. |
| 65 | NPlanes | 1 | Number of color planes |
| 66 | BytesPerLine | 2 | Number of bytes to allocate for a scanline plane. MUST be an EVEN number. Do NOT calculate from Xmax-Xmin. |
| 68 | PaletteInfo | 2 | How to interpret palette- 1 = Color/BW, 2 = Grayscale (ignored in PB IV/ IV +) |
| 70 | HscreenSize | 2 | Horizontal screen size in pixels. New field found only in PB IV/IV Plus |
| 72 | VscreenSize | 2 | Vertical screen size in pixels. New field found only in PB IV/IV Plus |
| 74 | Filler | 54 | Blank to fill out 128 byte header. Set all bytes to 0 |
NOTES:
- All sizes are measured in BYTES.
- All variables of SIZE 2 are integers.
- *) HDpi and VDpi represent the Horizontal and Vertical resolutions which the image was created (either printer or scanner); i.e. an image which was scanned might have 300 and 300 in each of these fields.
Decoding .PCX Files
First, find the pixel dimensions of the image by calculating
[XSIZE = Xmax - Xmin + 1] and [YSIZE = Ymax - Ymin + 1]. Then calculate how many bytes are required to hold one complete uncompressed scan line: TotalBytes = NPlanes * BytesPerLineNote that since there are always an even number of bytes per scan line, there will probably be unused data at the end of each scan line. TotalBytes shows how much storage must be available to decode each scan line, including any blank area on the right side of the image. You can now begin decoding the first scan line - read the first byte of data from the file. If the top two bits are set, the remaining six bits in the byte show how many times to duplicate the next byte in the file. If the top two bits are not set, the first byte is the data itself, with a count of one.
Continue decoding the rest of the line. Keep a running subtotal of how many bytes are moved and duplicated into the output buffer. When the subtotal equals TotalBytes, the scan line is complete. There should always be a decoding break at the end of each scan line. But there will not be a decoding break at the end of each plane within each scan line. When the scan line is completed, there may be extra blank data at the end of each plane within the scan line. Use the XSIZE and YSIZE values to find where the valid image data is. If the data is multi-plane, BytesPerLine shows where each plane ends within the scan line.
Continue decoding the remainder of the scan lines (do not just read to end-of-file). There may be additional data after the end of the image (palette, etc.)
Palette Information Description
EGA/VGA 16 Color Palette Information
In standard RGB format (IBM EGA, IBM VGA) the data is stored as 16 triples.
Each triple is a 3 byte quantity of Red, Green, Blue values. The values can
range from 0-255, so some interpretation may be necessary. On an IBM EGA,
for example, there are 4 possible levels of RGB for each color. Since
256/4 = 64, the following is a list of the settings and levels:
Setting Level
0-63 0
64-127 1
128-192 2
193-254 3
VGA 256 Color Palette Information
ZSoft has recently added the capability to store palettes containing more than 16 colors in the .PCX image file. The 256 color palette is formatted and treated the same as the 16 color palette, except that it is substantially longer. The palette (number of colors x 3 bytes in length) is appended to the end of the .PCX file, and is preceded by a 12 decimal. Since the VGA device expects a palette value to be 0-63 instead of 0-255, you need to divide the values read in the palette by 4.
To access a 256 color palette:
- First, check the version number in the header; if it contains a 5 there is
a palette. - Second, read to the end of the file and count back 769 bytes. The value you find should be a 12 decimal, showing the presence of a 256 color palette.
24-Bit .PCX Files
24 bit images are stored as version 5 or above as 8 bit, 3 plane images.
24 bit images do not contain a palette. Bit planes are ordered as lines of red, green, blue in that order.
CGA Color Palette Information
NOTE: This is no longer supported for PC Paintbrush IV/IV Plus.
For a standard IBM CGA board, the palette settings are a bit more complex.
Only the first byte of the triple is used. The first triple has a valid
first byte which represents the background color. To find the background,
take the (unsigned) byte value and divide by 16. This will give a result
between 0-15, hence the background color. The second triple has a valid
first byte, which represents the foreground palette. PC Paintbrush supports
8 possible CGA palettes, so when the foreground setting is encoded between
0 and 255, there are 8 ranges of numbers and the divisor is 32.
CGA Color Map
Header Byte #16: Background color is determined in the upper four bits.
Header Byte #19: Only upper 3 bits are used, lower 5 bits are ignored. The first three bits that are used are ordered C, P, I. These bits are interpreted as follows:
c: color burst enable - 0 = color; 1 = monochrome
p: palette - 0 = yellow; 1 = white
i: intensity - 0 = dim; 1 = bright
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