Basic Description

PCX files can hold the following Images: 4-color palletized CGA, 16-color palletized EGA, 256-color palletized VGA, and 24-bit truecolor.
Derived Formats: SCR .

Basic File Format

Name				Size	Description
Signature				1 byte	=$0A ($CD for SCR) >$0A: This is a PCX font file ! (not described here)
Version				1 byte	0= Version 2.5 2= Version 2.8 with Palette 3= Version 2.8 without Palette 5= Version 3.0
Compression				1 byte	1 = RLE compressed
BitsPerPixel				1 byte	number of bits per pixel per plane
Xmin				2 bytes	column of upper left corner of image. Most commonly = 0
Ymin				2 bytes	row of upper left corner of image. Most commonly = 0
Xmax				2 bytes	XSize = Xmax - Xmin + 1
Ymax				2 bytes	YSize = Ymax - Ymin + 1
Xdpi				2 bytes	used inconsistently across versions (info needed)
Ydpi				2 bytes	used inconsistently across versions (info needed)
ColorTable				48 bytes	Palette Information (one of the 3 following formats)
	CGA / Monochrome			48 bytes	see CGA Palette paragraph below
		BackgroundColor		1 byte	high-order 4 bits = Color (CGA hardware palette)
		filler		2 byte	unused
		PaletteInfo		1 byte	only 3 bits used
			MonoFlag	bit 7	0 = Color 1 = Monochrome
			Palette	bit 6	0 = Green, Red, Brown (Yellow) 1 = Cyan, Magenta, White
			Intensity	bit 5	0 = dark 1 = light
			reserved	bits 4-0	unused filler 44 bytes unused =0
	16 color			48 bytes	present, if NumColorPlanes = 1, and BitsPerPixel < 8
			Red	1 byte	Red intensity
			Green	1 byte	Green intensity
			Blue	1 byte	Blue intensity
		repeated 16 times
	256 color			48 bytes	present if BitsPerPixel =8
		filler		48 bytes	unused, but for 256 color images, the first 16color map entries of the image may be stored here in addition to the full palette at the end of this file.
reserved				1 byte	=0
NumColorplanes				1 byte	1 = Monochrome / Palletized 3 = RGB (valid ?) 4 = RGBI
BytesPerScanline				2 bytes	size of uncompressed data per scan line. must be even. (can be any number for SCRs)
ColorTableType				2 bytes	1 = Color or B&W 2 = Grayscale \|
filler				58 bytes	makes the header a total of 128 bytes
Raster Data				cannot be pre calculated	The pixel data
Palette Header				1 byte	=12, should be present if: Version = 2 or Version = 5, BitsPerPixel = 8, NumColorplanes = 1, ColorTableType = 1
Palette				768 bytes	present under same conditions as Palette Header

The CGA Palette

Since almost nobody has a CGA Card anymore, here are the two CGA Palettes in RGB. (Guesswork!, I neither have a CGA PCX file, nor a CGA Card anywhere)

CGA Palette 0, dark	Green	0,128,0
	Red	128,0,0
	Brown	128,128,0
CGA Palette 0, light	light Green	0,255,0
	light Red	255,0,0
	Yellow	255,255,0
CGA Palette 1, dark	Cyan	0,128,128
	Magenta	128,0,128
	Gray	128,128,128
CGA Palette 1, light	light Cyan	0,255,255
	light Magenta	255,0,255
	White	255,255,255

Raster Data encoding

The raster data immediately follows the 128byte header. It uses a run length encoding for compression purposes. Data is stored top-down, left-to-right, with a possible filler byte between scan lines. The encoding is dependent on the number of bits/pixel.

Raster Data encoding for 2bit 4-color images (CGA)

unknown. If you find out how this is done, please drop me a note.

Raster Data encoding for 4bit 16color images

unknown. If you find out how this is done, please drop me a note.

Raster Data encoding for 8bit 256color/256 grayscale images

Byte 1 (n)		Byte 2 (c)	Result
>=$C0		any	n consecutive pixels of color c
<$C0		-	a pixel with color n. (c is already part of the next code)

Every scan line with an odd number of bytes will have a zero appended, to match a 16 bit alignment. Single pixels with a color >= $C0 have to be stored as a ($C1, color) -tuple. Only pixels on the same line may be combined to form a 2-byte sequence.

Raster Data encoding for 24/32bit truecolor images

Every row of pixels is written as four 8bit single color component scan lines, each having the same scheme as the 8bit version. These scan lines have the following order: Red, Green, Blue, Alpha. The fifth scan line in the file will be the Red component of pixel row 2.
Whether it is valid to store only three scan lines, skipping the Alpha Channel, is unresolved.

Portability

The CGA Palette is portable via translation table. 2-byte values are stored in Intel order (Least significant byte first)

Trademarks, Patents and Royalties

To my knowledge: none.
(please read the disclaimer)

Cross-Checking Software

This section is for programmers, who wish to cross-check their implementation with others. This is an incomplete list of programs, which are available as freeware / shareware / try-before-buy etc.
The following software is able to decode PCXs:

MS-Paintbrush, MS-Paint (included in MS-Windows)
JASC Paintshop Pro

The following software is able to encode PCXs:

MS-Paintbrush, MS-Paint (included in MS-Windows)
JASC Paintshop Pro

Online Resources

Disclaimer

This is not the official format documentation. Although we're doing our best to keep this information as accurate as possible, there is no way of checking all of it under all possible circumstances. We're not taking any responsibility for the results of these information or lack thereof. The 'trademarks, patents and royalties' section is here just for your convenience, and is in no way complete. Please send us a note, if you find any incorrect of missing information.

PCX File Format