A PCX file contains a single image. It supports 1 bit, 2 bit, 4 bit and 8 bit indexed images as well as 24-bit rgb images. There are two possible palettes for indexed images in a pcx file: the 16 color palette in the header, which is used for the indexed images with <= 4 bits. For the 8 bit indexed images, there can be a separate 256 color palette of which presence can be detected from the header information. If there is a 256 color palette it is located at the end of the file.
A scanline contains the information for a one row of the image. A color plane represents a single component of an image. Indexed images have only one color plane, but the 24-bit rgb images have three: red, green and blue. Multiple color planes are stored successively inside the scanline (first the whole row of red, then green, then blue). Each color plane scanline must take an even number of bytes and so there might be a junk byte at the end of each.
Each scanline is stored run-length-encoded (RLE) in the following manner:
There is an encoding break after each scanline, but there are no encoding breaks between color planes in a scanline.
The header has a fixed size of 128 bytes and looks like this:
| Offset | Type | Description |
|---|---|---|
| 0000h | byte | Manufacturer. |
| 10 = ZSoft | ||
| 0001h | byte | Version information |
| 0 = PC Paintbrush v2.5 | ||
| 2 = PC Paintbrush v2.8 with palette information | ||
| 3 = PC Paintbrush v2.8 without palette information | ||
| 4 = PC Paintbrush/Windows | ||
| 5 = PC Paintbrush v3.0+ | ||
| 0002h | byte | Encoding scheme |
| 1 = RLE | ||
| 0003h | byte | Bits per pixel |
| 0004h | word | Left margin of image |
| 0006h | word | Upper margin of image |
| 0008h | word | Right margin of image |
| 000Ah | word | Lower margin of image |
| 000Ch | word | Horizontal DPI resolution |
| 000Eh | word | Vertical DPI resolution |
| 0010h | 48 bytes | Color palette for indexed images with <= 4 bits per pixel |
| 16 RGB triplets | ||
| 0040h | byte | Reserved |
| 0041h | byte | Number of color planes |
| =”NCP” | ||
| 0042h | word | Number of bytes per scanline (always even, |
| use instead of right margin-left margin). | ||
| =”NBS” | ||
| 0044h | word | Palette information |
| 1 = color/bw palette | ||
| 2 = grayscale image | ||
| 0046h | word | Horizontal screen size |
| 0048h | word | Vertical screen size |
| 004Ah | 54 bytes | Reserved; set to 0 |
| … | … | … |
| 0080h | Image data |
The space needed to decode a single scan line is NCP * NBS bytes, the last byte may be a junk byte which is not displayed.
After the image data, if the version number is 5 (or greater?) there possibly is a VGA color palette. The color ranges from 0 to 255; 0 is zero intensity, 255 is full intensity. The palette has the following format:
| Offset | Count | Type | Description |
|---|---|---|---|
| 0000h | 1 | byte | VGA palette integer (0Ch) |
| 0001h | 768 | byte | RGB triplets with palette information |