.PCX

Introduction

PCX is the format of PC Paintbrush, a paint program from ZSoft first released in 1985. It became one of the most common image formats on MS-DOS, used by scanners, fax software and many games, and grew through several versions from monochrome and 16-colour EGA images to 256-colour VGA and 24-bit colour.

Its compression is a simple run-length encoding, so it suits flat-coloured artwork far better than photographs.

Structure

A PCX file starts with a 128-byte header. Its first byte is always 0A, followed by the version, the encoding (1 for run-length encoding) and the bits per pixel in each plane. The image bounds come next as four little-endian numbers, so the width is xmax − xmin + 1. A 16-colour palette sits inside the header, and later fields give the number of colour planes and the bytes per line of each plane.

The image data follows as run-length encoded lines. A byte with its top two bits set (C0 or above) is a count in its low six bits, and the next byte is repeated that many times; any other byte is a single pixel value. A 256-colour image ends with the byte 0C and a 768-byte palette of 256 RGB colours.

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