.PI1 File Format

Introduction

DEGAS was a paint program for the Atari ST, written by Tom Hudson and sold by Batteries Included. Its picture files, and those of its successor DEGAS Elite, became the usual way to swap images on the ST, and many other programs read and write them.

A DEGAS file is a copy of the ST’s screen memory with the palette in front. The digit in the extension is the screen mode: .pi1 for low resolution (320 × 200, 16 colours), .pi2 for medium (640 × 200, 4 colours) and .pi3 for high (640 × 400, black and white). DEGAS Elite added compressed files with the extensions .pc1, .pc2 and .pc3.

Structure

An uncompressed file starts with a big-endian word giving the resolution (0 low, 1 medium, 2 high), then 16 palette words in the ST’s colour format, then 32000 bytes of screen memory: 32034 bytes in all. The screen is stored as the ST shows it, with the bitplanes interleaved word by word. DEGAS Elite adds 32 bytes of colour cycling settings at the end.

In a compressed file the top bit of the resolution word is set. The screen is then packed line by line, each bitplane of a line separately, with the same run-length scheme as PackBits.

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