.IMG

Introduction

GEM Image is the bitmap format of GEM, the graphical desktop from Digital Research that ran on MS-DOS PCs and was built into the Atari ST. It was the picture format of GEM Paint and of Ventura Publisher, and many Atari ST programs read and write it.

The original format holds one or more bitplanes without a palette, so colour files depend on the screen they were made on. A later extension, XIMG, adds a palette to the header. The .img extension is shared with disk images and other formats, so a reader has to check the header.

Structure

The header is a list of big-endian 16-bit words: the version, the header length in words (usually 8), the number of bitplanes, the pattern length in bytes, the pixel width and height in microns, the image width in pixels and the number of lines. A longer header can follow with more data; in an XIMG file it starts with the letters XIMG and holds the palette.

The pixels come next, line by line and plane by plane, compressed with a few simple codes. A byte 00 with a count starts a pattern run, which repeats the next pattern-length bytes. The sequence 00 00 FF with a count repeats the whole next line that many times. A byte 80 with a count is followed by that many literal bytes. Any other byte is a solid run: its top bit gives the colour, all set or all clear, and the low seven bits the number of bytes.

Also used for

• Raw disk image .img

Links