GEM Raster (or GEM VDI Bit Image, GEM Bit Image, GEM Image, GEM Bitmap, IMG) is an image file format originally associated with the GEM operating environment by Digital Research. It was used by GEM Paint and Ventura Publisher, and by a variety of Atari ST software.
The format of bi-level images is well standardized, but the specification is ambiguous as to the format of color images. As a result, there are several incompatible flavors of color GEM raster images, some of which have their own names, such as XIMG (Extended GEM Bit Image), TIMG (TrueColor IMG), STTT, and Hyperpaint.
Files begin with two bytes indicating the (big-endian) file version number. The version number is usually 1 (
0x00 0x01), but some XIMG files have a version number of 2, and version number 3 has also been used.
The next two bytes indicate the size of the header, in 2-byte words. For example, a value of 8 means the header is 16 bytes, measured from the beginning of the file.
The header size is at least 8 words, but extended versions of the format have a larger header.
XIMG files have the ASCII signature "
XIMG" at byte offset 16.
- GEM VDI Reference Guide (v2.0), Appendix G
- GEM/3 Programmer's Toolkit, Release 3.1 Supplement, sec. 9 p. 22
- Format of GEM bitmap
- Understanding color IMGs (This page might be broken -- more info here.)
- http://cd.textfiles.com/crawlycrypt2/crm_demo/images16/ (XIMG)
- http://sylvana.net/1stguide/snaps/ (Version 3 files)