GEM Raster
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), STTT, and Hyperpaint.
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Identification
Files begin with bytes 0x00 0x01, except that XIMG files can also begin with 0x00 0x02.
The next two bytes indicate the size of the header, in 2-byte words (i.e. a value of 8 means the header is 16 bytes, measured from the beginning of the file).
The header size is at least 8 (0x00 0x08), but extended versions of the format have a larger header.
XIMG files have an ASCII signature of "XIMG" at offset 16, and usually have a header size of 59 words (0x00 0x3B).
Specifications
- http://www.fileformat.info/format/gemraster/spec/index.htm
- 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
Software
- Netpbm: pbmtogem, gemtopnm
Sample files
- http://www.fileformat.info/format/gemraster/sample/index.htm
- http://cd.textfiles.com/clipart1996/BUSI/OTHER/
- http://cd.textfiles.com/clipart1996/TYPE/OTHER/
- http://cd.textfiles.com/fantaziasampler/CLIPART/IMG/
- http://cd.textfiles.com/crawlycrypt2/crm_demo/images16/ (XIMG)
