| Full Name | GEM Raster |
| Format ID | IG_FORMAT_GEM = 13 |
| File Extension(s) | *.gem |
| Data Type | Raster image |
| Data Encoding | Binary |
| Color Profile Support | No |
| Multi-Page Support | No |
| Alpha Channel Support | No |
N/A
IG_COMPRESSION_RLE - 1, 4, 8 bpp
None
None
A deal with the creators of Ventura Publisher made this a significant format in the desktop publishing arena. It was also distributed by PC systems manufacturers and was the native bitmap format of the Atari ST system.
The structure of a GEM Raster image file begins with a fixed-length header and is followed by the bitmap data.
The data fields of the header include the version number (always 1), image width (in pixels), and image height (in scan lines). A field named "Headerlength" contains a value of either 8 or 9, where 9 indicates that an optional field appears at the end, called "BitImageFlag". This field is directly tied with a "NumberOfPlanes" field. If "NumberOfPlanes" is greater than 1, the BitImageFlag indicates whether the image is color or grayscale. (BitImageFlag = 0 = color, BitImageFlag = 1 = grayscale).
Two fields in the header that are a little unusual among bitmap formats are the width and height of the pixels (in microns), and a field called "PatternLength". PatternLength is the length of any pattern that will be decoded. It is used by the RLE compression scheme, which assigns one of four different kind of codes to store each of the four different types of repeat data in the image. The "pattern code" indicates that a pattern is available. The pattern and repeat count are decoded while reading the file.
For more on RLE compression, see the ImageGear Supported Compressions Reference section.
Murray, James D. and William vanRyper. Encyclopedia of Graphics File Formats. Sebastopol, CA: O'Reilly & Associates, Inc., 1994.