https://www.temlib.org/AtariForumWiki/index.php?action=history&feed=atom&title=GEM_Bit_Image_file_formatGEM Bit Image file format - Revision history2026-07-26T17:08:39ZRevision history for this page on the wikiMediaWiki 1.39.2https://www.temlib.org/AtariForumWiki/index.php?title=GEM_Bit_Image_file_format&diff=13461&oldid=prev>Lp: New page2017-01-03T00:14:41Z<p>New page</p>
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GEM Bit Image *.IMG<br />
<br />
1 word version number of image file [1]<br />
1 word length of header in words [usually 8]<br />
1 word number of color planes [1 for monochrome]<br />
1 word pattern length in bytes [1-8, usually 2 for screen images]<br />
1 word pixel width in microns (1/1000 mm, 25400 microns per inch)<br />
1 word pixel height in microns<br />
1 word line width in pixels<br />
1 word number of lines<br />
-------<br />
? words header length defined in 2nd word of header<br />
<br />
? bytes data<br />
<br />
NOTES: If the image is a color image (planes > 1), the planes are stored<br />
separately starting with plane 0. There is, however, no standard way of<br />
storing the color palette. Some programs may save the palette in separate<br />
files, some may extend the header. For this reason, you should never<br />
assume the header is 8 words long, always get the header length from the<br />
2nd word of the header. Also, the line width in the 7th word is the number<br />
of pixels in a line. Since the data is encoded in byte-wide packets, the<br />
actual unpacked line width is always a multiple of 8, and may be 1-7 pixels<br />
longer than the length specified in the header.<br />
<br />
For each byte x in the data section,<br />
<br />
x = 0 Pattern/scanline run.<br />
Read the next byte, n (unsigned).<br />
<br />
If n > 0 then:<br />
Read a number of bytes equal to the "pattern<br />
length" word in the header. Repeat this<br />
pattern n times.<br />
<br />
If n = 0 then:<br />
Scanline run. Data for the next scanline<br />
is to be used multiple times. Read the<br />
following record:<br />
<br />
1 byte flag byte [$FF]<br />
1 byte number of times to use<br />
next scanline data<br />
<br />
The data for the next scanline follows,<br />
compressed normally.<br />
<br />
x = 80 (hex) Uncompressed bit string. The next byte<br />
determines the number of bytes to use<br />
literally. The literal data bytes follow.<br />
<br />
otherwise Solid run. The value of x determines<br />
what to draw. The high bit specifies whether<br />
the pixels are set or cleared. A 1 indicates<br />
a byte-run using $FF, a 0 indicates a byte-run<br />
using $00. The low 7 bits, taken as an unsigned<br />
quantity, specify the length of the run in bytes.<br />
</pre><br />
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