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AtariWikiSysop https://temlib.org/AtariForumWiki2018/index.php?title=GEM_Bit_Image_file_format&diff=4260&oldid=prev 2017-01-03T00:14:41Z

<p>New page</p> <p><b>New page</b></p><div>&lt;pre&gt;<br /> 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 &gt; 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 &gt; 0 then:<br /> Read a number of bytes equal to the &quot;pattern<br /> length&quot; 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 /> &lt;/pre&gt;<br /> Back to [[ST Picture Formats]]</div>

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