GEM Bit Image file format
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GEM Bit Image *.IMG
1 word version number of image file [1]
1 word length of header in words [usually 8]
1 word number of color planes [1 for monochrome]
1 word pattern length in bytes [1-8, usually 2 for screen images]
1 word pixel width in microns (1/1000 mm, 25400 microns per inch)
1 word pixel height in microns
1 word line width in pixels
1 word number of lines
-------
? words header length defined in 2nd word of header
? bytes data
NOTES: If the image is a color image (planes > 1), the planes are stored
separately starting with plane 0. There is, however, no standard way of
storing the color palette. Some programs may save the palette in separate
files, some may extend the header. For this reason, you should never
assume the header is 8 words long, always get the header length from the
2nd word of the header. Also, the line width in the 7th word is the number
of pixels in a line. Since the data is encoded in byte-wide packets, the
actual unpacked line width is always a multiple of 8, and may be 1-7 pixels
longer than the length specified in the header.
For each byte x in the data section,
x = 0 Pattern/scanline run.
Read the next byte, n (unsigned).
If n > 0 then:
Read a number of bytes equal to the "pattern
length" word in the header. Repeat this
pattern n times.
If n = 0 then:
Scanline run. Data for the next scanline
is to be used multiple times. Read the
following record:
1 byte flag byte [$FF]
1 byte number of times to use
next scanline data
The data for the next scanline follows,
compressed normally.
x = 80 (hex) Uncompressed bit string. The next byte
determines the number of bytes to use
literally. The literal data bytes follow.
otherwise Solid run. The value of x determines
what to draw. The high bit specifies whether
the pixels are set or cleared. A 1 indicates
a byte-run using $FF, a 0 indicates a byte-run
using $00. The low 7 bits, taken as an unsigned
quantity, specify the length of the run in bytes.
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