Spectrum 512 Compressed file format
Spectrum 512 (Compressed) *.SPC
1 word flag word [$5350 or "SP"]
1 word reserved for future use [always 0]
1 long length of data bit map
1 long length of color bit map
<= 32092 bytes compressed data bit map
<= 17910 bytes compressed color bit map
--------------
<= 50014 bytes total
Data compression:
Compression is via a modified run length encoding (RLE) scheme,
similar to DEGAS compressed and Tiny. The data map is stored as a
sequence of records. Each record consists of a header byte followed by
one or more data bytes. The meaning of the header byte is as follows:
For a given header byte, x:
0 <= x <= 127 Use the next x + 1 bytes literally (no repetition)
-128 <= x <= -1 Use the next byte -x + 2 times
The data appears in the following order:
1. Picture data, bit plane 0, scan lines 1 - 199
2. Picture data, bit plane 1, scan lines 1 - 199
3. Picture data, bit plane 2, scan lines 1 - 199
4. Picture data, bit plane 3, scan lines 1 - 199
Decompression of data ends when 31840 data bytes have been used.
Color map compression:
Each 16-word palette is compressed separately. There are three
palettes for each scan line (597 total). The color map is stored as a
sequence of records. Each record starts with a 1-word bit vector which
specifies which of the 16 palette entries are included in the data
following the bit vector (1 = included, 0 = not included). If a palette
entry is not included, it is assumed to be zero (black). The least
significant bit of the bit vector refers to palette entry zero, while the
most significant bit refers to palette entry 15. Bit 15 must be zero,
since Spectrum 512 does not use palette entry 15. Bit 0 should also be
zero, since Spectrum 512 always makes the background color black.
The words specifying the values for the palette entries indicated in
the bit vector follow the bit vector itself, in order (0 - 15).
NOTE: Regarding Spectrum pictures, Shamus McBride reports the following:
"... [The Picture Formats List] says bit 15 of the color map vector
must be zero. I've encountered quite a few files where [bit 15] is
set (with no associated palette entry)..."
See also Spectrum 512 file format
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