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Palettes.md

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Palettes

Amberstar on the Amiga uses (at least) two palette representations:

  • wide palettes, which include a header and represent each color by 4 bytes
  • compact palettes, which lack a header and represent each color by 2 bytes

In the Amiga version there are several palettes stored:

  • 10 wide palettes in COL_PALL.AMB, for labyrinth data. Color 0 is the color key for transparency.
  • 26 compact palettes in PICS80.AMB, each following a raw pixmap
  • 2 wide palettes stored internally in each tilesets

There may be other palettes stored elsewhere..

Wide Palettes

Wide palettes have a total size of 66 (0x42) bytes. They start with a word which seems to give the number of colors. In Amberstar I've only seen 00 10 (= 16 colors) so far.

After that the colors themselves follow. Each color is represented by 4 bytes. The first byte gives alpha, then red, then green and last blue. But alpha seems to be unused and always has a value of 0. The other color components are given by very low values in the range 0 to 7. You have to mulitply them by 32 to get the right color value. Maybe even add 16 in the end as well.

In the Amiga palette registers, the values are doubled, meaning that e.g. the black value 000 maps to RGB triple #000000, while the brightest possibel value 777 maps to RGB triple #EEEEEE.

Another approach would be (value | (value << 4)) << 2.

Compact palettes

Compact palettes have a size of 32 (0x20) bytes. They lack a header and store alpha, red, green, and blue information in one nibble (half-byte) each.

  • red is byte[0] & 0x07
  • green is byte[1] & 0x70 >> 4
  • blue is byte[1] & 0x07

Some palettes use values greater than 7, though the extra bit does not have any known meaning. To get the correct colour, it needs to be masked out (hence ... & 0x07 instead of ... & 0x0f).

Color component interpretation

First approach color components:

Stored value Result
00 10 (16)
01 30 (48)
02 50 (80)
03 70 (112)
04 90 (144)
05 B0 (176)
06 D0 (208)
07 F0 (240)

Second approach color components:

Stored value Result
00 00 (0)
01 22 (34)
02 44 (68)
03 66 (102)
04 88 (136)
05 AA (170)
06 CC (204)
07 EE (238)