Sorry to bump this, but due to the recent cracking of I.M.Meen's IMG format (Chill Manor being investigated since in is V2 IMG format) I have a renewed interest in cracking CMP to "finish it off". First off, I looked deeper into the format and it, like IMGs, contains a masking check for images with transparency, like cursors and inventory icons. It ALSO uses the external palette differently as was established here. I am almost positive that CMPs actually make a kind of... color map. Actually pick out colors in the palette instead of using them in a raw format unlike textures and other sprites. Why is beyond me, but tests point in this direction as well. The palettes at the end of each START_SCR.PCX are the only palette used and is the same for all PCX files in this game. Made sure on the face PCX files as well for player selection. Yes, the game uses a palette from a PCX as opposed to one shared palette. This supports my hypothesis.
So, part of the unknown data would be the masking, the other would be the colors picked out from the palette. The only other possibility in my mind would be an RLE palette per image which would be... kinda dumb if there's only one game palette and all colors are just in there.
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