CVPC — canvases

Implemented in colwin/formats/cvpc.py and colwin/formats/lzw.py. 96 resources across eight modules, from 10×10 buttons to a 1920×480 panorama.

Unlike a sprite, a canvas carries its own complete palette, so what you see when you open one is exactly what the game shows, and editing the PNG’s palette edits the game’s.

Layout

Big-endian dimensions in an x86 title are unusual, and they are what made the header legible in the first place.

The three bytes that cost a decode

A first reading took the header as … bpp, mask, ff ff ff, palette at 0x09, treating ff ff ff as padding. That decoded 14 pixels out of 307,200. The call site settled it: 1068:4c05 computes the decompressor’s source pointer as base + (mask + 1) * 3 + 6 and passes the palette itself as base + 6, so the palette starts at offset 6 and ff ff ff is entry 0, white. With that three-byte correction the decode went from 0/96 to 96/96.

The codestream is GIF’s LZW

Codes packed LSB-first, initial width bpp + 1, clear code 1 << bpp, end code + 1, first free entry + 2, width growing to 12, framed in length-prefixed sub-blocks. There is no zero-length terminator; the end code stops the stream.

None of that is a guess. The game’s decompressor is 1088:0000 — 1,002 bytes of hand-written 386 assembly in one of the only two code segments in the whole binary with no relocation records at all. It allocates three 4,096-entry WORD tables (prefix, suffix, length), initialises them as an LZW dictionary, and computes 2 << bpp, +2 and +4 as word offsets and bpp + 1 as the initial code width. Its caller carries the string “Error: Cannot allocate decompres…” — the authors naming it themselves.

CVPC is not a GIF file: no resource carries a GIF87a or GIF89a signature and the header above is nothing like GIF’s. But the binary holds a real GIF decoder, evidenced by its own error strings (“Error: Resource is not a GIF - “, “Error: GIF contains no global color map - “), and the developers’ build paths survive beside them: ..\resource\logo.gif, ..\resource\opencrd1.gif, ..\resource\ablogo.spr. The art was authored as GIF and converted at build time by people who already had GIF LZW in hand.

Encoding

colwin’s encoder mirrors that decoder’s state machine step for step, including when it widens a code: the decoder’s dictionary lags the encoder’s by one entry, so widening on the encoder’s own counter desynchronises the stream by one code. It tracks the decoder’s counter instead.

Re-encoding all 96 shipped canvases reproduces identical pixels and identical palettes, in files between 0.991× and 1.001× the original size. The bytes are not identical, because the two encoders place their clear codes differently — which is why colwin never re-encodes a canvas you have not edited.