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.