nx_pcx_decode.nx

nx_pcx_decode.nx -- ZSoft PCX reader (PC Paintbrush; the DOS-era standard). THE RLE IS PER-SCANLINE, NOT PER-IMAGE. A run may not cross a scanline boundary, and a decoder that treats the stream as one flat run-length sequence will still produce a plausible image on most files and shear on the ones that pad. We decode exactly nplanes*bytes_per_line per row and stop, which is also what makes a malformed row detectable. PLANES ARE SEPARATE, NOT INTERLEAVED. 24-bit PCX stores a whole row of R, then a whole row of G, then B -- not RGB triples. The same row buffer is therefore indexed plane*bytes_per_line + x, which is the one addressing mistake that yields a recognisable-but-colour-smeared image. 8-bit palette lives at the END of the file: the last 769 bytes, introduced by a 0x0C marker byte. Its absence on a 1-plane 8-bit file is a refusal, not a reason to invent a grayscale ramp. genealogy_id: zsoft_pcx_5 lineage_id: nx_pcx_v1 license_tier: ORIGINAL