WIM2 format

WIM2 format

WIM2 starts with a versioned fixed header, UTF-8 JSON metadata, and a fixed-size tile index. Every tile entry records geometry, codec mode, entropy ID, progressive layers, payload offset, compressed size, expanded size, and CRC32.

Tile modes

• Raw: fallback when coding overhead would increase size.
• Predictive: reversible spatial prediction and residual coding.
• Palette: exact local palettes of up to 256 colors.
• Wavelet: reversible CDF 5/3 for lossless output and quantized CDF 9/7 for lossy output.

Automatic lossless selection chooses the smallest exact candidate. Lossy selection balances reconstructed distortion and encoded size. The decoder reads the recorded mode and never repeats classification.

Independent tiles

Tiles are independently compressed and checksummed. ROI decoding can read only intersecting payloads, and corruption in one tile does not require trusting unrelated tiles.

Extensions

Optional chunks are appended after the base image and located through an XDIR directory and fixed XEND trailer.

• HIST: indexed WIM2 history states.
• AROT: checksums and two GF(256) parity shards for bounded recovery.

This wiki page is an overview, not yet a frozen normative specification.