wai-rs: jpegai_bitstream — tools-header (TON) post-filter chain parser #18
2026-07-17
wai-rs: jpegai_bitstream — tools-header (TON) post-filter chain parser
Decodes the post-filter chain of the TON substream into a ToolsHeader: EFE-linear (B1 plane means + two decode_filters groups), eICCI (per-tile use flags, short-list flag, and the signalled Y / joint-UV model indices), EFE-nonlinear (min/max symbol, mask flags, per-plane luma ranges and A1 weights), and LEF (enable + reference channel) — in filters-factory order, each gated by its recursive enable flag. Adds the header's universal width rule as a first-class primitive: BitReader::read_max(max) reads ceil(log2(max+1)) bits, per the reference HeaderCoder::calc_bits_count. No header field has a fixed width, and several are runtime-dependent: a just-decoded max_symbol sets the width of the weights that follow it (65535 -> 16 bits, then 99 -> 7 bits within the same EFE-linear header). Validated on real test256.bin, ending exactly on bit 973 (+3 pad closing the 63-byte TON) with every value matching the reference. Two independent cross-checks: the EFE nonlinear luma range (497/25320) and its 8 A1 weights are exactly the luma_min_q / luma_max_q / weights_q15 inputs of the byte-exact efn_filter, and B1/100 = 125.61 / 130.91 are the plane means efl_chroma consumes. start_bit is the first filter's enable flag, not the TON payload start: _stream_header_part defaults to "tool_header", so other tools emit into TON ahead of the chain (two such bits precede EFE-linear in the reference stream); locating that prefix generally requires walking the whole tool tree. Co-Authored-By: Claude Opus 4.8 <[email protected]>