nx_jpeg_decoder.nx

nx_jpeg_decoder.nx -- JPEG baseline (SOF0) 8-bit decoder. CAPABILITY_COMPLETENESS: PARTIAL MISSING_CAPABILITIES: - SOF1 extended sequential (rare; most encoders use baseline) - SOF2 progressive: queued; widely deployed but needs spectral- selection + successive-approximation decoder - SOFn arithmetic-coded variants (SOF9-SOF15): essentially never seen in the wild - 12-bit sample precision: ITU-T T.81 allows it; almost no real-world content uses it - hierarchical mode (SOF5-SOF7): rare - lossless JPEG (SOF3, SOF7, SOF11): different algorithm entirely; queued as a separate primitive - CMYK / YCCK 4-component output: substrate v1 emits RGB only COVERED: - SOF0 baseline DCT 8-bit (the >99% case) - 1-component (grayscale) and 3-component (YCbCr) frames - Chroma subsampling 4:4:4, 4:2:2, 4:2:0 (and 4:1:1 by virtue of the generalised H/V sampling-factor loop) - DC + AC Huffman entropy decoding - Restart markers RST0..RST7 (DRI segment defines interval) - Byte stuffing (FF 00 sequences in entropy data) - APPn / COM segment skipping - 8x8 inverse DCT via nx_dct8 (already shipped) - YCbCr -> RGB conversion (BT.601 in Q10 fixed-point) Composes: nx_bitstream (MSB-first bit reader), nx_huffman (DC + AC table decode), nx_dct8 (inverse 2D DCT 8x8), nx_jpeg_header (existing marker-walk helpers; used for the initial SOI + structure validation -- this brick implements the full marker walk inline for self-containment). genealogy_id: itu_t_t81_jpeg_baseline_1992 lineage_id: nx_jpeg_decoder_v1_baseline