nx_jpeg_marker.nx

nx_jpeg_marker.nx -- JPEG segment marker scanner. Arc C1.1 of the full JPEG decoder per ITU-T Rec. T.81 / ISO 10918-1 (1992). JPEG file structure (T.81 sec B.1): SOI marker (0xFF 0xD8) one or more frame segments: [misc tables: DQT, DHT, DRI, APPn, COM] SOF (Start of Frame: 0xC0 baseline, 0xC2 progressive, etc.) one or more scans: SOS (Start of Scan: 0xFF 0xDA, then header, then entropy-coded segment) [entropy-coded data with 0xFF 0x00 byte-stuffing] EOI marker (0xFF 0xD9) Segment format (variable-length kinds): 0xFF + marker_byte + 2-byte big-endian length (length INCLUDES the 2 length bytes but NOT the 0xFF + marker) + (length-2) payload bytes. Length-less markers: SOI (D8), EOI (D9), TEM (01), RSTn (D0..D7). CRITICAL byte-stuffing rule (T.81 sec F.1.2.3): inside the entropy-coded segment that follows SOS, a literal 0xFF in the compressed bitstream is followed by 0x00. A real marker is 0xFF nn where nn != 0x00 and nn is not in D0..D7 (those are RSTn markers that DO appear inside entropy data as restart points). This primitive scans markers BEFORE the entropy stream gets processed -- it walks from SOI through SOF/DQT/DHT segments and stops at SOS (returning the SOS segment so caller knows where the entropy stream begins). Decoding the entropy stream is the job of nx_jpeg_entropy.nx (next stone). nx_safety_envelope: intended_use: "JPEG marker scanning for the bits-up image decoder pipeline." sil_target: SIL1 evidence: [t81_section_b1_canonical_basis, sealed_marker_taxonomy, bounded_iteration] hazard_register: [bug-tape-jpeg-malformed-length-overflow, bug-tape-jpeg-marker-byte-stuffing-mishandled]