nx_jpeg_sos.nx

nx_jpeg_sos.nx -- JPEG Start-of-Scan header parser. Per ITU-T Rec. T.81 sec B.2.3. SOS segment payload (immediately after the 2-byte length the marker scanner already consumed): Ns (1 byte) number of components in this scan (1..4) per component (2 bytes each): Csj (1 byte) component selector (matches a Ci from the active SOF frame descriptor) TdjTaj (1 byte) high4 = Td DC Huffman table id (0..3), low4 = Ta AC Huffman table id (0..3) Ss (1 byte) start of spectral selection. Baseline JPEG always 0. Se (1 byte) end of spectral selection. Baseline JPEG always 63. AhAl (1 byte) high4 = Ah successive-approximation high, low4 = Al successive-approximation low. Baseline JPEG always 0x00. Total: 1 + 2 * Ns + 3 bytes. After this header, the entropy-coded segment begins (handled by nx_jpeg_entropy). nx_safety_envelope: intended_use: "Scan-header parsing for JPEG decode." sil_target: SIL1 evidence: [t81_section_b_2_3_canonical_basis, bounded_4_component_max] hazard_register: [bug-tape-sos-bad-ns, bug-tape-sos-progressive-mistaken-baseline] residual_risk: "Progressive JPEG (Ss != 0 or Se != 63 or AhAl != 0) is detected but not decoded by this primitive." verdict: NOT_YET_EVALUATED