nx_jpeg_zigzag.nx

nx_jpeg_zigzag.nx -- inverse zig-zag scan order for JPEG 8x8 blocks. Per ITU-T Rec. T.81 Fig. A.6 / table H.1. JPEG stores DCT coefficients in zig-zag order to cluster non-zero coefficients near the start of the stream (improves run-length coding efficiency). Decoding pipeline needs the inverse: natural_block[un_zigzag[k]] = zigzag_block[k] for k in 0..63. This file ships the 64-entry inverse-zigzag table AND a primitive that applies it to an 8x8 i64 coefficient block. The zig-zag scan visits 8x8 block positions in this order (T.81 Fig. A.6): 0 1 5 6 14 15 27 28 2 4 7 13 16 26 29 42 3 8 12 17 25 30 41 43 9 11 18 24 31 40 44 53 10 19 23 32 39 45 52 54 20 22 33 38 46 51 55 60 21 34 37 47 50 56 59 61 35 36 48 49 57 58 62 63 nx_safety_envelope: intended_use: "8x8 DCT-block un-zigzag for JPEG decode." sil_target: SIL1 evidence: [t81_figure_a6_canonical_basis, fixed_64_entry_table_bit_exact_reproducible] verdict: NOT_YET_EVALUATED