nx_av1_tile.nx

nx_av1_tile.nx -- AV1/AV2 tile geometry and tile_info, writer and reader. Layer four, on nx_av1_ec.nx / nx_av1_obu.nx / nx_av1_seq.nx. Tiles are how AV1 and AV2 parallelise: a frame is split into independently decodable rectangles, and every tile group in the bitstream is indexed against this geometry. Get the geometry wrong and the tile data is read at the wrong offsets -- the entropy decoder then produces confident nonsense. THE UNITS CHANGE THREE TIMES. Pixels -> 4x4 mode-info units -> superblocks, and the superblock step is 64 or 128 depending on a sequence-header flag. MiCols is 2 * ((width + 7) >> 3), which is NOT (width + 3) >> 2: the frame is rounded to an 8-pixel boundary FIRST and then counted in 4x4 units, so odd sizes round differently than the obvious formula gives. A 1x1 frame has 2 mode-info columns, not 1. ns() IS NOT f(n). The non-symmetric code spends w-1 bits on small values and w bits on large ones, so that an alphabet whose size is not a power of two costs no wasted bits. Reading it as a plain fixed-width field consumes one bit too few on every large value and desynchronises everything after. genealogy_id: av1_spec_5_9_15_tile_info lineage_id: nx_av1_tile_v1 license_tier: ORIGINAL