Every AV1 frame either decode rung has ever been measured against is `tile_cols =
tile_rows = 1`. The vendored vector is single-tile on all 274 of its frames, so every
tile array the conversions fill — `tiles.widths`, `tiles.heights`, the per-tile records
— had only ever been written at index 0, and a conversion that wrote tile 0 and left
the rest zero would pass the whole suite. Our encoder splits 4K into TWO TILE ROWS.
**The fixture.** `lowdelay-3840x2160.ivf.av1`, 261 KB, 60 frames — `punktfunk-host
spike --source synthetic --codec av1 --width 3840 --height 2160 --fps 60 --seconds 1
--bitrate 1` on .21 (NVENC, RTX 5070 Ti), wrapped to IVF with `ffmpeg -f obu … -c copy`
so `common::split_av1_aus` (the vendored parser's own `IvfIterator`) frames it exactly
as it frames the vector, with no second splitter that could disagree.
**4K is not a size choice, it is the only shape with the property.** Measured on the
same box with the same command: 1280x720, 1920x1080 and 2560x1440 all give `tile_cols =
tile_rows = 1`; 3840x2160 gives `tile_cols = 1, tile_rows = 2` with
`width_in_sbs_minus_1 = [59]`, `height_in_sbs_minus_1 = [16, 16]`, and both tiles in ONE
Tile Group OBU. 60 frames instead of 120 pays for the resolution: 261 KB, under both the
282 KB H.264 and 270 KB H.265 low-delay fixtures.
Goldens are libavcodec's software decode, cross-checked between ffmpeg n8.1.2 (Arch
x86_64, libdav1d) and 8.1.1 (Homebrew, macOS arm64, libdav1d) whose 746,496,000-byte raw
outputs are BYTE-IDENTICAL, not merely equal per frame. 60 of 60 digests distinct.
**AV1's frame accounting is asserted, never derived.** The vendored vector is 250
temporal units carrying 274 coded frames of which 24 are hidden; this stream is 60 units,
60 coded, 60 shown, 0 hidden, 0 `show_existing_frame`, 1 key frame. Neither is the
general case, so both parity harnesses now take units / decoded / shown as three
independent parameters instead of computing one from another, and the CPU guard states
all six numbers.
**A CPU gate that needed no hardware at all.** `pic_av1`'s new
`a_two_tile_frame_fills_both_row_entries_and_leaves_the_rest_zero` pins the second row
entry against its OWN `height_in_sbs_minus_1`, requires the two rows to tile the frame
exactly, and requires TWO tile RECORDS out of ONE tile group with rows (0,0) and (1,0) —
the transposition a square grid could never reveal — each spanning real bytes. The
existing one-tile test asserts index 0 is right and `1..` are zero, which a broken
multi-tile conversion also satisfies.
⚠⚠ **This is a file, and on AV1 that distinction has already cost a release.** "250/250
delivered frames bit-identical to libavcodec" was true for the entire period the host was
shipping only the FIRST TILE of every 4K frame: the verification ran against a vendored
file while the truncation lived in packetisation, and the suite stayed green throughout.
This fixture closes the multi-tile gap on the DECODE rungs and closes nothing about
fragmentation, reassembly, loss or AU boundaries — the golden header, both module docs
and the leg docs all say so, at length, so the next reader does not inherit the same
false confidence.
Legs: `low_delay_host_av1_every_frame_hashes_bit_identical_to_libavcodec` on the Vulkan
rung (11 ignored legs now) and on the D3D11VA rung, plus two non-ignored CPU tests.
Verified: 11/11 Vulkan parity legs on .21 (RTX 5070 Ti, 610.57.04), the new one 60/60
bit-identical; workspace clippy `-D warnings` and `cargo fmt --all --check` clean on .21.
enricobuehler
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