`fd6241a2` made HEVC's freedom from the release-ordering defect falsifiable on CPU and
recorded what was still missing: no low-delay HEVC stream was vendored, so the exemption
rested on a structural argument plus one throwaway measurement. This vendors the stream,
and the exemption HELD.
**The fixture.** `lowdelay-640x480.h265`, 270 KB, 120 pictures — `punktfunk-host spike
--source synthetic --codec h265 --width 640 --height 480 --fps 60 --seconds 2 --bitrate 1`
on .21 (NVENC, RTX 5070 Ti, driver 610.57.04). Deliberately the H.264 sibling's resolution
and frame count: the two are then directly comparable, 640 and 480 are both multiples of
MinCbSizeY so there is no conformance window and a hash mismatch can only be decode rather
than readback geometry, and 270 KB sits alongside the 282 KB already accepted for H.264.
Goldens are libavcodec's software decode, cross-checked BIT-IDENTICAL across ffmpeg n8.1.2
(Arch, x86_64) and 8.1.1 (Homebrew, macOS arm64), 120 of 120 digests distinct.
**The exemption held, measured rather than argued.** `sps_max_dec_pic_buffering_minus1 = 4`
against the four pictures 8.3.2 keeps marked in steady state, `sps_max_num_reorder_pics = 0`,
`numRefL0 = 1` — a five-picture DPB filled exactly by four references plus the current
picture. 115 of the 120 access units retire a picture, and `removed ∩ dpb_refs` is **0 of
120**. A 300-picture 1080p stream from the same host reports the same shape: 295
retirements, 0 intersections. It is the encoder and not the resolution, exactly as for
H.264.
**A zero proves nothing on its own, so the fixture is pinned by its counterfactual.**
`test-25fps.h264` reported zero for two milestones while every stream we ship aliased on
99% of its frames. So the guarantee here is not "we looked and it was fine": hand
`plan_to_dxva_h265` the marked DPB as it stood BEFORE `decode_rps` — the mutation a
snapshot move would cause, reconstructed exactly as `dpb_refs(N-1) ∪ {stored(N-1)}` — and
the alias appears on **115 of 120** access units, driven through the real conversion rather
than through planner arithmetic. If a regeneration ever produced a stream that reordered,
or a DPB deeper than its reference count, that 115 collapses to 0 and the tests say so
instead of continuing to pass.
**The two rungs are exempt for different reasons, and the asymmetry is now a gate.** DXVA
binds the whole marked DPB — `RefPicList` is spec-defined that way, and an RFI long-term
anchor has to survive in it — so its exemption really is `H265Planner`'s snapshot ordering,
one call away from being untrue. `plan_to_vk_h265` never reads `dpb_refs` at all:
`pReferenceSlots` is the slots the operation uses, so it binds the current RPS sets, which
`decode_rps` itself derives and which therefore cannot name a picture that same RPS just
dropped. A new test feeds that conversion the identical widened snapshot and asserts
nothing changes, so a future change making the Vulkan rung bind the marked DPB — a
legitimate thing to want, since a *Foll* anchor invisible to the hardware is the RFI
failure shape — fails loudly instead of silently acquiring the defect.
What the Vulkan pixel leg adds is therefore NOT aliasing coverage, and its docs say so:
it is the first HEVC frame either rung has decoded from our own encoder, under a DPB that
retires and reissues a slot on 115 of 120 access units back to back, where the vendored
vector's reordering keeps that eviction slack.
Legs: `low_delay_host_h265_every_frame_hashes_bit_identical_to_libavcodec` on the Vulkan
rung (10 ignored legs now, up from 9) and on the D3D11VA rung, plus three non-ignored CPU
guards that run in ordinary CI.
Verified: 10/10 Vulkan parity legs on .21 (RTX 5070 Ti, 610.57.04), the new one 120/120
bit-identical; workspace clippy `-D warnings` and `cargo fmt --all --check` clean on .21.
enricobuehler
f0702f3e06