Second critico pass on the functional probe.
- The probe does real Vulkan device init, which can transiently fail when the
box is busy (notably the startup warm racing the encode benchmark). Caching
that timeout as a permanent 'no' would pin HDR to the zscale CPU chain until
daemon restart. Now a deadline is NOT cached — only a clean non-zero exit
(filter absent / no ICD), which is a stable result. zscale stays cached as
before (cheap deterministic grep, can't flake).
- Surface the exec error when ffmpeg never produced stderr (timeout / ENOENT):
the fallback log now shows err.Error() instead of a blank tail, so 'no
Vulkan' is distinguishable from 'ffmpeg never ran'.
- Dockerfile comment: clarify the Vulkan ICD (not GLX) is the load-bearing
mount that 'graphics' adds; 'compute' alone doesn't mount it.
Probe still returns true on a Vulkan host (verified); engine tests green.
Review (critico) caught a regression: the prod agent image ships a BtbN GPL
ffmpeg with libplacebo COMPILED IN but no Vulkan runtime (debian-slim, no
libvulkan1/mesa-vulkan-drivers/nvidia ICD). The presence probe (ffmpeg
-filters) would flip HasLibplacebo on, the filter's Vulkan device creation
would fail at runtime, and HDR sources that previously tonemapped via zscale
would break.
- FFmpegSupportsLibplacebo now RUNS the filter on one synthetic frame and
requires a clean exit (forces Vulkan device init + filtergraph negotiation),
so it is honest about THIS host: works on Vulkan-capable hosts, falls back to
zscale where Vulkan is absent. Logs the real ffmpeg error on failure.
- Warm the libplacebo (Vulkan init ~1.7s) + zscale caches in a background
goroutine at startup so the first stream session doesn't pay the probe and
risk its setup timeout.
- Benchmark: margin 1.5x -> 2.0x (the probe measures encode only; real decode
of HEVC/10-bit + busier content needs more headroom), per-probe timeout
12s -> 6s + overall 45s -> 20s (it blocks registration on software hosts),
and a 'no rung measured' case (missing lavfi/wedged ffmpeg) now keeps the
1080 default instead of flooring at 480 — an infra failure isn't a slow host.
Verified e2e on the fixed binary: LOTR Two Towers (HEVC 3840x1608 10-bit
HDR10/PQ, 12GB) on desktop-Chrome caps -> hls, ffmpeg runs h264_nvenc with
-vf ...,libplacebo=...:format=yuv420p:tonemapping=bt.2390 (zscale chain
replaced), 45 fMP4 segments, ffprobe confirms output h264 yuv420p bt709
(tonemapped from bt2020/smpte2084), no ffmpeg errors.
Prefer the single-pass Vulkan libplacebo filter over the CPU zscale chain
for HDR->SDR tonemapping when the agent ffmpeg has it. One GPU pass does
tonemap + BT.709 primaries/transfer/matrix + 8-bit yuv420p, replacing the
four-stage zscale chain and its trailing format=/setparams. Higher quality,
far cheaper than the CPU path, and present on builds that lack zscale.
- FFmpegSupportsLibplacebo probe (cached, mirrors FFmpegSupportsZscale)
- HasLibplacebo on TranscodeRuntime, wired from buildTranscodeRuntime
- hls.go: videoTail picks libplacebo when present (not h264_vaapi), else
keeps the zscale tonemap + format chain
- test: libplacebo replaces the zscale chain, never runs alongside it
HDR (HDR10/HLG/Dolby Vision) transcoded to SDR came out washed-out and
desaturated because the filter chain never tonemapped. buildHLSFFmpegArgsAt now
inserts a zscale linearise -> hable tonemap -> BT.709 chain after the scale and
before format=, but only when the source is HDR and the ffmpeg build has zscale
(FFmpegSupportsZscale, cached). Builds without zimg keep the old behaviour
(plays, just desaturated) instead of erroring.
It's a CPU filter, valid for every encoder here: the decode hwaccel deliberately
leaves frames in system memory (no -hwaccel_output_format), so zscale runs ahead
of format=/hwupload exactly like the existing scale filter. Verified on a real
4K HDR10 file — vivid colour and deep blacks vs the washed-out baseline.
