e4373454ba
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.
68 lines
2.7 KiB
Go
68 lines
2.7 KiB
Go
package engine
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import (
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"bytes"
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"context"
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"log"
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"os/exec"
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"sync"
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"time"
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)
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// hdrTonemapChain is the ffmpeg filter segment that maps an HDR source
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// (HDR10/HLG, or a Dolby Vision base layer) down to SDR BT.709: linearise the
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// PQ/HLG signal, tonemap the highlights (hable), then re-encode to BT.709
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// transfer/matrix/primaries in limited range. Without it an HDR source
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// transcoded to an SDR encode keeps wide-gamut/PQ data the SDR player can't
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// interpret, so colour looks washed-out / desaturated.
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//
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// Requires the zscale filter (libzimg) in the ffmpeg build — gate on
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// FFmpegSupportsZscale. Trailing comma so it slots in front of the chain's
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// `format=` stage. CPU filter: valid for every encoder here because the decode
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// hwaccel intentionally leaves frames in system memory (see buildHLSFFmpegArgsAt).
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//
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// Tuned for HDR10/PQ (npl=100) and the common DV+HDR10 case. HLG and bare-DV
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// (Profile 5, no PQ signalling) get an approximate mapping — zscale linearises
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// from whatever transfer the stream declares — but the result is still clearly
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// better than the untonemapped washed-out baseline. A per-transfer chain is a
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// possible follow-up if HLG/DV-only sources become common.
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const hdrTonemapChain = "zscale=t=linear:npl=100,format=gbrpf32le,zscale=p=bt709,tonemap=tonemap=hable:desat=0,zscale=t=bt709:m=bt709:r=tv,"
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var (
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zscaleCacheMu sync.Mutex
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zscaleCache = map[string]bool{}
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)
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// FFmpegSupportsZscale reports whether the ffmpeg binary at path was built with
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// the zscale filter (libzimg), required for HDR→SDR tonemapping. Cached per
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// path. A detection failure (binary missing, exec error) is treated as "no" so
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// tonemapping is simply skipped — the source still plays, just without it.
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func FFmpegSupportsZscale(ffmpegPath string) bool {
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if ffmpegPath == "" {
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return false
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}
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zscaleCacheMu.Lock()
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if v, ok := zscaleCache[ffmpegPath]; ok {
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zscaleCacheMu.Unlock()
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return v
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}
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zscaleCacheMu.Unlock()
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// Probe OUTSIDE the lock: `ffmpeg -filters` can take a beat, and holding the
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// mutex across it would stall a concurrent session start. Worst case two
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// cold callers probe the same binary at once — both write the same bool.
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ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
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defer cancel()
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out, err := exec.CommandContext(ctx, ffmpegPath, "-hide_banner", "-filters").Output()
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supported := err == nil && bytes.Contains(out, []byte("zscale"))
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zscaleCacheMu.Lock()
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zscaleCache[ffmpegPath] = supported
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zscaleCacheMu.Unlock()
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if supported {
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log.Printf("[tonemap] ffmpeg has zscale — HDR sources will be tonemapped to SDR")
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} else {
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log.Printf("[tonemap] ffmpeg %q lacks zscale — HDR sources play without tonemapping (desaturated)", ffmpegPath)
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}
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return supported
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}
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