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fix(stream): clean HLS segments — no B-frames, no scene-cut, CFR

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Slightly-VFR / B-frame MKV sources made ffmpeg's fMP4 muxer emit a continuous
"Packet duration is out of range" flood and produce uneven segment lengths the
web player stuttered on. Add, on the two main encoders + globally:

- libx264: -bf 0 -sc_threshold 0
- h264_nvenc: -bf 0 -no-scenecut 1
- -fps_mode cfr (force constant frame rate)

Keyframe cadence stays driven by -force_key_frames, so every segment is exactly
hls_time long. Verified: the warning flood drops from dozens/sec to ~1 per 80s
of transcoded content (cosmetic), segments stay valid fMP4.
This commit is contained in:
Deivid Soto 2026-06-03 09:12:05 +02:00
parent ea152a2276
commit 325c11c1eb
1 changed files with 19 additions and 2 deletions
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21
internal/engine/hls.go
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@ -1236,7 +1236,13 @@ func buildHLSFFmpegArgsAt(cfg HLSSessionConfig, probe *StreamProbe, tmpDir strin
// this is the difference between ~3 s and ~2.5 s per segment on a // this is the difference between ~3 s and ~2.5 s per segment on a
// recent x86 CPU. `-threads 0` is libx264's default but explicit // recent x86 CPU. `-threads 0` is libx264's default but explicit
// helps when the user has set GOMAXPROCS. // helps when the user has set GOMAXPROCS.
args = append(args, "-preset", profile.Preset, "-threads", "0") // -bf 0 (no B-frames) + -sc_threshold 0 (no scene-cut keyframes): both
// remove the timestamp irregularities that make ffmpeg's HLS muxer emit
// "Packet duration is out of range" on slightly-VFR / B-frame sources and
// produce uneven segment lengths the player stutters on. Keyframe cadence
// is driven by -force_key_frames below, so disabling scene-cut keeps every
// segment exactly hls_time long.
args = append(args, "-preset", profile.Preset, "-threads", "0", "-bf", "0", "-sc_threshold", "0")
case "h264_nvenc": case "h264_nvenc":
// p3 + vbr keeps NVENC fast (~1.5 s seg-0) without the segmentation // p3 + vbr keeps NVENC fast (~1.5 s seg-0) without the segmentation
// breakage `-tune ll` introduced in 0.9.9: with -tune=ll the NVENC // breakage `-tune ll` introduced in 0.9.9: with -tune=ll the NVENC
@ -1245,7 +1251,11 @@ func buildHLSFFmpegArgsAt(cfg HLSSessionConfig, probe *StreamProbe, tmpDir strin
// "preparando sesión" until the 60 s mark-ready timeout. Verified on // "preparando sesión" until the 60 s mark-ready timeout. Verified on
// ffmpeg 6.1.1 + driver 580 / RTX-class GPUs: dropping -tune ll // ffmpeg 6.1.1 + driver 580 / RTX-class GPUs: dropping -tune ll
// restores per-segment cuts at 27x real-time vs 28x with -tune ll. // restores per-segment cuts at 27x real-time vs 28x with -tune ll.
args = append(args, "-preset", profile.Preset, "-rc", "vbr") // -bf 0 + -no-scenecut: same rationale as libx264 (NVENC's own flag for
// scene-cut). No B-frame reorder → monotonic DTS → uniform segments, no
// "Packet duration is out of range" flood. Safe with -force_key_frames
// (unlike -tune ll, which broke per-segment cuts — see note above).
args = append(args, "-preset", profile.Preset, "-rc", "vbr", "-bf", "0", "-no-scenecut", "1")
case "h264_qsv": case "h264_qsv":
// veryfast is the fastest realistic QSV preset; medium was too // veryfast is the fastest realistic QSV preset; medium was too
// conservative for first-start. look_ahead=0 keeps the encoder // conservative for first-start. look_ahead=0 keeps the encoder
@ -1394,6 +1404,13 @@ func buildHLSFFmpegArgsAt(cfg HLSSessionConfig, probe *StreamProbe, tmpDir strin
"-ac", "2", "-ac", "2",
) )
// Force constant frame rate. Many MKV rips are slightly variable-frame-rate
// (or carry irregular PTS); muxed to fMP4 that produces non-monotonic packet
// durations ("Packet duration is out of range") and uneven segment lengths
// the player stutters on. CFR resamples to a steady cadence → uniform
// segments. Near-CFR sources (23.976/24/25) are essentially untouched.
args = append(args, "-fps_mode", "cfr")
// HLS muxer — fmp4 segments with pre-computed segment count. // HLS muxer — fmp4 segments with pre-computed segment count.
// `-start_number` slots seg-N.m4s where N matches the segment index in // `-start_number` slots seg-N.m4s where N matches the segment index in
// the pre-rendered manifest. Each ffmpeg writes its own ffmpeg.m3u8 but // the pre-rendered manifest. Each ffmpeg writes its own ffmpeg.m3u8 but