Two transcoder fixes for browser MediaSource Extensions parsing:
1. -ar 48000 -ac 2 on the audio output. Source 5.1 / 7.1 streams produced
a moov atom Chrome CHUNK_DEMUXER refuses to parse, even when the video
metadata is fine and a non-MSE video element accepts the same file.
Forcing AAC-LC stereo 48 kHz makes the moov shape MSE-compatible.
2. -frag_duration 1000000 (1 second) so each moof+mdat fragment caps at
~1s of media. Without it, ffmpeg only splits at keyframes and high-
bitrate 1080p produces 8 MiB+ mdat boxes — MSE waits for the whole
mdat before parsing the first fragment, so playback never starts.
3. -movflags +negative_cts_offsets so b-frames carry the right pts/dts
offsets and the playhead doesn't reset every fragment.
4. New range_req debug log to make sizing bugs greppable.
1. -profile:v main + -level:v 4.0 to avoid Chrome's HW decoder path that
failed with "VaapiWrapper: failed initializing for h264 high" on Linux.
2. setparams to rewrite HDR HEVC color metadata to SDR Rec.709 so browsers
don't reject wide-gamut output.
3. serveRange caps `want` by estimated final size (not current). ReadAt
blocks until ffmpeg catches up — that's the right behaviour. Returning
RangeEnd inmediato was making the browser abort with "Format error".
4. Debug log on every range_req.
The previous scale expression `min(iw,iw*H/ih)':'min(ih,H)` produced odd
widths (e.g. 1425×720 for a 16:9 source capped at 720p) which libx264
refuses with `width not divisible by 2`, killing the encoder before a
single byte was written.
Switch to `scale=-2:H:force_original_aspect_ratio=decrease`, which
derives a width that preserves aspect ratio AND is rounded to a multiple
of 2. Always set `-pix_fmt yuv420p` so 10-bit HEVC sources are downcast
to the 8-bit format browser <video> elements actually decode.
Also add `-y`, guard nil pipe in Close(), and the related transcode
plumbing for browser-decided per-session quality.
Source files in HEVC, AV1, AC3, DTS, EAC3, etc. now transcode through ffmpeg
to fragmented MP4 (h264 + aac) on-the-fly when the browser would otherwise
play silent black. Decision matrix lives in engine.DecideAction:
passthrough → remux → audio-transcode → full video-transcode.
Architecture — temp file + growing-size source:
- engine.streamSource interface abstracts byte source. Two impls:
* diskFileSource: passthrough when codecs are already browser-friendly.
* transcodeSource: spawns ffmpeg writing to a /tmp/tc-stream-*.mp4 file.
A ticker polls file size and wakes blocked ReadAt callers as ffmpeg
produces output. Estimate of final size (bitrate × duration) is
announced over the wire so the browser's scrubber has something to
anchor on.
- dataChannelPump now reads from streamSource instead of *os.File. HELLO
carries Transcoding=true + an estimated total size; Seekable=true (we
read random-access from the temp file even while writing).
- Transcoder runtime resolved per session by buildTranscodeRuntime in
cmd/daemon: ffmpeg/ffprobe path lookup + HWAccel auto-detection
(NVENC/QSV/VAAPI/VideoToolbox).
- New [downloads.transcode] TOML section: enabled (default true), hw_accel
(auto), preset (veryfast), video_bitrate (5M), audio_bitrate (192k),
max_height (optional downscale), max_concurrent (safety cap).
Falls back to passthrough if ffprobe is missing, fails, or codecs are
already browser-friendly. tmp file is cleaned up on session shutdown.
