A transcoded episode stalled every few seconds and seeking took five to
nine seconds to produce a frame. Neither was a seek bug: both seeks in the
capture landed correctly. The stream itself could not keep up.
The episode was HEVC video, E-AC-3 audio, and a PGSSUB subtitle track.
Only the audio needed transcoding — the device profile supports HEVC and
the server would have remuxed the video untouched. But the PlaybackInfo
request omitted SubtitleStreamIndex, and omitting it does not mean "no
subtitles": the server then honours the source's default/forced flag and
picks a track itself. It picked the PGS one. PGS is a bitmap, and the
profile advertised only srt/vtt as External, so it could not go out as a
sidecar — leaving SubtitleMethod=Encode, burn-in.
Burn-in is a video cost, not a subtitle cost. Compositing rules out
remuxing, so the whole HEVC stream was re-encoded to h264 frame by frame.
The server could not sustain that in real time: the buffer never grew past
one segment and playback ran waiting -> HLS error -> canplay -> three
seconds of picture, indefinitely, while each seek restarted the encoder
from scratch. TranscodeReasons named it — SubtitleCodecNotSupported — but
nothing in the log connected that to the stall, so the diagnostic now says
which track it is declining and why.
Ask for SubtitleStreamIndex=-1 explicitly, and advertise every text format
we can render (srt/subrip/ass/ssa/vtt) as External so a subtitle can only
ever arrive as a sidecar. Nothing is lost: the app already fetches subtitle
tracks itself and draws them over the video (UR-020), so the server's
composited copy was always redundant. Image-based tracks are consequently
not offered, which is honest rather than a regression — the renderer cannot
composite a bitmap, and the previous behaviour paid for them by making the
stream unwatchable.
The policy lives beside the other device-profile rules in Rust, where it is
testable without a device.
TRACES: UR-020, UR-004 | DR-176 | UT-168
dtourolle
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