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Document subtitle-burn 500's real root cause: Emby's fontconfig, not us
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Once the hls.js timeout fix stopped masking it, the actual segment
request came back with a real 500. Emby's own server log (shared by the
user, not from this app) shows exactly why: ffmpeg's subtitles filter
failing with "Failed to load fontconfig fonts!" inside StartFfMpeg -
a font/fontconfig-cache problem in whatever environment Emby's
transcoder runs in, not anything this app's request could cause.

No code change: this app deliberately forces SubtitleMethod=Encode (see
applyPlaybackOverrides) since it never wires up hls.js's rendition-based
subtitle API - dropping that to dodge the 500 would just bring back the
original "subtitle picked, never shown" bug instead of fixing anything.
Recorded as Round 4 of the player-track-selection saga so this doesn't
get re-investigated from scratch later.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
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Nicolas 2026-09-14 13:54:54 +02:00
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@ -510,6 +510,29 @@ real `Hls` instance from the exact pinned CDN build
`hls.config.fragLoadingTimeOut` etc. - all four came back as set, not as
hls.js's own defaults.
**Round 4: a real 500, once the timeout was no longer masking it - traced
to Emby, not this app.** Exactly what Round 2 said would happen "if
subtitles still don't show once this log line confirms `subtitleMethod=
Encode` really was sent": with Round 3's timeout fix giving Emby enough
rope to actually answer instead of hls.js giving up first, the segment
request came back with a real, fast 500 - and Emby's own server log
(shared directly, not this app's) shows exactly why: `[subtitles@f2] Failed
to load fontconfig fonts!`, thrown out of `StartFfMpeg`/
`GetDynamicSegment` before a response body ever exists. That's ffmpeg's
`subtitles` filter (burning text into the video via `libass`) failing to
find usable fonts/a fontconfig cache in whatever environment Emby's
transcoder runs in - a deployment/environment issue on the Emby side
(commonly a container image missing font packages, or a fontconfig cache
directory that isn't writable), with nothing in this app's request that
could cause or fix it. Confirmed not a regression to work around here:
this app deliberately forces `SubtitleMethod=Encode` (see
`applyPlaybackOverrides`'s own comment) specifically because it never
wires up `hls.js`'s rendition-based subtitle API - dropping that override
to dodge this 500 would silently bring back the very first version of
this bug (a subtitle picked, never shown) instead of fixing anything.
Nothing to change in this app for this one; it needs fonts/fontconfig
fixed wherever Emby's own container/host runs.
## Assumptions to verify
**Confirmed against the real server** (not a guess — actually exercised,