Embedded subtitles on mkvmerge-muxed files with compressed subtitle tracks
(the anime-release convention, reported against AV1 encodes) never rendered:
the track selected fine but nothing ever appeared on screen.
Two defects compounded. ContentCompAlgo 0 (zlib) is the Matroska default
value, so mkvmerge omits the element and the explicit-value detection never
engaged - media3 ignores an empty ContentCompression and silently emits
compressed samples. And for text subtitle tracks TrackOutput-level inflation
can never work: MatroskaExtractor prepends the plaintext timecode prefix to
the still-compressed payload and truncates the sample at the first NUL byte
before any TrackOutput runs, while AssTrackOutput feeds that same internal
buffer to libass, which then parses zero events and reports every frame as a
changed empty render.
Treat the presence of ContentCompression as zlib until an explicit
ContentCompAlgo says otherwise, and for SRT/ASS/SSA/VTT tracks inflate the
block frame payload before the parent's subtitle sample assembly, leaving the
TrackOutput wrapper to the formats it can actually handle. Blocks that are
laced, corrupt, or over-bound pass through byte-identical. The detection fix
also makes the existing whole-sample TrackOutput inflation engage for
PGS/VobSub zlib tracks, which media3 does not truncate.
Verified with mkvmerge zlib/plain fixture extraction tests and end-to-end on
an API 36 emulator against a Jellyfin direct-play mux mirroring the reported
file.
close#2023close#2022
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