mux: detect forced PGS subtitles from the stream
Flag a PGS subtitle track FlagForced when it displays subtitles and every one carries the HDMV forced_on_flag (a dedicated forced/narrative track), independent of the disc's vendor label metadata. The track header reserves a FlagForced byte up front and it is promoted at finish() from the accumulated display-set state. Only ever promotes — a track already forced from the playlist metadata is never demoted.
This commit is contained in:
Matthew Jackson
+186
-5
@@ -672,6 +672,14 @@ pub struct MkvMuxer<W: Write + Seek> {
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/// (to patch in place) and the IFO-claimed count (to warn on disagreement);
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/// `corrected` flips once patched so we only act on the first frame.
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ac3_channel_fixups: std::collections::HashMap<usize, Ac3ChannelFixup>,
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/// Deferred PGS forced-subtitle detection. A PGS subtitle track reserves a
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/// 1-byte `FlagForced` value up-front; as its display sets are written, the
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/// track is judged forced iff it displayed at least one subtitle and EVERY
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/// display set carried the HDMV `forced_on_flag` (a dedicated forced/narrative
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/// track). At `finish()` the reserved byte is promoted to 1 for such tracks —
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/// so forced subs are flagged even on discs without vendor label metadata.
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/// Only ever promotes (0→1); a scan/vendor forced flag is never demoted.
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pgs_forced_fixups: std::collections::HashMap<usize, PgsForcedFixup>,
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/// `--log-level 3` opening-frame capture: the first ~100 coded frames per
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/// track are written (raw) to a `<output>.opening.bin` side file with a
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/// per-frame summary logged, so an opening-GOP / menu / mid-GOP-open issue is
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@@ -692,6 +700,18 @@ struct Ac3ChannelFixup {
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corrected: bool,
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}
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/// Deferred PGS forced-subtitle detection state for one PGS subtitle track.
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struct PgsForcedFixup {
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/// Absolute file offset of the 1-byte `FlagForced` value in the Tracks element.
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value_offset: u64,
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/// Whether the track has displayed at least one subtitle (a display PCS).
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has_display: bool,
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/// Whether EVERY display set so far carried the forced_on_flag. Starts true;
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/// cleared by the first non-forced display set. With `has_display`, a value of
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/// true at `finish()` means the whole track is forced narrative.
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all_forced: bool,
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}
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/// TimestampScale: nanoseconds per Matroska timestamp tick. 0.1 ms (100_000 ns).
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///
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/// The classic 1 ms scale truncates two distinct cadences onto the same tick:
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@@ -917,6 +937,8 @@ impl<W: Write + Seek> MkvMuxer<W> {
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let mut track_uids: Vec<u64> = Vec::with_capacity(tracks.len());
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let mut ac3_channel_fixups: std::collections::HashMap<usize, Ac3ChannelFixup> =
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std::collections::HashMap::new();
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let mut pgs_forced_fixups: std::collections::HashMap<usize, PgsForcedFixup> =
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std::collections::HashMap::new();
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// Per track: whether it emitted a conforming `mvcC` BlockAdditionMapping.
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// Filled below from the SAME built record that drives the CodecPrivate
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// mvcC extension, so the three MVC signals never diverge.
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@@ -948,7 +970,25 @@ impl<W: Write + Seek> MkvMuxer<W> {
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if !track.is_default {
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ebml::write_uint(&mut writer, ebml::FLAG_DEFAULT, 0)?;
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}
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if track.is_forced {
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if track.track_type == ebml::TRACK_TYPE_SUBTITLE && track.codec_id == ebml::CODEC_PGS {
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// Reserve a 1-byte FlagForced (initial value = the scan/vendor
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// flag) and record its offset, so PGS content can promote it to 1
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// at finish() if the track proves to be forced narrative. Written
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// explicitly (ID + size + value) so the value is a single,
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// in-place-patchable byte — same idiom as the Channels fixup.
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ebml::write_id(&mut writer, ebml::FLAG_FORCED)?;
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ebml::write_size(&mut writer, 1)?;
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let value_offset = writer.stream_position()?;
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writer.write_all(&[track.is_forced as u8])?;
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pgs_forced_fixups.insert(
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i,
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PgsForcedFixup {
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value_offset,
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has_display: false,
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all_forced: true,
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},
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);
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} else if track.is_forced {
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ebml::write_uint(&mut writer, ebml::FLAG_FORCED, 1)?;
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}
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@@ -1203,6 +1243,7 @@ impl<W: Write + Seek> MkvMuxer<W> {
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max_block_ticks: 0,
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last_video_keyframe_ticks: None,
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ac3_channel_fixups,
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pgs_forced_fixups,
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opening_capture: None,
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})
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}
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@@ -1514,6 +1555,17 @@ impl<W: Write + Seek> MkvMuxer<W> {
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}
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}
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// Accumulate PGS forced-subtitle state: a display set marks the track as
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// having shown a subtitle, and clears `all_forced` the moment a
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// non-forced set appears. `finish()` promotes FlagForced only for a track
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// that displayed subtitles and had every one forced.
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if let Some(fixup) = self.pgs_forced_fixups.get_mut(&track_idx) {
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if let Some(forced) = super::codec::pgs::display_set_is_forced(data) {
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fixup.has_display = true;
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fixup.all_forced &= forced;
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}
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}
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Ok(())
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}
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@@ -1546,6 +1598,26 @@ impl<W: Write + Seek> MkvMuxer<W> {
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// Close final cluster
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self.end_cluster()?;
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// Promote FlagForced for PGS subtitle tracks that proved to be forced
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// narrative (displayed subtitles, every one forced). In-place single-byte
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// rewrite of the reserved value, then restore the append position for the
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// Cues that follow. Only promotes (0→1); a track already forced from the
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// scan/vendor flag stays forced.
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let forced_offsets: Vec<u64> = self
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.pgs_forced_fixups
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.values()
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.filter(|f| f.has_display && f.all_forced)
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.map(|f| f.value_offset)
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.collect();
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if !forced_offsets.is_empty() {
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let here = self.writer.stream_position()?;
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for off in forced_offsets {
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self.writer.seek(std::io::SeekFrom::Start(off))?;
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self.writer.write_all(&[1u8])?;
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}
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self.writer.seek(std::io::SeekFrom::Start(here))?;
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}
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// Write Cues
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let cues_start = self.writer.stream_position()?;
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let cues_offset = cues_start - self.segment_start;
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@@ -2600,13 +2672,122 @@ mod tests {
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let muxer = MkvMuxer::new(buf, &tracks, None, 60.0, &[]).unwrap();
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let data = muxer.writer.into_inner();
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// FlagForced should NOT be written for non-forced tracks
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assert!(
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find_id(&data, ebml::FLAG_FORCED).is_none(),
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"FlagForced element should not be present for non-forced subtitle"
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// A PGS subtitle track now RESERVES a FlagForced element (so PGS content
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// can promote it later), but for a non-forced track its value is 0. The
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// value byte sits after the 2-byte ID (0x55AA) + 1-byte size.
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let pos =
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find_id(&data, ebml::FLAG_FORCED).expect("PGS subtitle reserves a FlagForced element");
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assert_eq!(
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data[pos + 3],
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0,
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"reserved FlagForced value must be 0 for a non-forced subtitle"
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);
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}
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/// A PGS display-set PES block (PCS) with one composition object, whose
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/// forced_on_flag is set per `forced`.
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#[cfg(test)]
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fn pgs_display_set(forced: bool) -> Vec<u8> {
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let mut pcs = vec![0u8; 18];
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pcs[0] = 0x16; // segment type PCS
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pcs[13] = 1; // number_of_composition_objects
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pcs[17] = if forced { 0x40 } else { 0x00 }; // first object flags
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pcs
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}
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#[test]
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fn mkv_pgs_forced_promoted_when_all_display_sets_forced() {
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// End-to-end: a PGS subtitle track whose every display set is forced is
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// promoted to FlagForced=1 at finish(), even though the scan flag was
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// false (no vendor metadata).
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use crate::disc::SubtitleStream;
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use std::sync::{Arc, Mutex};
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let shared = Arc::new(Mutex::new(Cursor::new(Vec::new())));
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let sub = MkvTrack::subtitle(&SubtitleStream {
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pid: 0x1200,
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codec: Codec::Pgs,
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language: "eng".into(),
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forced: false,
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qualifier: crate::disc::LabelQualifier::None,
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codec_data: None,
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});
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let tracks = [make_video_track(), sub];
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let mut muxer =
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MkvMuxer::new(SharedWriter(shared.clone()), &tracks, None, 60.0, &[]).unwrap();
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muxer
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.write_frame(0, 0, true, &[0u8; 16], Some(40_000_000), None)
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.unwrap(); // video keyframe opens a cluster
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muxer
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.write_frame(
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1,
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1_000_000,
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true,
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&pgs_display_set(true),
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Some(2_000_000),
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None,
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)
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.unwrap();
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muxer.finish().unwrap();
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let data = shared.lock().unwrap().clone().into_inner();
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let pos = find_id(&data, ebml::FLAG_FORCED).expect("reserved FlagForced");
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assert_eq!(
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data[pos + 3],
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1,
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"all-forced PGS track promoted to FlagForced=1"
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);
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}
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#[test]
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fn mkv_pgs_not_promoted_when_a_display_set_is_not_forced() {
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// A track with ANY non-forced display set is a full track, not forced
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// narrative — FlagForced stays 0.
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use crate::disc::SubtitleStream;
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use std::sync::{Arc, Mutex};
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let shared = Arc::new(Mutex::new(Cursor::new(Vec::new())));
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let sub = MkvTrack::subtitle(&SubtitleStream {
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pid: 0x1200,
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codec: Codec::Pgs,
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language: "eng".into(),
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forced: false,
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qualifier: crate::disc::LabelQualifier::None,
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codec_data: None,
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});
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let tracks = [make_video_track(), sub];
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let mut muxer =
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MkvMuxer::new(SharedWriter(shared.clone()), &tracks, None, 60.0, &[]).unwrap();
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muxer
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.write_frame(0, 0, true, &[0u8; 16], Some(40_000_000), None)
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.unwrap();
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muxer
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.write_frame(
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1,
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1_000_000,
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true,
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&pgs_display_set(true),
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Some(2_000_000),
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None,
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)
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.unwrap();
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muxer
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.write_frame(
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1,
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3_000_000,
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true,
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&pgs_display_set(false),
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Some(4_000_000),
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None,
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)
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.unwrap(); // a normal (non-forced) subtitle → not a forced track
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muxer.finish().unwrap();
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let data = shared.lock().unwrap().clone().into_inner();
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let pos = find_id(&data, ebml::FLAG_FORCED).expect("reserved FlagForced");
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assert_eq!(data[pos + 3], 0, "mixed PGS track stays FlagForced=0");
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}
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// ============================================================
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// Seekability tests: SeekHead, keyframe-aligned clusters, Cues
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// ============================================================
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