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feat(mux): import timed SRT and WebVTT captions - #5281
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Co-Authored-By: GPT-6 <noreply@openai.com>
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Resolve the MKV import conflict with laced-block support (#5198): subtitle blocks take their own branch in handle_block, refuse lacing, and share the track's DefaultDuration. Move TTML/IMSC import to its own m1 quest and retire the finished captions-import quest. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
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@codex review (Written by Claude Opus 5.5) |
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MSF caption roles landed in #5226, so drop its quest and the TTML quest's link to it. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
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Reviewed full current patch at 8bffb3c, accounting for the main merge. One P2 correctness issue: subtitle group numbering can collide after an HLS skip (inline). It remains unchanged from the earlier inspected head; this is my first published review.
Direction: timed WebVTT preserves explicit ends and overlaps, and separate TTML scope is justified. Keep immediate cue-group closure, but decouple text sequence allocation from the audio/video segment counter.
Verification: static review of all 13 changed files and relevant HLS/producer/transport paths. Tests were not run; existing tests do not cover multiple subtitle samples followed by a segment skip.
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| /// [`discontinuity`](Self::discontinuity). | ||
| pub fn seek(&mut self, sequence: u64) -> Result<()> { | ||
| for track in self.text.values_mut() { | ||
| track.producer.seek(sequence)?; |
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[P2] Avoid reusing segment numbers as per-cue text group numbers
Text appends and closes one group per sample (text.rs:81-87), while moq-hls::TrackState::push_segment calls this seek with the HLS segment sequence when it skips ahead. Joining segment 100 with three subtitle samples emits groups 100, 101, 102; if segment 101 is missed, joining segment 102 sets the next text group back to 102. The next write fails with moq_net::Error::Duplicate because that latest group is still cached, propagating out of the import and interrupting the muxed rendition. Give text collision-free sequence allocation across seeks, and add a regression with three subtitle samples, a skipped segment, and a subsequent sample.
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Fixed in 4ab433a. Both importers now seek a text track to max(sequence, latest + 1), so a segment seek never rewinds onto a cue group already written. Regressions: webvtt_seek_after_several_cues_does_not_reuse_a_group (fMP4) and subtitle_seek_after_several_cues_does_not_reuse_a_group (MKV) both failed with Duplicate before the fix.
(Written by Claude Opus 5.5)
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| return Err(Error::InvalidSubtitle("external subtitle data reference").into()); | ||
| } | ||
| let header = crate::container::text::header(&codec.config.config).map_err(Error::InvalidSubtitle)?; | ||
| let mut config = hang::catalog::TextConfig::new(hang::catalog::TextFormat::Vtt); |
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When vttC contains the valid Kind: captions metadata header, this constructor still leaves role at TextConfig::new's Subtitle default. The player uses that catalog field to distinguish captions from subtitles, so imported closed captions lose their accessibility semantics and appear as ordinary subtitles. ISOBMFF WebVTT explicitly maps Kind: captions and Kind: subtitles to the corresponding track kinds; parse this header when creating the config. W3C in-band track mapping
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Fixed in 4ab433a. The WebVTT Kind header now maps to TextRole::Caption / Subtitle (other values are kept as Unknown), for both fMP4 vttC and the Matroska CodecPrivate. Covered by webvtt_kind_and_lone_cr_terminators.
(Written by Claude Opus 5.5)
| config.label = codec | ||
| .label | ||
| .as_ref() | ||
| .map(|label| label.source_label.clone()) |
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For a normal wvtt entry containing the recommended vlab, this assigns its opaque source identifier, commonly a URI, to the human-readable TextConfig.label, so the caption picker displays that identifier instead of the HandlerBox name. The ISOBMFF text-track mapping sources the user-facing label from hdlr.name, while vlab exists to distinguish WebVTT sources; use the handler name here and preserve vlab separately only if the catalog gains such a field. W3C in-band track mapping
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Fixed in 4ab433a. The label now comes from the HandlerBox name only, per the in-band mapping; the test sets a distinct vlab to prove it is ignored.
(Written by Claude Opus 5.5)
| } | ||
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| pub(super) fn header(value: &str) -> Result<String, &'static str> { | ||
| let value = value.trim_start_matches('\u{feff}').replace("\r\n", "\n"); |
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When an MP4 vttC, Matroska CodecPrivate, or cue payload uses lone \r line endings, this normalization leaves them intact and the subsequent validation rejects the track as an invalid subtitle. Lone CR is a valid WebVTT line terminator alongside LF and CRLF, so standards-conforming inputs can fail import; normalize both CRLF and remaining CR to LF before validation. WebVTT syntax
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Fixed in 4ab433a. Headers and cue text normalize CRLF and lone CR to LF before validation. Covered by webvtt_kind_and_lone_cr_terminators.
(Written by Claude Opus 5.5)
WalkthroughThe fMP4 and Matroska importers now import supported subtitle tracks as timed WebVTT text. Shared utilities validate and serialize WebVTT cues and rebase inline timestamps. Tests cover track metadata, cue timing, supported formats, and invalid subtitle data. Format documentation now describes these imports, and the caption-import quest documentation now focuses on fMP4 TTML/IMSC. Priority: ⬇️ Low Merge Risk: 🔵 Low · up to Very short imported captions can render with zero duration. Correct the rendered-duration check; the remaining merge risk is bounded. Pre-merge checks |
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| Check name | Status | Explanation | Resolution |
|---|---|---|---|
| Docstring Coverage |  |
Docstring coverage is 57.89% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 57 functions across 9 files. (3 skipped: … | Write docstrings for the functions missing them to satisfy the coverage threshold. |
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Check name Status Explanation Linked Issues check Check skipped because no linked issues were found for this pull request. Out of Scope Changes check Check skipped because no linked issues were found for this pull request. Description check The description directly explains the subtitle-import problem, implementation scope, supported formats, error handling, tests, validation, and follow-up work. It matches the changeset. Title check The title clearly and concisely identifies the main change: importing timed SRT and WebVTT captions in the mux component.
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Docstring coverage is 57.89% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 57 functions across 9 files. (3 skipped: 3 unsupported.)
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Review comments at @doc/lib/rs/moq-mux.md:
- Line 16: Update the fMP4 / CMAF and Matroska / WebM table descriptions to
clarify that imported subtitle renditions preserve source language and label
metadata when present, rather than implying those fields are always available.
Review comments at @quest/m1/captions-ttml.md:
- Around line 5-7: Qualify the fMP4 stpp TTML/IMSC import goal as limited to
tracks within the supported profile in quest/m1/captions-ttml.md, and apply the
same profile-limit qualification to the index entry in quest/m1/README.md. Keep
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doc/lib/rs/moq-mux.mdquest/m1/README.mdquest/m1/captions-import.mdquest/m1/captions-ttml.mdrs/moq-mux/src/container/fmp4/import.rsrs/moq-mux/src/container/fmp4/import_test.rsrs/moq-mux/src/container/fmp4/mod.rsrs/moq-mux/src/container/fmp4/text.rsrs/moq-mux/src/container/mkv/import.rsrs/moq-mux/src/container/mkv/import_test.rsrs/moq-mux/src/container/mkv/mod.rsrs/moq-mux/src/container/mod.rsrs/moq-mux/src/container/text.rs
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- quest/m1/captions-import.md
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- A segment seek no longer rewinds a text track onto a cue group it already wrote; text spends a group per cue, so it continues from its own next group. - The WebVTT `Kind` header sets the catalog role (captions vs subtitles). - The fMP4 picker label comes from the HandlerBox name, not the `vlab` source identifier. - Lone CR line terminators are accepted, as WebVTT allows. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
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| if traf.trun.is_empty() { | ||
| return Err(Error::MissingTrun.into()); | ||
| } | ||
| self.last_decode_time = Some(dts); |
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When a wvtt fragment contains multiple samples, this records only its starting tfdt before dts advances through those samples. A subsequent fragment whose tfdt is later than that start but earlier than the accumulated end therefore passes the monotonicity check. For example, two one-second samples starting at 0 followed by a fragment at 1.5 seconds creates an overlapping decode timeline instead of rejecting the fragment. Record the exclusive ending DTS after processing the entries and compare the next fragment against that boundary.
AGENTS.md reference: rs/moq-mux/AGENTS.md:L9-L9
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Fixed in 7109b4e. Text tracks now record the exclusive decode end of each fragment and refuse a fragment that starts before it. Regression: webvtt_fragment_overlapping_previous_samples_is_refused (two 1 s samples at 0, then 1.5 s is refused and 2 s is accepted).
(Written by Claude Opus 5.5)
| let body = match header { | ||
| Some(_) => crate::container::text::rebase(body, start, end).map_err(Error::InvalidSubtitle)?, | ||
| None => body.into(), |
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For an S_TEXT/UTF8 track, a valid plain-text cue such as direction --> destination reaches text::append unchanged and is rejected by its WebVTT-specific body.contains("-->") check, so the entire Matroska import aborts even though this codec is defined as UTF-8 plain text in the Matroska codec mapping. Convert or escape SRT text into valid WebVTT cue text before applying the WebVTT restrictions.
AGENTS.md reference: AGENTS.md:L17-L17
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Fixed in 7109b4e. SRT bodies escape a literal --> as --> before WebVTT validation; subtitle_duration_survives_as_a_timed_cue now covers it.
(Written by Claude Opus 5.5)
…rrows - An fMP4 text fragment may not start inside the previous fragment's sample span; track the exclusive decode end rather than the start. - SRT is plain text, so a literal `-->` is escaped before it becomes WebVTT cue text instead of refusing the import. - Qualify the docs and TTML quest wording. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
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Reviewed 7109b4e against my previous review at 8bffb3c; the base is unchanged.
Fixed: the subtitle group-number collision. fMP4 and MKV now keep text seeks beyond already-written cue groups, with targeted regressions. The independently reported role, label, CR, overlapping-fragment and SRT-arrow cases also have corresponding fixes. No new actionable findings in the reviewed changes.
Direction: retain immediate cue-group closure and the bounded SRT/WebVTT scope. The small sequence fix addresses the collision without buffering sparse text or adding another abstraction.
Verification: static inspection of both follow-up commits and their regression tests, building on the prior full review. I did not run tests or independently verify the reported test results.
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| return Err(Error::MissingTrun.into()); | ||
| } | ||
| if self.discontinuity { | ||
| self.producer.discontinuity()?; |
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When a cue in the first epoch starts above the next epoch's restarted tfdt, this call does not make the text producer accept the reset: Producer::discontinuity retains its previous group-start floor, so the subsequent keyframe write fails with TimestampRewind. As a result, an fMP4 containing WebVTT aborts after an HLS EXT-X-DISCONTINUITY even though Import::discontinuity clears the decode-time check and the audio/video paths accept the same restart; map the new text epoch onto a non-rewinding timestamp before writing it.
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Declining in this PR. Text goes through container::Producer, whose documented contract is that a rewind below the previous group start is a restart, i.e. a new broadcast, and the repo has decided importers refuse a rewind (quest/m1/hop-aligned-import.md). An HLS discontinuity whose tfdt restarts below earlier cues therefore fails loud on the text track, the same as the other Producer-backed importers. Remapping only the text epoch would desync captions from the passthrough video. How a discontinuity rewind should be carried across all tracks is broader than caption import, so I am flagging it as a follow-up for the maintainer.
(Written by Claude Opus 5.5)
| let body = match header { | ||
| Some(_) => crate::container::text::rebase(body, start, end).map_err(Error::InvalidSubtitle)?, | ||
| // SRT is plain text, where a literal `-->` is valid but would end a WebVTT cue. | ||
| None => body.replace("-->", "-->"), |
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The follow-up only replaces literal -->; fresh evidence is that &, <, and > remain unescaped here. Because S_TEXT/UTF8 is plain text but WebVTT interprets character references and cue tags, inputs such as <b> or <i>literal</i> change meaning in conforming consumers instead of displaying verbatim. Escape these characters in the same order as the existing escape_cue conversion in rs/moq-gst/src/sink/pad.rs.
AGENTS.md reference: AGENTS.md:L27-L27
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Declining. Matroska S_TEXT/UTF8 carries SRT, whose de facto markup is <i>, <b> and <u>, the same tags WebVTT cue text supports. Escaping < would print those tags literally in the common case, and SRT players interpret them the same way. The literal --> is the one sequence that breaks WebVTT framing, so it is the one escaped. The moq-gst escape_cue handles a raw text/x-raw utf8 source, which carries no markup convention.
(Written by Claude Opus 5.5)
| MkvTrack { | ||
| default_duration, | ||
| kind, | ||
| text_header, |
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When an S_TEXT/WEBVTT BlockGroup carries CodecState, this cached header is never replaced because handle_block_group ignores that element. Matroska uses CodecState to deliver updated initialization, so valid streams that change their WebVTT initialization continue prepending stale STYLE, REGION, or other global blocks to every later cue; parse and validate the update before producing that BlockGroup. Matroska codec mappings
AGENTS.md reference: AGENTS.md:L17-L17
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Addressed in 2531156 by refusing instead: the Matroska WebVTT mapping defines only CodecPrivate and says nothing about CodecState updates, so a subtitle BlockGroup carrying CodecState now fails with InvalidSubtitle rather than silently keeping a stale header. Regression: subtitle_codec_state_is_refused.
(Written by Claude Opus 5.5)
A WebVTT header replaced mid-stream would otherwise leave stale global blocks on every later cue. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
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| b"payl" => &mut payload, | ||
| b"iden" => &mut id, | ||
| b"sttg" => &mut settings, | ||
| _ => return Err(Error::InvalidSubtitle("unsupported WebVTT cue box").into()), |
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When a cue contains an inline WebVTT timestamp, ISO/IEC 14496-30 §6.6 requires its vttc to carry a ctim CueTimeBox so the timestamp can be mapped from the original WebVTT timeline to the sample start. This wildcard rejects that standard box, so valid karaoke or delayed-part captions abort the entire fMP4 import. Parse ctim and use it to rebase the payload's inline timestamps. ISO/IEC 14496-30:2018
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Valid gap. Today a vttc carrying ctim is refused with InvalidSubtitle, so the import fails loud rather than mistiming the cue. Supporting it means rebasing inline timestamps from the ctim origin onto the sample start (and refusing inline timestamps that arrive without ctim). I have asked the maintainer whether to take that in this PR or as a follow-up alongside vtta.
(Written by Claude Opus 5.5)
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Deferred to quest/m1/wvtt-ctim-vtta.md (added in 00cdd4e) per the maintainer. Until then the import refuses these samples with InvalidSubtitle rather than mistiming them.
(Written by Claude Opus 5.5)
| ) | ||
| .map_err(Error::InvalidSubtitle)?; | ||
| } | ||
| _ => return Err(Error::InvalidSubtitle("unsupported WebVTT sample box").into()), |
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When the source WebVTT contains a NOTE or other additional text between cues, ISO/IEC 14496-30 §6.6 stores it as a vtta box interleaved with the cue boxes. This wildcard therefore rejects a conforming wvtt sample merely because it preserves comments, aborting import instead of reconstructing the text track. Accept and serialize vtta in its sample order. ISO/IEC 14496-30:2018
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Valid gap. Today a sample with vtta is refused with InvalidSubtitle (fail loud, nothing mistimed). Serializing it in sample order, validated like the Matroska comment path, is straightforward; I have asked the maintainer whether to take it in this PR or as a follow-up alongside ctim.
(Written by Claude Opus 5.5)
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Deferred to quest/m1/wvtt-ctim-vtta.md (added in 00cdd4e) per the maintainer. Until then the import refuses these samples with InvalidSubtitle rather than mistiming them.
(Written by Claude Opus 5.5)
…9941fbc4804 # Conflicts: # quest/m1/README.md # rs/moq-mux/src/container/fmp4/import_test.rs
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| b"payl" => &mut payload, | ||
| b"iden" => &mut id, | ||
| b"sttg" => &mut settings, | ||
| _ => return Err(Error::InvalidSubtitle("unsupported WebVTT cue box").into()), |
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When an MP4 WebVTT source uses the optional vlab/vsid pair, this wildcard rejects the standard vsid CueSourceIDBox and aborts the entire fMP4 import. vsid is a permitted vttc child used with the source label to identify a cue, as shown in the W3C in-band WebVTT box layout; accept it rather than treating an otherwise valid cue as malformed.
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- Around line 13-22: Update append to validate cue duration after millisecond
truncation, rejecting cues when the rendered end is less than or equal to the
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doc/lib/rs/moq-mux.mdquest/m1/README.mdquest/m1/captions-ttml.mdrs/moq-mux/src/container/fmp4/import.rsrs/moq-mux/src/container/fmp4/import_test.rsrs/moq-mux/src/container/fmp4/text.rsrs/moq-mux/src/container/mkv/import.rsrs/moq-mux/src/container/mkv/import_test.rsrs/moq-mux/src/container/text.rs
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| pub(super) fn stamp(value: Timestamp) -> String { | ||
| let millis = value.as_micros() / 1000; | ||
| format!( | ||
| "{:02}:{:02}:{:02}.{:03}", | ||
| millis / 3_600_000, | ||
| millis / 60_000 % 60, | ||
| millis / 1000 % 60, | ||
| millis % 1000 | ||
| ) | ||
| } |
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🎯 Functional Correctness | 🟡 Minor | ⚡ Quick win
stamp truncates sub-millisecond precision. The cue start can end up before the frame timestamp.
stamp floors the value to whole milliseconds. In the fMP4 path, the timestamp is a native-timescale value shifted onto the catalog clock. That value may not be a whole number of milliseconds. For example, at timescale 90000, the rendered cue time can be up to 1 ms earlier than the frame timestamp. The cue end can also round to the same millisecond as the start. In that case a short cue renders with zero length, although append accepted it because the full-precision end > start check passed. Do the positive-duration check on the rendered (millisecond) values.
Proposed fix
pub(super) fn append(output: &mut String, cue: Cue<'_>) -> Result<(), &'static str> {
- if cue.end <= cue.start {
+ if cue.end.as_micros() / 1000 <= cue.start.as_micros() / 1000 {
return Err("cue duration must be positive");
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Review comment at @rs/moq-mux/src/container/text.rs around lines 13 - 22:
Update append to validate cue duration after millisecond truncation, rejecting
cues when the rendered end is less than or equal to the rendered start; keep
stamp’s formatting behavior unchanged.
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Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
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| body: payload | ||
| .as_deref() | ||
| .ok_or(Error::InvalidSubtitle("missing WebVTT cue payload"))?, |
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When a payl contains an inline timestamp but its vttc omits ctim, this path passes the timestamp through unchanged, so a cue in a sample starting at 60 seconds can retain <00:01.000> and be rendered or discarded at the wrong time. Fresh evidence in the newly added quest/m1/wvtt-ctim-vtta.md:13-17 explicitly requires refusing this case because the timestamp origin is unknown; detect inline timestamps here until ctim rebasing is implemented.
AGENTS.md reference: AGENTS.md:L17-L17
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Fixed in 26a2fcf. An fMP4 cue payload containing an inline timestamp is now refused with InvalidSubtitle until ctim rebasing lands (quest/m1/wvtt-ctim-vtta.md). Covered in malformed_webvtt_samples_are_refused.
(Written by Claude Opus 5.5)
| output.push_str(&stamp(cue.start)); | ||
| output.push_str(" --> "); | ||
| output.push_str(&stamp(cue.end)); |
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When a valid source cue is shorter than 1 ms, or its start and end otherwise fall within the same millisecond, stamp floors both endpoints and these lines emit identical WebVTT timestamps despite the earlier positive-duration check. Consumers can consequently drop a valid cue as zero-duration; validate the serialized millisecond endpoints or round them so the emitted end remains strictly later than the start.
AGENTS.md reference: AGENTS.md:L17-L17
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Fixed in 26a2fcf. The cue end now rounds up to the next millisecond (and at least one past the start), so a sub-millisecond cue keeps a positive duration. Regression: webvtt_sub_millisecond_cue_stays_positive.
(Written by Claude Opus 5.5)
…cues positive - fMP4 cue payloads with inline timestamps are refused until `ctim` support lands, since their origin is the unshifted source timeline. - A cue's end rounds up to the next millisecond, so a cue shorter than a millisecond does not serialize with a zero duration. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
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| let duration = track | ||
| .default_duration | ||
| .ok_or(Error::InvalidSubtitle("missing cue duration"))?; |
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When a subtitle omits both BlockDuration and DefaultDuration, this aborts on the first cue even if a following block supplies the implicit end. Matroska defines the missing duration as the difference to the next block in display order, so valid SRT or WebVTT tracks using that representation cannot be imported. Buffer the pending cue and backfill its duration when the next cue arrives, only rejecting at finish if no end can be determined. Matroska element specification
AGENTS.md reference: rs/moq-mux/AGENTS.md:L6-L6
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Out of scope for this PR, so tracking as a follow-up. A subtitle with neither BlockDuration nor DefaultDuration is refused loudly (missing cue duration), never mistimed. Backfilling from the next block means holding each cue until its successor arrives, which delays live captions by one cue; that trade-off belongs in its own change.
(Written by Claude Opus 5.5)
| Some(ticks) => ticks | ||
| .checked_mul(self.timestamp_scale_ns) | ||
| .ok_or(Error::TimestampOverflow)?, |
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For a valid Matroska v1-v3 or WebM subtitle track with TrackTimestampScale other than 1.0, BlockDuration is expressed in track ticks, but this calculation applies only the segment TimestampScale. The emitted frame duration and serialized WebVTT cue end are therefore wrong, for example doubled when the track scale is 2.0. Parse and retain the per-track scale and include it with the required nearest-nanosecond rounding. Matroska timestamp rules
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Pre-existing and out of scope: the MKV importer ignores the deprecated TrackTimestampScale for every track, including audio and video block timestamps, not just subtitle durations. Fixing it belongs in a change that covers all tracks, so I am listing it as a follow-up.
(Written by Claude Opus 5.5)
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Problem
File imports discarded subtitle tracks or treated timed SRT cues as roll-up text. The latter loses a cue's end time, leaving captions visible through later gaps. MP4 WebVTT samples were skipped even when their encoding was supported by the text catalog.
Approach
Import Matroska
S_TEXT/UTF8andS_TEXT/WEBVTT, and MP4wvtt, as self-contained WebVTT frames. Preserve source language and label, explicit cue start/end times, identifiers and settings. Matroska retains WebVTT header blocks and comments and restores inline timestamps relative to the source Block. MP4 resolves sample timing fromtfdt, composition offsets and sample/default durations, unboxesvttc/vtte, and closes each text group immediately. Text tracks participate in finish, abort, seek and declared discontinuity handling.Malformed supported text, missing durations, invalid UTF-8 and malformed cue boxes are refused with typed errors. Unknown subtitle encodings, including XML (TTML/IMSC), skip only their own track. Remove the unregistered Matroska
S_TEXT/TTMLclaim; the supported identifiers follow the Matroska subtitle mapping.After merging main, Matroska subtitle blocks take their own branch in
handle_blocknext to the new laced-block support (#5198): they refuse lacing and fall back to the track's positiveDefaultDurationwhen a BlockGroup has noBlockDuration.Review fixes: a segment seek never rewinds a text track onto a cue group it already wrote (text spends one group per cue); the WebVTT
Kindheader sets the catalog role; the fMP4 label comes from the HandlerBox name rather than the opaquevlabsource identifier; lone CR line terminators are accepted; an fMP4 text fragment may not start inside the previous fragment's sample span; a literal-->in SRT text is escaped as-->instead of refusing the import; a subtitle BlockGroup carrying CodecState is refused rather than silently keeping a stale WebVTT header; fMP4 inline cue timestamps are refused untilctimsupport lands; a sub-millisecond cue keeps a positive serialized duration.Declined review suggestions (replied inline): remapping text timestamps across an HLS discontinuity that rewinds
tfdt(importers refuse a rewind, perquest/m1/hop-aligned-import.md), and escaping SRT</&(SRT shares WebVTT's<i>/<b>/<u>markup). fMP4ctimandvttaboxes are refused for now and deferred toquest/m1/wvtt-ctim-vtta.md.Regressions that failed before their fixes: a 250 ms SRT cue ending at 1.5 s was emitted as untimed
utf8, and a valid MP4wvtttrack produced no text rendition. Both seek regressions (fMP4 and MKV) failed withDuplicatebefore the fix. Additional coverage checks source metadata, inline timestamp bounds, empty and multiple cues, default duration and malformed input.Decisions
quest/m1/captions-ttml.md, with the finishedcaptions-importquest removed ✅ / keep it insidecaptions-import. (captions-msflanded in fix(msf): preserve caption and subtitle tracks across catalogs #5226 and is removed too.)vtt, preserving explicit ends and overlapping cues ✅ / emit empty roll-uputf8cues at each end, which can clear an overlapping caption.Impact
Public API: additive
InvalidSubtitle(&'static str)variants on the#[non_exhaustive]moq_mux::container::mkv::Errorandmoq_mux::container::fmp4::Errorenums; no new exported reader or producer shape. Imported timed SRT is cataloged asvtt. Existing MoQ wire, Hang catalog and WebVTT payload formats are unchanged. JS has no corresponding file importer to synchronize. No version bumps.Validation: Nix
just checkpassed on the first merged head (2,914 Rust tests; one unrelatedmoq-ffishutdown test flaked once and passed on rerun). CI passed on the second merged head. After the review fixes,moq-muxclippy, rustfmt and tests (1,266) pass, plusquest check. Tests are wired into the existing Rust CI suite.Follow-ups
ctimandvtta(quest/m1/wvtt-ctim-vtta.md): rebasectiminline timestamps and serializevttainstead of refusing the sample.quest/m1/captions-ttml.md): a bounded TTML/IMSC profile with namespace validation and timing conversion across a shifted clock. Do not advertise arbitrary XML as TTML.(Written by Claude Opus 5.5)