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LibVideo: Calculate Block timestamps for Matroska according to spec

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Tracks have a timestamp scale value that should be present which scales
each block's timestamp offset to allow video to be synced with audio.
They should also contain a CodecDelay element and may also contain a
TrackOffset that offsets the block timestamps.
This commit is contained in:
Zaggy1024 2022-11-12 13:09:07 -06:00 • committed by Andreas Kling
parent ceb7632862
commit a58bf7c3d1
4 changed files with 49 additions and 12 deletions
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40
Userland/Libraries/LibVideo/Containers/Matroska/Reader.cpp
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@ -44,6 +44,8 @@ constexpr u32 TRACK_UID_ID = 0x73C5;
constexpr u32 TRACK_TYPE_ID = 0x83;
constexpr u32 TRACK_LANGUAGE_ID = 0x22B59C;
constexpr u32 TRACK_CODEC_ID = 0x86;
constexpr u32 TRACK_TIMESTAMP_SCALE_ID = 0x23314F;
constexpr u32 TRACK_OFFSET_ID = 0x537F;
constexpr u32 TRACK_VIDEO_ID = 0xE0;
constexpr u32 TRACK_AUDIO_ID = 0xE1;
@ -416,6 +418,14 @@ static DecoderErrorOr<TrackEntry> parse_track_entry(Streamer& streamer)
track_entry.set_codec_id(TRY_READ(streamer.read_string()));
dbgln_if(MATROSKA_TRACE_DEBUG, "Read Track's CodecID attribute: {}", track_entry.codec_id());
break;
case TRACK_TIMESTAMP_SCALE_ID:
track_entry.set_timestamp_scale(TRY_READ(streamer.read_float()));
dbgln_if(MATROSKA_TRACE_DEBUG, "Read Track's TrackTimestampScale attribute: {}", track_entry.timestamp_scale());
break;
case TRACK_OFFSET_ID:
track_entry.set_timestamp_offset(TRY_READ(streamer.read_variable_size_signed_integer()));
dbgln_if(MATROSKA_TRACE_DEBUG, "Read Track's TrackOffset attribute: {}", track_entry.timestamp_offset());
break;
case TRACK_VIDEO_ID:
track_entry.set_video_track(TRY(parse_video_track_information(streamer)));
break;
@ -520,7 +530,7 @@ static DecoderErrorOr<Cluster> parse_cluster(Streamer& streamer, u64 timestamp_s
return cluster;
}
static DecoderErrorOr<Block> parse_simple_block(Streamer& streamer, Time cluster_timestamp, u64 timestamp_scale)
static DecoderErrorOr<Block> parse_simple_block(Streamer& streamer, Time cluster_timestamp, u64 segment_timestamp_scale, TrackEntry track)
{
Block block;
@ -529,7 +539,23 @@ static DecoderErrorOr<Block> parse_simple_block(Streamer& streamer, Time cluster
auto position_before_track_number = streamer.position();
block.set_track_number(TRY_READ(streamer.read_variable_size_integer()));
block.set_timestamp(cluster_timestamp + Time::from_nanoseconds(TRY_READ(streamer.read_i16()) * timestamp_scale));
// https://www.matroska.org/technical/notes.html
// Block Timestamps:
// The Block Element and SimpleBlock Element store their timestamps as signed integers,
// relative to the Cluster\Timestamp value of the Cluster they are stored in. To get the
// timestamp of a Block or SimpleBlock in nanoseconds you have to use the following formula:
// `( Cluster\Timestamp + ( block timestamp * TrackTimestampScale ) ) * TimestampScale`
//
// When a CodecDelay Element is set, its value MUST be substracted from each Block timestamp
// of that track. To get the timestamp in nanoseconds of the first frame in a Block or
// SimpleBlock, the formula becomes:
// `( ( Cluster\Timestamp + ( block timestamp * TrackTimestampScale ) ) * TimestampScale ) - CodecDelay`
Time timestamp_offset = Time::from_nanoseconds(static_cast<i64>(static_cast<double>(TRY_READ(streamer.read_i16()) * segment_timestamp_scale) * track.timestamp_scale()));
timestamp_offset -= Time::from_nanoseconds(static_cast<i64>(track.codec_delay()));
// This is only mentioned in the elements specification under TrackOffset.
// https://www.matroska.org/technical/elements.html
timestamp_offset += Time::from_nanoseconds(static_cast<i64>(track.timestamp_offset()));
block.set_timestamp(cluster_timestamp + timestamp_offset);
auto flags = TRY_READ(streamer.read_octet());
block.set_only_keyframes((flags & (1u << 7u)) != 0);
@ -596,7 +622,7 @@ DecoderErrorOr<SampleIterator> Reader::create_sample_iterator(u64 track_number)
auto position = optional_position.value() - get_element_id_size(CLUSTER_ELEMENT_ID) - m_segment_contents_position;
dbgln_if(MATROSKA_DEBUG, "Creating sample iterator starting at {} relative to segment at {}", position, m_segment_contents_position);
return SampleIterator(this->m_mapped_file, segment_view, track_number, position, TRY(segment_information()).timestamp_scale());
return SampleIterator(this->m_mapped_file, segment_view, TRY(track_for_track_number(track_number)), TRY(segment_information()).timestamp_scale(), position);
}
static DecoderErrorOr<bool> find_keyframe_before_timestamp(SampleIterator& iterator, Time const& timestamp)
@ -643,7 +669,7 @@ DecoderErrorOr<void> Reader::seek_to_random_access_point(SampleIterator& iterato
// FIXME: This could cache the keyframes it finds. Is it worth doing? Probably not, most files will have Cues :^)
if (timestamp < iterator.last_timestamp() || iterator.last_timestamp().is_negative()) {
// If the timestamp is before the iterator's current position, then we need to start from the beginning of the Segment.
iterator = TRY(create_sample_iterator(iterator.m_track_id));
iterator = TRY(create_sample_iterator(iterator.m_track.track_number()));
if (!TRY(find_keyframe_before_timestamp(iterator, timestamp)))
return DecoderError::corrupted("No random access points found"sv);
@ -678,11 +704,11 @@ DecoderErrorOr<Block> SampleIterator::next_block()
if (element_id == CLUSTER_ELEMENT_ID) {
dbgln_if(MATROSKA_DEBUG, " Iterator is parsing new cluster.");
m_current_cluster = TRY(parse_cluster(streamer, m_timestamp_scale));
m_current_cluster = TRY(parse_cluster(streamer, m_segment_timestamp_scale));
} else if (element_id == SIMPLE_BLOCK_ID) {
dbgln_if(MATROSKA_TRACE_DEBUG, " Iterator is parsing new block.");
auto candidate_block = TRY(parse_simple_block(streamer, m_current_cluster->timestamp(), m_timestamp_scale));
if (candidate_block.track_number() == m_track_id)
auto candidate_block = TRY(parse_simple_block(streamer, m_current_cluster->timestamp(), m_segment_timestamp_scale, m_track));
if (candidate_block.track_number() == m_track.track_number())
block = move(candidate_block);
} else {
dbgln_if(MATROSKA_TRACE_DEBUG, " Iterator is skipping unknown element with ID {:#010x}.", element_id);