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Add support for Matroska zlib compressed subtitle tracks #3211

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6874267
Add zlib compressed text tracks support for Matroska files
inorichi May 8, 2026
cda36af
Add tests that validate zlib compressed text tracks support for Matro…
inorichi May 8, 2026
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3 changes: 3 additions & 0 deletions libraries/exoplayer/src/test/java/androidx/media3/exoplayer/e2etest/MkvPlaybackTest.java
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Original file line number Diff line number Diff line change
Expand Up @@ -47,13 +47,16 @@ public static ImmutableList<String> mediaSamples() {
"sample_with_htc_rotation_track_name.mkv",
"sample_with_pgs_subtitles.mkv",
"sample_with_ssa_subtitles.mkv",
"sample_with_zlib_ass_subtitles.mkv",
"sample_with_null_terminated_ssa_subtitles.mkv",
"sample_with_overlapping_ssa_subtitles.mkv",
"sample_with_srt.mkv",
"sample_with_null_terminated_srt.mkv",
"sample_with_zlib_null_terminated_srt.mkv",
"sample_with_overlapping_srt.mkv",
"sample_with_vtt_subtitles.mkv",
"sample_with_null_terminated_vtt_subtitles.mkv",
"sample_with_zlib_null_terminated_vtt_subtitles.mkv",
"sample_with_vobsub.mkv");
}

Expand Down
42 changes: 36 additions & 6 deletions libraries/extractor/src/main/java/androidx/media3/extractor/mkv/MatroskaExtractor.java
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Original file line number Diff line number Diff line change
Expand Up @@ -84,6 +84,7 @@
import java.util.Map;
import java.util.Objects;
import java.util.UUID;
import java.util.zip.Inflater;
import org.checkerframework.checker.nullness.qual.EnsuresNonNull;
import org.checkerframework.checker.nullness.qual.MonotonicNonNull;
import org.checkerframework.checker.nullness.qual.RequiresNonNull;
Expand Down Expand Up @@ -463,6 +464,7 @@ public static ExtractorsFactory newFactory(SubtitleParser.Factory subtitleParser
private final ParsableByteArray seekEntryIdBytes;
private final ParsableByteArray sampleStrippedBytes;
private final ParsableByteArray subtitleSample;
private final ParsableByteArray inflatedSubtitleSample;
private final ParsableByteArray encryptionInitializationVector;
private final ParsableByteArray encryptionSubsampleData;
private final ParsableByteArray supplementalData;
Expand All @@ -476,6 +478,9 @@ public static ExtractorsFactory newFactory(SubtitleParser.Factory subtitleParser
private boolean isWebm;
private boolean pendingEndTracks;

// Inflater for zlib compressed tracks
@Nullable private Inflater inflater;

// The chapter corresponding to the current EditionEntry element, or null.
@Nullable private ChapterEntry currentChapter;

Expand Down Expand Up @@ -591,6 +596,7 @@ public MatroskaExtractor(SubtitleParser.Factory subtitleParserFactory, @Flags in
nalLength = new ParsableByteArray(4);
sampleStrippedBytes = new ParsableByteArray();
subtitleSample = new ParsableByteArray();
inflatedSubtitleSample = new ParsableByteArray();
encryptionInitializationVector = new ParsableByteArray(ENCRYPTION_IV_SIZE);
encryptionSubsampleData = new ParsableByteArray();
supplementalData = new ParsableByteArray();
Expand Down Expand Up @@ -863,6 +869,9 @@ protected void startMasterElement(int id, long contentPosition, long contentSize
case ID_CONTENT_ENCODING:
// TODO: check and fail if more than one content encoding is present.
break;
case ID_CONTENT_COMPRESSION:
getCurrentTrack(id).hasContentCompression = true;
break;
case ID_CONTENT_ENCRYPTION:
getCurrentTrack(id).hasContentEncryption = true;
break;
Expand Down Expand Up @@ -1256,8 +1265,8 @@ protected void integerElement(int id, long value) throws ParserException {
}
break;
case ID_CONTENT_COMPRESSION_ALGORITHM:
// This extractor only supports header stripping.
if (value != 3) {
// This extractor only supports header stripping and zlib compressed text tracks.
if (value != 3 && (value != 0 || getCurrentTrack(id).type != C.TRACK_TYPE_TEXT)) {
throw ParserException.createForMalformedContainer(
"ContentCompAlgo " + value + " not supported", /* cause= */ null);
}
Expand Down Expand Up @@ -1834,13 +1843,13 @@ private void readScratch(ExtractorInput input, int requiredLength) throws IOExce
private int writeSampleData(ExtractorInput input, Track track, int size, boolean isBlockGroup)
throws IOException {
if (CODEC_ID_SUBRIP.equals(track.codecId)) {
writeSubtitleSampleData(input, SUBRIP_PREFIX, size);
writeSubtitleSampleData(input, SUBRIP_PREFIX, size, track.hasContentCompression);
return finishWriteSampleData();
} else if (CODEC_ID_ASS.equals(track.codecId) || CODEC_ID_SSA.equals(track.codecId)) {
writeSubtitleSampleData(input, SSA_PREFIX, size);
writeSubtitleSampleData(input, SSA_PREFIX, size, track.hasContentCompression);
return finishWriteSampleData();
} else if (CODEC_ID_VTT.equals(track.codecId)) {
writeSubtitleSampleData(input, VTT_PREFIX, size);
writeSubtitleSampleData(input, VTT_PREFIX, size, track.hasContentCompression);
return finishWriteSampleData();
}

Expand Down Expand Up @@ -2055,7 +2064,8 @@ private void resetWriteSampleData() {
sampleStrippedBytes.reset(/* limit= */ 0);
}

private void writeSubtitleSampleData(ExtractorInput input, byte[] samplePrefix, int size)
private void writeSubtitleSampleData(
ExtractorInput input, byte[] samplePrefix, int size, boolean hasContentCompression)
throws IOException {
int sizeWithPrefix = samplePrefix.length + size;
if (subtitleSample.capacity() < sizeWithPrefix) {
Expand All @@ -2066,6 +2076,25 @@ private void writeSubtitleSampleData(ExtractorInput input, byte[] samplePrefix,
System.arraycopy(samplePrefix, 0, subtitleSample.getData(), 0, samplePrefix.length);
}
input.readFully(subtitleSample.getData(), samplePrefix.length, size);

if (hasContentCompression) {
if (inflater == null) {
inflater = new Inflater();
}
subtitleSample.setPosition(samplePrefix.length);
subtitleSample.setLimit(sizeWithPrefix);
if (Util.maybeInflate(subtitleSample, inflatedSubtitleSample, inflater)) {
sizeWithPrefix = samplePrefix.length + inflatedSubtitleSample.limit();
subtitleSample.ensureCapacity(sizeWithPrefix);
System.arraycopy(
inflatedSubtitleSample.getData(),
0,
subtitleSample.getData(),
samplePrefix.length,
inflatedSubtitleSample.limit());
}
}

subtitleSample.setPosition(0);
subtitleSample.setLimit(sizeWithPrefix);
// Defer writing the data to the track output. We need to modify the sample data by setting
Expand Down Expand Up @@ -2359,6 +2388,7 @@ protected static final class Track {
public int defaultSampleDurationNs;
public int maxBlockAdditionId;
private int blockAddIdType;
public boolean hasContentCompression;
public boolean hasContentEncryption;
public byte @MonotonicNonNull [] sampleStrippedBytes;
public TrackOutput.@MonotonicNonNull CryptoData cryptoData;
Expand Down
40 changes: 40 additions & 0 deletions libraries/extractor/src/test/java/androidx/media3/extractor/mkv/MatroskaExtractorTest.java
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Expand Up @@ -92,6 +92,19 @@ public void mkvSample_withNullTerminatedSubripSubtitles() throws Exception {
simulationConfig);
}

@Test
// TODO: b/507050745 - Suppressed due to failure with subtitlesParsedDuringExtraction=true on SDK
// >= 32.
@Config(maxSdk = 31)
public void mkvSample_withZlibNullTerminatedSubripSubtitles() throws Exception {
ExtractorAsserts.assertBehavior(
getExtractorFactory(subtitlesParsedDuringExtraction),
"media/mkv/sample_with_zlib_null_terminated_srt.mkv",
getAssertionConfigWithPrefix(
"media/mkv/sample_with_zlib_null_terminated_srt.mkv", subtitlesParsedDuringExtraction),
simulationConfig);
}

@Test
// TODO: b/507050745 - Suppressed due to failure with subtitlesParsedDuringExtraction=true on SDK
// >= 32.
Expand Down Expand Up @@ -143,6 +156,19 @@ public void mkvSample_withAssSubtitles() throws Exception {
simulationConfig);
}

@Test
// TODO: b/507050745 - Suppressed due to failure with subtitlesParsedDuringExtraction=true on SDK
// >= 32.
@Config(maxSdk = 31)
public void mkvSample_withZlibAssSubtitles() throws Exception {
ExtractorAsserts.assertBehavior(
getExtractorFactory(subtitlesParsedDuringExtraction),
"media/mkv/sample_with_zlib_ass_subtitles.mkv",
getAssertionConfigWithPrefix(
"media/mkv/sample_with_zlib_ass_subtitles.mkv", subtitlesParsedDuringExtraction),
simulationConfig);
}

// https://github.com/google/ExoPlayer/pull/8265
@Test
// TODO: b/507050745 - Suppressed due to failure with subtitlesParsedDuringExtraction=true on SDK
Expand Down Expand Up @@ -198,6 +224,20 @@ public void mkvSample_withNullTerminatedVttSubtitles() throws Exception {
simulationConfig);
}

@Test
// TODO: b/507050745 - Suppressed due to failure with subtitlesParsedDuringExtraction=true on SDK
// >= 32.
@Config(maxSdk = 31)
public void mkvSample_withZlibNullTerminatedVttSubtitles() throws Exception {
ExtractorAsserts.assertBehavior(
getExtractorFactory(subtitlesParsedDuringExtraction),
"media/mkv/sample_with_zlib_null_terminated_vtt_subtitles.mkv",
getAssertionConfigWithPrefix(
"media/mkv/sample_with_zlib_null_terminated_vtt_subtitles.mkv",
subtitlesParsedDuringExtraction),
simulationConfig);
}

@Test
public void mkvSample_withVorbisAudio() throws Exception {
ExtractorAsserts.assertBehavior(
Expand Down
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