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Clarify SSA and SubRip docs in MatroskaExtractor

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The handling of times wasn't really clear to me, hopefully this more
exhaustive documentation helps a bit.

Also assert the end timecode is the 'correct' length for the format.

PiperOrigin-RevId: 279922369
This commit is contained in:
ibaker 2019-11-12 10:54:49 +00:00 committed by Oliver Woodman
parent 2faef48302
commit b43db3bceb
1 changed files with 50 additions and 19 deletions
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69
library/core/src/main/java/com/google/android/exoplayer2/extractor/mkv/MatroskaExtractor.java
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@ -236,14 +236,21 @@ public class MatroskaExtractor implements Extractor {
private static final int FOURCC_COMPRESSION_VC1 = 0x31435657;
/**
* A template for the prefix that must be added to each subrip sample. The 12 byte end timecode
* starting at {@link #SUBRIP_PREFIX_END_TIMECODE_OFFSET} is set to a dummy value, and must be
* replaced with the duration of the subtitle.
* <p>
* Equivalent to the UTF-8 string: "1\n00:00:00,000 --> 00:00:00,000\n".
* A template for the prefix that must be added to each subrip sample.
*
* <p>The display time of each subtitle is passed as {@code timeUs} to {@link
* TrackOutput#sampleMetadata}. The start and end timecodes in this template are relative to
* {@code timeUs}. Hence the start timecode is always zero. The 12 byte end timecode starting at
* {@link #SUBRIP_PREFIX_END_TIMECODE_OFFSET} is set to a dummy value, and must be replaced with
* the duration of the subtitle.
*
* <p>Equivalent to the UTF-8 string: "1\n00:00:00,000 --> 00:00:00,000\n".
*/
private static final byte[] SUBRIP_PREFIX = new byte[] {49, 10, 48, 48, 58, 48, 48, 58, 48, 48,
44, 48, 48, 48, 32, 45, 45, 62, 32, 48, 48, 58, 48, 48, 58, 48, 48, 44, 48, 48, 48, 10};
private static final byte[] SUBRIP_PREFIX =
new byte[] {
49, 10, 48, 48, 58, 48, 48, 58, 48, 48, 44, 48, 48, 48, 32, 45, 45, 62, 32, 48, 48, 58, 48,
48, 58, 48, 48, 44, 48, 48, 48, 10
};
/**
* The byte offset of the end timecode in {@link #SUBRIP_PREFIX}.
*/
@ -272,14 +279,21 @@ public class MatroskaExtractor implements Extractor {
private static final byte[] SSA_DIALOGUE_FORMAT = Util.getUtf8Bytes("Format: Start, End, "
+ "ReadOrder, Layer, Style, Name, MarginL, MarginR, MarginV, Effect, Text");
/**
* A template for the prefix that must be added to each SSA sample. The 10 byte end timecode
* starting at {@link #SSA_PREFIX_END_TIMECODE_OFFSET} is set to a dummy value, and must be
* replaced with the duration of the subtitle.
* <p>
* Equivalent to the UTF-8 string: "Dialogue: 0:00:00:00,0:00:00:00,".
* A template for the prefix that must be added to each SSA sample.
*
* <p>The display time of each subtitle is passed as {@code timeUs} to {@link
* TrackOutput#sampleMetadata}. The start and end timecodes in this template are relative to
* {@code timeUs}. Hence the start timecode is always zero. The 12 byte end timecode starting at
* {@link #SUBRIP_PREFIX_END_TIMECODE_OFFSET} is set to a dummy value, and must be replaced with
* the duration of the subtitle.
*
* <p>Equivalent to the UTF-8 string: "Dialogue: 0:00:00:00,0:00:00:00,".
*/
private static final byte[] SSA_PREFIX = new byte[] {68, 105, 97, 108, 111, 103, 117, 101, 58, 32,
48, 58, 48, 48, 58, 48, 48, 58, 48, 48, 44, 48, 58, 48, 48, 58, 48, 48, 58, 48, 48, 44};
private static final byte[] SSA_PREFIX =
new byte[] {
68, 105, 97, 108, 111, 103, 117, 101, 58, 32, 48, 58, 48, 48, 58, 48, 48, 58, 48, 48, 44,
48, 58, 48, 48, 58, 48, 48, 58, 48, 48, 44
};
/**
* The byte offset of the end timecode in {@link #SSA_PREFIX}.
*/
@ -1468,16 +1482,32 @@ public class MatroskaExtractor implements Extractor {
private void commitSubtitleSample(Track track, String timecodeFormat, int endTimecodeOffset,
long lastTimecodeValueScalingFactor, byte[] emptyTimecode) {
setSampleDuration(subtitleSample.data, blockDurationUs, timecodeFormat, endTimecodeOffset,
lastTimecodeValueScalingFactor, emptyTimecode);
setSubtitleSampleDuration(
subtitleSample.data,
blockDurationUs,
timecodeFormat,
endTimecodeOffset,
lastTimecodeValueScalingFactor,
emptyTimecode);
// Note: If we ever want to support DRM protected subtitles then we'll need to output the
// appropriate encryption data here.
track.output.sampleData(subtitleSample, subtitleSample.limit());
sampleBytesWritten += subtitleSample.limit();
}
private static void setSampleDuration(byte[] subripSampleData, long durationUs,
String timecodeFormat, int endTimecodeOffset, long lastTimecodeValueScalingFactor,
/**
* Formats {@code durationUs} using {@code timecodeFormat}, and sets it as the end timecode in
* {@code subtitleSampleData}.
*
* <p>See documentation on {@link #SSA_DIALOGUE_FORMAT} and {@link #SUBRIP_PREFIX} for why we use
* the duration as the end timecode.
*/
private static void setSubtitleSampleDuration(
byte[] subtitleSampleData,
long durationUs,
String timecodeFormat,
int endTimecodeOffset,
long lastTimecodeValueScalingFactor,
byte[] emptyTimecode) {
byte[] timeCodeData;
if (durationUs == C.TIME_UNSET) {
@ -1493,7 +1523,8 @@ public class MatroskaExtractor implements Extractor {
timeCodeData = Util.getUtf8Bytes(String.format(Locale.US, timecodeFormat, hours, minutes,
seconds, lastValue));
}
System.arraycopy(timeCodeData, 0, subripSampleData, endTimecodeOffset, emptyTimecode.length);
Assertions.checkState(timeCodeData.length == emptyTimecode.length);
System.arraycopy(timeCodeData, 0, subtitleSampleData, endTimecodeOffset, timeCodeData.length);
}
/**