MPEG4Writer: Fix the bug in MPEG4 metadata track.
HACK to make the metadata track compliant with the iso standard.
Metadata track is added from API 26 and the original implementation does not
fully followed the TextMetaDataSampleEntry specified in ISO/IEC 14496-12-2015
in that only the mime_format is written out. content_encoding and
data_reference_index have not been written out. This leads to the failure
when some MP4 parser tries to parse the metadata track according to the
standard. The hack here will make the metadata track compliant with the
standard while still maintaining backwards compatibility. This would enable
Android versions before API 29 still read out the standard compliant Metadata track
generated with Android API 29 and upward. The trick is based on the fact that
the Metadata track must start with prefix “application/” and those missing
fields are not used in Android's Metadata track. By writting out the
mime_format twice, the first mime_format will be used to fill out the missing
reserved, data_reference_index and content encoding fields. On the parser side, the
extracter before API 29 will read out the first mime_format correctly and
drop the second mime_format. The extractor from API 29 will check if the
reserved, data_reference_index and content encoding are filled with
“application” to detect if this is a standard compliant metadata track and
read out the data accordingly.
Bug: 117950811
Test: CTS test.
Change-Id: I405216c08e2f8cb28c985d9460d8cf2b16ba1931
diff --git a/media/libstagefright/MPEG4Writer.cpp b/media/libstagefright/MPEG4Writer.cpp
index 6ff3d78..a48466a 100644
--- a/media/libstagefright/MPEG4Writer.cpp
+++ b/media/libstagefright/MPEG4Writer.cpp
@@ -3674,6 +3674,29 @@
TRESPASS();
}
mOwner->beginBox(fourcc); // TextMetaDataSampleEntry
+
+ // HACK to make the metadata track compliant with the ISO standard.
+ //
+ // Metadata track is added from API 26 and the original implementation does not
+ // fully followed the TextMetaDataSampleEntry specified in ISO/IEC 14496-12-2015
+ // in that only the mime_format is written out. content_encoding and
+ // data_reference_index have not been written out. This leads to the failure
+ // when some MP4 parser tries to parse the metadata track according to the
+ // standard. The hack here will make the metadata track compliant with the
+ // standard while still maintaining backwards compatibility. This would enable
+ // Android versions before API 29 to be able to read out the standard compliant
+ // Metadata track generated with Android API 29 and upward. The trick is based
+ // on the fact that the Metadata track must start with prefix “application/” and
+ // those missing fields are not used in Android's Metadata track. By writting
+ // out the mime_format twice, the first mime_format will be used to fill out the
+ // missing reserved, data_reference_index and content encoding fields. On the
+ // parser side, the extracter before API 29 will read out the first mime_format
+ // correctly and drop the second mime_format. The extractor from API 29 will
+ // check if the reserved, data_reference_index and content encoding are filled
+ // with “application” to detect if this is a standard compliant metadata track
+ // and read out the data accordingly.
+ mOwner->writeCString(mime);
+
mOwner->writeCString(mime); // metadata mime_format
mOwner->endBox(); // mett
}