Some matroska files embed ASS format subtitles. The header for said subtitles include the header for the subtitle stream in the "codec private data" section. It appears to be optional whether or not the last byte of this data is 0, i.e. a null terminator for the string data. Using ffmpeg to extract subtitles for such a file, this header is copied directly to the output file, including the null terminator, if it was present. This results in a file in which there is a null terminator after the header, but preceeding the actual content of the subtitle file. Obviously this is not correct.
As a data point, of the ~600 mkvs I have locally, 22 of them have ASS subtitles, and of them 20 include the null terminator, so it doesn't appear to be a rare phenomenon. As another data point, the tool mkvextract from mkvtoolnix avoids the ambiguity by first assuming that the source buffer is *not* null terminated, and then manually adding a (possibly second) null terminator. The buffer is then interpreted as a null terminated string and processed that way. (https://gitlab.com/mbunkus/mkvtoolnix/-/blob/main/src/extract/xtr_textsubs.cpp#L117) My change here simply avoids copying the trailing null terminator(s) present. FATE succeeds as there are no mkvs in the suite that have ASS subtitles embedded. Tim Angus (1): avformat/assenc: fix incorrect copy of null terminator libavformat/assenc.c | 6 ++++++ 1 file changed, 6 insertions(+) -- 2.25.1 _______________________________________________ ffmpeg-devel mailing list ffmpeg-devel@ffmpeg.org https://ffmpeg.org/mailman/listinfo/ffmpeg-devel To unsubscribe, visit link above, or email ffmpeg-devel-requ...@ffmpeg.org with subject "unsubscribe".
