ffmpeg | branch: master | Andreas Rheinhardt <andreas.rheinha...@gmail.com> | Sat Dec 28 10:00:37 2019 +0100| [e6ea75a38de192c3133b9eebe603979bffeed20b] | committer: Andreas Rheinhardt
avformat/matroskaenc: Improve calculating EBML ID size When the Matroska muxer writes an EBML ID, it calculates the length of said ID before; and it does this as if this were a number that needs to be encoded as EBML number: The formula used is (av_log2(id + 1) - 1) / 7 + 1. But the constants used already contain the VINT_MARKER (the leading bit indicating the length of the EBML number) and therefore the algorithm used makes no sense. Instead the position of the most significant byte set gives the desired length. The algorithm used until now worked because EBML numbers are subject to restrictions: If the EBML number takes up k bytes, then the bit 1 << (7 * k) is set and av_log2(id) is 7 * k. So the current algorithm produces the correct result unless the EBML ID is of the form 7 * k - 1 because of the "id + 1". But contrary to encoding lengths as EBML number (where the + 1 exists to avoid the encodings reserved for unknown length), such EBML numbers are simply forbidden as EBML IDs and as such none of them were ever written. Signed-off-by: Andreas Rheinhardt <andreas.rheinha...@gmail.com> > http://git.videolan.org/gitweb.cgi/ffmpeg.git/?a=commit;h=e6ea75a38de192c3133b9eebe603979bffeed20b --- libavformat/matroskaenc.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/libavformat/matroskaenc.c b/libavformat/matroskaenc.c index f9e69c6c89..c484d95c2f 100644 --- a/libavformat/matroskaenc.c +++ b/libavformat/matroskaenc.c @@ -183,7 +183,7 @@ typedef struct MatroskaMuxContext { static int ebml_id_size(uint32_t id) { - return (av_log2(id + 1) - 1) / 7 + 1; + return (av_log2(id) + 7U) / 8; } static void put_ebml_id(AVIOContext *pb, uint32_t id) _______________________________________________ ffmpeg-cvslog mailing list ffmpeg-cvslog@ffmpeg.org https://ffmpeg.org/mailman/listinfo/ffmpeg-cvslog To unsubscribe, visit link above, or email ffmpeg-cvslog-requ...@ffmpeg.org with subject "unsubscribe".
