Re: Question about Test Suite
wm4 <[email protected]>
| Newsgroups | gmane.comp.multimedia.matroska.devel |
|---|---|
| Message-ID | <20150727211427.736798ee@miuna> |
On Mon, 27 Jul 2015 09:58:40 -0700 Vignesh Venkatasubramanian <[email protected]> wrote: > On Fri, Jul 24, 2015 at 12:00 AM, Moritz Bunkus <[email protected]> wrote: > > Hey, > > > >> How does mkvinfo get to 451452 to begin reading the next element > >> instead? > > > > mkvinfo uses libebml's EbmlMaster::Read() function for reading a cluster > > en bloc and then outputs which elements libebml has found. > > > > EbmlMaster::Read() uses EbmlEmenet::FindNextElement() under the hood > > for finding the next ID. And that function reads as many bytes as > > needed for forming a valid ID. > > > > So if the byte at position 451451 happens to be 0 (which I haven't > > verified, mind you) then it will probably be skipped as an EBML ID's > > first byte is never 0. Similar reasoning for other values for the > > first byte, e.g. anything smaller than 0x10 would be invalid as EBML > > IDs can only be four bytes long, and the position of first bit set to > > 1 determines the size of the EBML ID. > > Thanks for the explanation. But the problem is, the byte at position > 451451 is 0xEA which is a valid EBML ID right? How does mkvinfo skip > just that byte? FYI, here are the 100 bytes starting from position > 451417: http://pastebin.com/jKxthKwF > > I ran the file through ffmpeg's matroska parser. That resyncs to the > next level 1 element as soon as it sees the invalid track number (i.e. > position 451518) by doing a byte-by-byte search. So it merely ignores > the value at 451451 as it is not a valid level 1 id. Yes, it probably tries "resyncing" by trying the next cluster. Some examples: http://git.videolan.org/?p=ffmpeg.git;a=blob;f=libavformat/matroskadec.c;h=b54679811854ba7fb42225bf086d8b737d0f2d4a;hb=HEAD#l661 https://github.com/mpv-player/mpv/blob/master/demux/ebml.c#L225 http://git.1f0.de/gitweb?p=ffmpeg.git;a=blob;f=libavformat/MatroskaParser.c;h=094fc4f282562702ddafb8fc5765e6facd7ce28c;hb=8b3deb5221b265723fc356add5d06a066bc8e50e#l2482 _______________________________________________ Matroska-devel mailing list [email protected] http://lists.matroska.org/cgi-bin/mailman/listinfo/matroska-devel Read Matroska-Devel on GMane: http://dir.gmane.org/gmane.comp.multimedia.matroska.devel