On Fri, Oct 16, 2015 at 08:04:08PM +0200, wm4 wrote: > Commit 2b3e9bbfb529e6bde238aeb511b55ebe461664c8 caused problems for a > certain API user: > > https://code.google.com/p/chromium/issues/detail?id=537725 > https://code.google.com/p/chromium/issues/detail?id=542032 > > The problem seems rather arbitrary, because if there's junk, anything > can happen. In this case, the imperfect junk skipping just caused it to > read different junk, from what I can see. > > We can improve the accuracy of junk detection by a lot by checking if 2 > consecutive frames use the same configuration. While in theory it might > be completely fine for the 1st frame to have a different format than the > 2nd frame, it's exceedingly unlikely, and I can't think of a legitimate > use-case. > > This is approximately the same mpg123 does for junk skipping, and the > set of compared header bits is exactly the same. > --- > The reason Chromium had so much problems with this is that they don't > allow mid-stream format changes at all, and the junk triggered format > changes. > > (Would be nice if Google paid me for this.) > --- > libavformat/mp3dec.c | 25 ++++++++++++++++++------- > 1 file changed, 18 insertions(+), 7 deletions(-) > > diff --git a/libavformat/mp3dec.c b/libavformat/mp3dec.c > index d3080d7..e7b6234 100644 > --- a/libavformat/mp3dec.c > +++ b/libavformat/mp3dec.c > @@ -54,7 +54,7 @@ typedef struct { > int is_cbr; > } MP3DecContext; > > -static int check(AVIOContext *pb, int64_t pos); > +static int check(AVIOContext *pb, int64_t pos, uint32_t *header); > > /* mp3 read */ > > @@ -374,12 +374,21 @@ static int mp3_read_header(AVFormatContext *s) > > off = avio_tell(s->pb); > for (i = 0; i < 64 * 1024; i++) { > + uint32_t header, header2; > + int frame_size; > if (!(i&1023)) > ffio_ensure_seekback(s->pb, i + 1024 + 4); > - if (check(s->pb, off + i) >= 0) { > - av_log(s, AV_LOG_INFO, "Skipping %d bytes of junk at > %"PRId64".\n", i, off); > - avio_seek(s->pb, off + i, SEEK_SET); > - break; > + frame_size = check(s->pb, off + i, &header); > + if (frame_size > 0) { > + avio_seek(s->pb, off, SEEK_SET); > + ffio_ensure_seekback(s->pb, i + 1024 + frame_size + 4); > + if (check(s->pb, off + i + frame_size, &header2) >= 0 && > + (header & 0xFFFEFCF0) == (header2 & 0xFFFEFCF0))
This also requires the bitrate and stereo type to match teh bitrate can change in VBR, the stereo coding type can also change i think [...] -- Michael GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB Frequently ignored answer#1 FFmpeg bugs should be sent to our bugtracker. User questions about the command line tools should be sent to the ffmpeg-user ML. And questions about how to use libav* should be sent to the libav-user ML.
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