[FFmpeg-devel] [PATCH] avformat: Fix probing on some JPEGs

[Michael Niedermayer]

On Fri, Aug 23, 2019 at 04:03:10PM -0700, Niki Bowe wrote:
> On Thu, Aug 22, 2019 at 2:30 AM Paul B Mahol <onemda at gmail.com> wrote:
> 
> > On Thu, Aug 22, 2019 at 11:19 AM Carl Eugen Hoyos <ceffmpeg at gmail.com>
> > wrote:
> >
> > > Am Mi., 21. Aug. 2019 um 23:05 Uhr schrieb Niki Bowe
> > > <nbowe-at-google.com at ffmpeg.org>:
> > > >
> > > > On Mon, Aug 19, 2019 at 7:22 PM Carl Eugen Hoyos <ceffmpeg at gmail.com>
> > > wrote:
> > > >
> > > > >
> > > > > This score would mean that mjpeg can never be detected.
> > > > > I suspect you have to reduce one of the demuxers to "- 1".
> > > > >
> > > > >
> > > > Thanks Carl.
> > > > Attached patch to reduce mpeg probe by -1, which also fixes the issue.
> > >
> > > Sorry, I misread the original report, it looked to me as if mJpeg was
> > > the culprit.
> > >
> > > Imo, the mpeg probing should be fixed (return a smaller value) for your
> > > sample
> > > by detecting that it is not mpeg, not by returning a smaller value for
> > > all samples.
> > >
> >
> > 1000% agree.
> >
> >
> It didn't return a smaller value for all samples, only the "invalid VDR
> files and short PES streams" case.
> Most mpegps files still return 26 immediately, because they have pack
> headers.
> 
> However, here is another patch where I try to limit it to only changing
> score for these jpegs.
> I noticed that these jpegs had a lot of 0x00000100 sequences, which matches
> mpeg picture header start code. I added another heuristic which matches
> these jpegs, but not any mpegps files I could find.
> 
> Alternatively I could make reduce score if it doesn't start with a start
> code? At the moment its happy to search until it finds start codes.
> 
> 
> Is everyone really sure the best approach is to modify mpegps_probe for
> this?
> The mpegps_probe function returns 25 in many instances where it may not be
> mpegps. It does only minimal structural checking, and allows invalid data
> to still classify as mpegps.
> jpeg probing returns 25 in some cases where it is almost certainly a jpeg
> (Has to go through multiple tags to get to SOS, many of which early out if
> they find invalid data).
> Note that 25 is still treated as "low confidence" for jpeg. It logs "Format
> jpeg_pipe detected only with low score of 25, misdetection possible!" for
> these jpegs.
> So I still think a score of 25 is too low for these jpegs, and that a
> better fix would be to return 26 for jpeg_pipe and mjpeg if it makes it
> past multiple tags to SOS.

jpegs can be in other container formats its not jpeg in that case but the
other container format

about this patch
it breaks this:

./ffplay tickets//3327/issue3327-libc-2.17.so

https://trac.ffmpeg.org/raw-attachment/ticket/3327/issue3327-libc-2.17.so

which is detected as mpeg after the patch.
really nothing should be detected as mpeg after this that was not before
the idea IIUC is make something that was detected as mpeg to be not anymore

Thanks

[...]


-- 
Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

Awnsering whenever a program halts or runs forever is
On a turing machine, in general impossible (turings halting problem).
On any real computer, always possible as a real computer has a finite number
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