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

[Niki Bowe]

On Sun, Aug 25, 2019 at 11:39 AM Michael Niedermayer <michael at niedermayer.cc>
wrote:

> 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. Good find.
Attached patch which only applies the extra sanity checks to the "Invalid
VDR files and short PES streams" path.



> 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
> of states N, and will either halt in less than N cycles or never halt.
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-- 

Nikolas Bowe |  SWE |  nbowe at google.com |  408-565-5137
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