[FFmpeg-devel] [RFC] How to fix DR+lavfi+vflip crash

[Michael Niedermayer]

On Wed, Dec 01, 2010 at 04:53:59PM -0800, Jason Garrett-Glaser wrote:
> On Tue, Nov 30, 2010 at 2:26 PM, Stefano Sabatini
> <stefano.sabatini-lala at poste.it> wrote:
> > On date Saturday 2010-11-27 16:53:51 +0100, Stefano Sabatini encoded:
> >> On date Saturday 2010-11-06 18:21:55 +0100, Stefano Sabatini encoded:
> >> > On date Saturday 2010-11-06 18:10:04 +0100, Stefano Sabatini encoded:
> >> > > Hi,
> >> > >
> >> > > as you may know the command:
> >> > > ffplay INPUT -vf vflip
> >> > >
> >> > > crashes with many video codecs. This is a regression introduced by the
> >> > > direct rendering feature, since the codec request the frame to use for
> >> > > putting the decoded frame, it gets a frame with negative linesizes and
> >> > > crash
> >> > >
> >> > > (BTW the smacker regression also seems to depend on diect
> >> > > rendering, and precisely with the way the palette is initialized in
> >> > > avfilter_default_get_buffer, which doesn't use ff_systematic_pal()).
> >> > >
> >> > > A possible first step would be to define a CODEC_CAP_NEG_LINESIZES
> >> > > capability (suggest a better name), and set it in all the codecs which
> >> > > currently support this feature (I have no idea which of them, do
> >> > > you?).
> >> > >
> >> > > At this point I see two solutions. One solution would be to change
> >> > > get_video_buffer(), and make it invert the buffer when it detects the
> >> > > negative linesizes && the NEG_LINESIZES capability is not
> >> > > supported, *or* auto-add another filter just before the ffplay source.
> >> > >
> >> > > Such a filter (vflipfix - suggest better name) would work as a null
> >> > > filter if the frame is not inverted, and would readjust the frame if
> >> > > the linesizes are inverted.
> >> > >
> >> > > The second solution seems simpler and cleaner.
> >> >
> >> > To make it even more useful, we may add a capability to the filters,
> >> > and auto-add the vflip-fix filter when building the filterchain, and
> >> > fix all the filters which doesn't support negative linesizes.
> >>
> >> Patchset attached.
> >
> > Ping.
> 
> I don't think this extra complexity is a good idea.  I'd rather just
> modify vflip to do things the slow way (it's NOT an important filter)
> and just officially drop support for negative linesizes.
> 
> This extra complexity would be worth it if it affected any use-case
> that matters.  It doesn't.

imagine a linear filter chain, apply each 2nd filter bottom to top.
This should reduce cache misses.

[...]
-- 
Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

In a rich man's house there is no place to spit but his face.
-- Diogenes of Sinope
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