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

[Jason Garrett-Glaser]

On Wed, Dec 1, 2010 at 5:07 PM, Michael Niedermayer <michaelni at gmx.at> wrote:
> 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.

Raster-order accesses across a video frame don't cause cache misses on
any CPU with hardware prefetch.

Jason