[FFmpeg-devel] [PATCH] mov: Skip computing SAR from invalid display matrix elements

[Vittorio Giovara]

On Wed, Mar 31, 2021 at 2:41 PM Anton Khirnov <anton at khirnov.net> wrote:

> Quoting Vittorio Giovara (2021-03-30 18:55:27)
> > Hello,
> > I was debugging an issue with a video file containing an invalid
> > display matrix, probably produced by a non conforming software.
> >
> > The content of the matrix is:
> > 00000000:            0       65536           0
> > 00000001:           -1           0           0
> > 00000002:            0           0  1073741824
> >
> > The -1 (stored as 4294967295) was probably a 1 shifted 32 times instead
> > of 16. The problem is that this value is bypassing the validation check
> > in the code below, and the resulting computed SAR value becomes 1:65536.
> >
> > This change interprets extremely low entries as invalid and makes sure
> > to skip them in the SAR computation. This passes fate, but I haven't been
> > able to test this extensively.
> > Please see the attached patch, any feedback or better solution is
> welcome.
> > --
> > Vittorio
> >
> > From 54ec72276cbb6f2536e73ff81b7d49a736ec1900 Mon Sep 17 00:00:00 2001
> > From: Vittorio Giovara <vittorio.giovara at gmail.com>
> > Date: Tue, 30 Mar 2021 16:47:39 +0200
> > Subject: [PATCH] mov: Skip computing SAR from invalid display matrix
> elements
> >
> > ---
>
> I'm wondering if that code should set sample_aspect_ratio at all. There
> are two other bits of code in mov.c that may set sample_aspect_ratio,
> and it's not clear whether it applies _in addition_ to the display
> matrix or not.
>

Unfortunately there are many (old-ish) samples that rely on the display
matrix to adjust the aspect ratio.
We added a test for this case in particular, check out fate-mov-zombie if
you have some time.
-- 
Vittorio