[FFmpeg-devel] [PATCH] Ensure sample_aspect_ratio.den is nonzero

David Conrad lessen42
Wed Apr 21 16:02:43 CEST 2010

On Apr 21, 2010, at 9:57 AM, Michael Niedermayer wrote:

> On Wed, Apr 21, 2010 at 09:37:29AM -0400, David Conrad wrote:
>> On Apr 21, 2010, at 9:11 AM, Michael Niedermayer wrote:
>>> On Wed, Apr 21, 2010 at 06:32:58AM -0400, David Conrad wrote:
>>>> Hi,
>>>> Currently no decoders check for validity of sample_aspect_ratio, specifically, whether sample_aspect_ratio.den is nonzero. This leads to an issue with theora files, which have 0/0 written a lot of the time, causing ffmpeg to calculate an aspect ratio of 4/1 after some math involving NaNs.
>>> some?
>>> elaborate please
>> av_q2d(sar) return NaN for 0/0, ffmpeg does some multiplications so it's still a NaN, then av_d2q returns 4/1 for a NaN due to how it's implemented.
>>>> Since "unknown" is the only valid interpretation of a denominator of zero, this adds a check to both avcodec_open() and avcodec_decode_video2() to set {0,1} which is a safer value to give to user applications.
>>>> Alternatively, I could put the check in each codec that sets it directly from the bitstream, or clarify the API that a zero denominator is allowed and fix ffmpeg.
>>> I would have assumed that 0/0 is allowed and meant unknown aspect but i
>>> need to first know where the problem you are working on is to be able
>>> to decide what would be the best approuch to fix that or if we should
>>> avoid 0/0
>> Alternate patch to return 0/1 from av_d2q for NaN attached, which also fixes the issue.
> 0/1 is not nan 0/1 is 0
> nan should become 0/0 again

That works too, I was just worried about random people dividing by 0.

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