[FFmpeg-devel] Fix for incorrect Vorbis decoding.

[Alex Converse]

On Thu, Nov 11, 2010 at 1:25 PM, David Conrad <lessen42 at gmail.com> wrote:
> On Nov 11, 2010, at 1:15 PM, Gregory Maxwell wrote:
>
>> Last night Alex Converse mentioned on IRC that a Ogg/Theora+Vorbis
>> file on Wikipedia was producing a garbled decode
>> (http://upload.wikimedia.org/wikipedia/commons/7/79/Big_Buck_Bunny_small.ogv).
>> ?I performed an automated sweep and identified many files for which
>> the ffmpeg decode produced bad sounding output.
>>
>> e.g. compare
>> http://myrandomnode.dyndns.org:8080/~gmaxwell/ffmpeg_broken_decode/f065.wav
>> http://myrandomnode.dyndns.org:8080/~gmaxwell/ffmpeg_broken_decode/v065.wav
>>
>> I didn't have a chance to look at it more in depth at the time. But
>> Monty listened to the files and said they sounded like an error
>> decoding the floor slope. Fortunately the logic in the ffmpeg vorbis
>> decoder is mostly identical to libvorbis, so it was fairly simple to
>> compare it.
>>
>> The root cause turned out to be an overflow in the floor 1
>> computation. I've attached a working fix, though my feelings won't be
>> bruised if you prefer another fix for stylistic reasons.
>
> It it's probably better to use plain int rather than the fast types; IIRC on some platforms they're mapped to 64-bit types which is certainly not the fastest for division.
>
> But the patch looks OK.
>

Applied r25724


...

Regards,
Alex Converse

Sorry about so many messages