[FFmpeg-devel] [PATCH] Make decoding alpha optional for some codecs.

Paul B Mahol onemda at gmail.com
Wed Sep 18 13:00:00 CEST 2013

On 9/18/13, Michael Niedermayer <michaelni at gmx.at> wrote:
> On Wed, Sep 18, 2013 at 06:04:59AM -0400, compn wrote:
>> On Wed, 18 Sep 2013 08:37:51 +0000, Paul B Mahol wrote:
>> >On 9/17/13, Reimar Doeffinger <Reimar.Doeffinger at gmx.de> wrote:
>> >> For codecs where decoding of a whole plane can simply
>> >> be skipped, we should offer applications to not decode
>> >> alpha for better performance (ca. 30% less CPU usage
>> >> and 40% reduced memory bandwidth).
>> >> It also means applications do not need to implement support
>> >> (even if it is rather simple) for YUVA formats in order to be
>> >> able to play these files.
>> >> Tested by manually hacking avcodec_default_get_format,
>> >> suggestions for how to test in FATE welcome.
>> >>
>> >
>> >Why you think this change is so important that should be
>> >commited?
>> it sounds similar to other cpu saving decoder features that ffmpeg has.
>> like 'skiploopfilter' or 'skipnonref' or 'fast'.
>> do those features take similar amounts of code?
>> do you think skipping alpha could be done in a simpler way?
> what about if this alpha skip is done similarly to gray only decoding?
> that is a AVOption setable skip_alpha instead of get_format()
> One advantage would be that it would not need any changes on the
> user application side, the user (or app) could just set skip alpha
> and the alpha decoding would be skiped
> to keep the code simple tha format could be left as a YUVA format
> only skiping the code that fills the alpha plane
> this should avoid the messy pixfmt setup code

I prefer patch that touch codecs code as little as possible.
So your proposal is perfect to me.

> [...]
> --
> Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB
> it is not once nor twice but times without number that the same ideas make
> their appearance in the world. -- Aristotle

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