[FFmpeg-devel] [PATCH 0/2] VDA decoder for ffmpeg

[Reimar Döffinger]

On 21 Aug 2012, at 10:02, Xidorn Quan <quanxunzhen at gmail.com> wrote:
> On Tue, Aug 21, 2012 at 3:00 PM, Reimar Döffinger
> <Reimar.Doeffinger at gmx.de>wrote:
> 
>> Thanks, I really like the feature, but why can't you instantiate a full
>> H.264 decoder with VDA HWAccel like an external application would instead
>> of reimplementing lots of the header parsing?
>> 
> 
> I have considered the solution you said, and obviously, life would
> be a lot more simple if the decoder be implemented that way. But for
> two reasons I decided not to use it:
> 
> 1. Origin H.264 decoder never guarantees that HWAccel will be used.
> 
>   For some cases, the decoder doesn't call get_format, and we will
>   have no chance to ask it to use HWAccel. If it happens,
>   the h264_vda decoder will automatically fall back to software
>   decoding without any warning or something like this.
> 
>   I don't think it is a proper behavior for this decoder. It should
>   either succeed to initialize and then use VDA to decode, or fail
>   if it can't and let caller to fall back itself.
> 
>   In fact, sometimes, VDA can decode a video but H.264 decoder
>   refuses to use it and do decoding itself.
> 
> 2. It's a trivial reason that, the VDA HWAccel will always copy
>   frame data to its own buffer before decoding, it wastes CPU time.
>   It can be more efficient in an independent decoder.

Both of those very much seem to be bugs to me though.
The second is more understandable considering othe HWAccel APIs that I think need it (still not good though), but the first one seems rather serious to me.