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

Don Moir donmoir at comcast.net
Fri Sep 20 08:19:11 CEST 2013

----- Original Message ----- 
From: "Reimar Döffinger" <Reimar.Doeffinger at gmx.de>
To: "FFmpeg development discussions and patches" <ffmpeg-devel at ffmpeg.org>
Sent: Friday, September 20, 2013 1:54 AM
Subject: Re: [FFmpeg-devel] [PATCH] Make decoding alpha optional for somecodecs.

> On 20.09.2013, at 00:50, "Don Moir" <donmoir at comcast.net> wrote:
>> ----- Original Message ----- From: "compn" <tempn at twmi.rr.com>
>> To: <ffmpeg-devel at ffmpeg.org>
>> Sent: Thursday, September 19, 2013 6:26 PM
>> Subject: Re: [FFmpeg-devel] [PATCH] Make decoding alpha optional for some codecs.
>>> On Thu, 19 Sep 2013 18:05:21 -0400, Don Moir wrote:
>>>> You could get lucky and some might look ok, but then alpha can give a smooth anti-aliasing effect and that would be lost. So 
>>>> effort
>>>> is add clutter to save some CPU cycles at the possible expense of a bad display.
>>> since ffplay doesnt handle alpha now, do your videos look bad in ffplay?
>>> i mean, not decoding alpha vs decoding alpha, does it make any
>>> difference when you dont display alpha anyways?
>> I pretty much don't use ffplay for anything. In my own app, alpha makes a great deal of difference, used as overlays and even 
>> when video is over black, the smoothness with alpha is very apparent. It's like take a alpha png and remove the alpha. Looks bad. 
>> stair stepped
> I just realize there is a possible issue with the whole argument, and if you mostly look at intermediate data you might get the 
> wrong impression.
> You assume that the color values need to be multiplied with alpha.
> However except for intermediates that makes no sense.
> The color values should be pre-multiplied with alpha after processing, since it makes the features more natural and avoid sharp 
> edges, which improves visual quality. It also means you will not waste bandwidth storing data that will be blended away anyway, 
> and it saves you one multiplication when displaying.
> In this case, just discarding alpha will actually display an image that looks exactly like against black background.
> Now the interesting question would be whether flashplayer does it like that, which would explain why I could never actually see 
> these issues...

Yeah I was thinking that... But I don't think you could actually know on a general basis. All depends on how it might be encoded 
given various formats. It could be that it is pretty much assumed some would not display the alpha and so maybe pre-multiplied 

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