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

Reimar Döffinger Reimar.Doeffinger at gmx.de
Thu Sep 19 23:56:16 CEST 2013

On Thu, Sep 19, 2013 at 05:37:29PM -0400, Don Moir wrote:
> >>> and basically every video player. E.g. quicktime displays full
> >>> transparency in Prores4444 as black and so does FCP.
> >>
> >>You have to decode the alpha to use the alpha mask any picture content
> >>which is under transparent regions.
> >>
> >>You cannot display the video correctly without multiplying the alpha
> >>with the video content - unless the alpha is 100% across the whole
> >>picture, which you don't know until you've decoded the alpha..
> >Well, as you said yourself later strictly speaking you can't
> >decode it without the proper background image.
> >And there can be some value to see just the color part,
> >even if in some cases it looks quite bad.
> >For quite a few of the flash videos they look just fine
> >after throwing away alpha.
> >I admit that you are right in principle, but for a lot
> >of players discarding alpha will not make anything worse,
> >and for players that multiply the color with alpha an
> >option to discard the alpha becomes even more valuable IMHO,
> >since it can save even more processing in cases where
> >"correctness" (in quotation marks because 100% correct needs the
> >intended background after all and players usually don't have that)
> >isn't critical.
> If you have say an ARGB value of  0,255,0,0  alpha value 0, and red
> value of 255, and assuming black background, what does disrecarding
> the alpha value mean?  Does it mean the value is now a solid red of
> 255 or does it mean value is black ?

Solid red. (and there obviously is no "background" color)

> If alpha is completely
> disregarded display will be terrible and mostly useless.

I don't know what kind of material you work with.
For the ones I could find they looked all completely normal.
Yes, you might have bad luck and come over the one that is
unwatchable without it (though on the other hand, if the video
has significant areas of low alpha, it will look mostly black
and probably not be worth watching either way), but that's also true for
options like skiploopfilter that I remember as being quite popular.

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