[FFmpeg-devel] [PATCH 2/4] lavfi/opencl: Handle overlay input formats correctly.

[Gyan]

On Wed, Nov 14, 2018 at 7:00 AM Song, Ruiling <ruiling.song at intel.com>
wrote:

>
>
> > -----Original Message-----
> > From: ffmpeg-devel [mailto:ffmpeg-devel-bounces at ffmpeg.org] On Behalf Of
> > Mark Thompson
> > Sent: Sunday, November 11, 2018 9:55 PM
> > To: ffmpeg-devel at ffmpeg.org
> > Subject: Re: [FFmpeg-devel] [PATCH 2/4] lavfi/opencl: Handle overlay
> input
> > formats correctly.
> >
> > On 29/10/18 05:56, Ruiling Song wrote:
> > > The main input may have alpha channel, we just ignore it.
> >
> > This doesn't ignore it - it leaves it uninitialised in the output, so a
> YUVA or GBRAP
> > output will never write to the A plane.  I don't think that's what
> you're intending.
> What I wanted to say is ignoring main input alpha channel.
> The question is what the user would expect the result alpha channel
> contains?
> I don't have a clear answer to it, so I just keep it uninitialized.


The s/w overlay filter does this to produce the output alpha:

    main_alpha += (1-main_alpha) * overlay_alpha

Better to keep it consistent, IMHO.

Gyan