Luma -> Alpha with v1.18

[amindfv at mailbox.org]

On Thu, Jan 06, 2022 at 11:30:25AM -0500, Nicolas Dufresne wrote:
> Le mercredi 05 janvier 2022 à 14:19 -0800, amindfv--- via gstreamer-devel a
> écrit :
> > I'm looking for a way to use the luma plane of one video as the alpha plane of another.
> > 
> > It appears the "alphacombine" element (https://gstreamer.freedesktop.org/documentation/codecalpha/alphacombine.html?gi-language=c) can do this, but it's too new even for the unstable release of GStreamer, and for a variety of reasons I'm stuck on the latest stable (1.18).
> > 
> > What I'm trying to do is also possible in ffmpeg with "extractplanes=y" + "alphamerge".
> > 
> > I'd welcome any way to do this, no matter how much of a hack. Big pipeline? Fine. Somehow using GPU/shaders? No prob.
> 
> Perhaps you can copy and build the codecalpha plugin into your project ? Note
> that alphacombine is a "simple" implementation, not suitable for every use
> cases, as it does not synchronize the inputs (this is to avoid adding large
> threading overhead introduce by the sycnhronization).
> 

When you say it doesn't perform synchronization, that wouldn't prevent me from doing precise synchronization elsewhere, right?

In other words, I can be sure that frame number N of stream #1 is paired with frame number N of stream #2 (assuming identical frame rates), it's just that alphacombine won't be the element doing that synchronization? Or are there cases where I can't be sure the frames don't get out of sync?

> All it does is take two I420 buffers, append then and declare this as A420
> (updating the GstVideoMeta).

That's actually perfect for my needs, I think (hope!).

Tom