[Bug 797169] compositor: Remove crossfade property and implement a more extensible interface like in glvideomixer

[GStreamer (GNOME Bugzilla)]

https://bugzilla.gnome.org/show_bug.cgi?id=797169

--- Comment #29 from Thibault Saunier <tsaunier at gnome.org> ---
(In reply to Sebastian Dröge (slomo) from comment #28)
> (In reply to Thibault Saunier from comment #26)
> > (In reply to Sebastian Dröge (slomo) from comment #25)
> > > (In reply to Thibault Saunier from comment #23)
> > > 
> > > > I do not think we want this kind of restriction at all, the end user can
> > > > transition whatever he want, no reason to not let it do that!
> > > 
> > > Not having such restrictions means more complicated internals and it
> > > restricts how you can change the internals / extend the internals :) But
> > > that's for somewhere else to discuss I guess.
> > 
> > Well, this issue is exactly about that... you want to simplify the
> > internals, at the cost of flexibility and even removing useful features (2
> > pads only (which is arguably not a mandatory feature), and now adding
> > restriction which must *not* exist from a end user video editing app
> > perspective).
> 
> My point here about compositor is not that. Half of the story is the code
> complexity, the other part is a non-intuitive interface that you wouldn't
> expect from something called a "compositor".
>
> We generally build elements that are simple and do one thing, and then you
> can combine them with others or use them multiple times even. And by doing
> that you can still get whatever blending behaviour you want without making
> the API weird.

This is just a compositing mode to me, I am not sure what is weird about that
API but I do believe it is very simple to use and generally useful to have.

> For cross-fading multiple pads might make sense, thinking of something like
> a crossfader+concat for example but that wouldn't be useful for pitivi. I
> don't know what pitivi exactly needs for cross-fading but if there's a
> use-case for cross-fading the first two pads, and then the result and a
> third pad, then sure, go for it. But that would also be implementable by two
> 2-pad crossfade elements then and would perform exactly the same operations
> then (and be more or less the same performance-wise).

It is not specifically Pitivi but GES, basically in there we use the compositor
as a layer based compositor, transition being part of a compositing mode
between two of them basically.

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