[Bug 668483] video: add support for global-alpha multiplicator for overlay rectangles

[GStreamer (bugzilla.gnome.org)]

https://bugzilla.gnome.org/show_bug.cgi?id=668483
  GStreamer | gst-plugins-base | git

Tim-Philipp Müller <t.i.m> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |t.i.m at zen.co.uk
            Summary|gstvideo: add support for   |video: add support for
                   |global-alpha multiplicator  |global-alpha multiplicator
                   |for overlay rectangles      |for overlay rectangles

--- Comment #3 from Tim-Philipp Müller <t.i.m at zen.co.uk> 2012-02-05 13:54:52 UTC ---
Thanks for working on this.

> We were not always sure whether the modification of the API calls where
> in line with the API design principles.

The API hasn't been released yet, so we could still make changes like this if
we wanted to. However, in this case I'm not sure if that's really necessary to
add this parameter to the constructor. I think for most use cases defaulting to
1.0 is sufficient, and if anyone wants to set a different global alpha they can
use the setter API after creating the rectangle, no?

There's one bit that I think might be missing from the patch. At first glance
it looks like the patch only caters for a consumer/renderer that supports
global alpha, but it's not clear what happens if the overlay consumer/renderer
doesn't support global alpha. If I'm not mistaken, it currently just ignores
the global alpha parameter then. What I think should happen is that it
automagically modifies the overlay pixel data (making it writable first if
needed) and multiplies all the alpha values in it with the global alpha if the
caller doesn't support global alpha. We'll need to add a flag to signal whether
a non-1.0 global alpha is set, and for the consumer/renderer to signal to us
whether they can handle it or not (in _get_pixels*()).

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