[Bug 759867] gl: Add support to compose VideoOverlayCompositionMeta before the display sink

[GStreamer (GNOME Bugzilla)]

https://bugzilla.gnome.org/show_bug.cgi?id=759867

--- Comment #6 from Florent Thiery <florent.thiery at ubicast.eu> ---
(In reply to Matthew Waters (ystreet00) from comment #1)
> > gst-launch-1.0 videotestsrc ! glupload ! timeoverlay ! glfiltercube ! glimagesink
> 
> This doesn't work because timeoverlay doesn't support memory:GLMemory *or*
> glfiltercube doesn't support meta:GstVideoOverlayComposition.  Which is
> expected at this point.

Yes, but then why is glcolorconvert allowing this to link/work in 1.6 but not
master ?

gst-launch-1.0 videotestsrc ! glupload ! timeoverlay ! glcolorconvert !
glfiltercube ! glimagesink

In 1.6, what was actually performing the text rendering (pango in the
timeoverlay element ?). Was timeoverlay generating small (i.e. resolution of
just the label dimensions) raw video buffers (RGBA) which are then passed
downstream as GstVideoOverlayCompositionMeta, and the first downstream element
that supports GstVideoOverlayCompositionMeta uploads the buffer to the GL
context and blends it ?

Documentation states "it can be used to attach overlay information (subtitles
or logos) to non-raw video buffers such as GL/VAAPI/VDPAU surfaces. The actual
blending of the overlay can then be done by e.g. the video sink that processes
these non-raw buffers"; by having the timeoverlay in the middle of the GL
pipeline, what is actually happening with regards to text rendering ? Is the
time overlay considered a subtitle, or a texture ? 

Sure, accelerated font rendering would require dedicated gltext/timeoverlay
elements (pango/cairo...).

As a workaround, i could render text in software to RGBA/PNG and then overlay
manually using gloverlay. Not really an option for time overlays but maybe for
OSD stuff...

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