http://groups.google.com/group/android-framework/browse_thread/thread/ffe7dacf83567b7b

Sparks..
For overlay interface, the hardware driver is responsible for scaling,
rotation, etc. SurfaceFlinger just leaves a whole in the 2D composited
frame (black pixels with alpha = 0) to allow overlay surface to show
through the 2D surfaces.

To be clear, this means that the video overlay is behind the 2D
overlay. I believe there are provisions for chroma-key if the overlay
doesn't support an alpha channel.

Hi It appears that I can't reply to the discussion above previously
called 'About the libhardware and overlay hardware interface.'  From
the discussion it appears that a video overlay is displayed behind the
sufaceflinger's composited frame.  When using overlays, the discussion
states that the surfaceflinger leaves a hole with alpha=0 and pixels =
black, but the discussion it also implies that an overlay may have an
alpha channel.

If the surfaceflinger leaves a hole, then what use is the overlay
alpha because there will be no colors to multiply.  Surely the
surfaceflinger does not always leave a hole? and can have color pixels
so that the surfaceflinger's composted frame can be alpha blended with
an overlay frame using hardware?

If this is the case can a composted frame have an alpha value so that
the hardware knows how to multiply the foreground surface with the
background video overlay? or does the overlay always have to have the
alpha?

Thanks

Chris
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