Soon we will be using GL (and its Windows equivalent) on most platforms
to implement a hardware accelerated compositor. We draw into a back
buffer and with up to 60hz we perform a buffer swap to display the back
buffer and make the front buffer the new back buffer (double buffering).
As a result, we have to recomposite the entire window with up to 60hz,
even if we are only animating a single pixel.
On desktop, this is merely bad for battery life. On mobile, this can
genuinely hit hardware limits and we won't hit 60 fps because we waste a
lot of time recompositing pixels that don't change, sucking up memory
bandwidth.
Most platforms support some way to only update a partial rect of the
frame buffer (AGL_SWAP_RECT on Mac, eglPostSubBufferNVfor Linux,
setUpdateRect for Gonk/JB).
I would like to add a protocol to layers to indicate that the layer has
changed since the last composition (or not). I propose the following API:
void ClearDamage(); // called by the compositor after the buffer swap
void NotifyDamage(Rect); // called for every update to the layer, in
window coordinate space (is that a good choice?)
I am using Damage here to avoid overloading Invalidate. Bike shedding
welcome. I would put these directly on Layer. When a color layer
changes, we damage the whole layer. Thebes layers receive damage as the
underlying buffer is updated.
The compositor accumulates damage rects during composition and then does
a buffer swap of that rect only, if supported by the driver.
Damage rects could also be used to shrink the scissor rect when drawing
the layer. I am not sure yet whether its easily doable to take advantage
of this, but we can try as a follow-up patch.
Feedback very welcome.
Thanks,
Andreas
PS: Does anyone know how this works on Windows?
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