Redraw only parts that have changed

[GoogleCodeExporter]

...Speculation and hypothesis

To turn an arbitrary animated 2D scene into one that uses layer
compositing, you have to segment it into layers. A layer is a part of the
scene for which it's possible to draw an image that can substitute for the
part.

State partitioning

First, collect drawn objects into a list by drawing order. Then segment the
list. If an object has a different blend mode from the previous, start a
new segment. If an object has changed during the last N frames, start a new
segment. If a segment has been static for M frames and it takes longer than
X milliseconds to draw, cache it into a layer.

Now we have the hard layer limits, but each layer is the size of the whole
screen, which is suboptimal. Optimally, each layer would be the size of its
bounding box, which brings us to...

Spatial partitioning

Compute the screen-aligned bounding boxes of each object in the layer. Then
go through the bounding boxes and merge all intersecting bounding boxes.
Now we have the disjoint bounding boxes of the layer, or spatial sublayers.

Cache each sublayer to an offscreen image, along with a hash of the objects
it depicts.

Scene compression, maybe?

If the draw list contains segments with the same segment hash, they can be
drawn using the same layer image (draw largest version, use possibly scaled
down version for others.) The segment hash should be incremental to make it
possible to discard non-matching segments as soon as possible.

Original issue reported on code.google.com by Ilmari.H...@gmail.com on 12 Mar 2008 at 5:57

[GoogleCodeExporter]

The above is from:

As CPU renderers are usually fill-rate limited (CPUs suck at SIMD and memory
bandwidth), they tend to segment the scene spatially into dirty areas, and 
redraw
only those. After all, if you're only highlighting a button, it's a waste to 
redraw
the whole scene. But a scene with an animated background and translucent 
foreground
objects has effectively the whole drawing area as its dirty area (e.g. animated
wallpaper, transparent windows in front.)

The usual way the speed up full-screen animation is to segment the scene into 
layers
and composite the layers using a GPU. Compositing the layers will be very fast 
since
GPUs have massive fillrate, and you only have to redraw changed layers. Compiz, 
Aero
and Quartz Extreme all use this method - the compositor draws all window images 
to
screen on every frame, but window images are only updated when the window 
contents
have changed.

For a window manager, doing layer compositing is easy, because the layers are
pre-defined. For an arbitrary 2D scene, it's more difficult.

Original comment by Ilmari.H...@gmail.com on 12 Mar 2008 at 5:58

[GoogleCodeExporter]

I would think this should be too hard... i might try to work on a patch

Original comment by crgod...@gmail.com on 17 Jul 2011 at 5:32