Unresponsive applications

Bill Spitzak spitzak at gmail.com
Thu Sep 22 15:33:12 PDT 2011

William Swanson wrote:

> draw_compositor_frame()
> {
>  foreach (window in windows) {
>    copy_buffer(window.buffer, window.x, window.y, window.w, window.h);
>    if (window.needs_border)
>      draw_border(window.x - 8, window.y - 24, window.w + 20, window.h + 32);
>  }
> }

That copied the buffer, thus all the drawing was done by the compositor, 
so of course it works. What is needed is a method such that either both 
draw into the same buffer or the compositor composites the two images 

It can be done but only by adding latency, as there will be several 
round trips before a resize requested by the user produces the resized 
image on the screen. Client-side decorations get rid of most of this 
round trip (there may be one after the full image is done).

> * Actually, doing the Windows Vista "glass" effect requires the
> compositor to get involved with borders. The glass is not a simple
> alpha-blending trick; it actually involves blurring the contents of
> the windows underneath. The compositor is the only one who can perform
> the blur, since it is the only process with access to the
> partially-composited desktop on each frame.

The blurry pixels do not have to be in the buffer. Instead the 
compositor actually blurs them in-place on the screen (modern gpu can 
certainly do this easily) and then composites the window, with partial 
transparency, above it. This is in fact how Windows does this.

There is a question of how to control the blur. Blurring everything will 
make non-rectangular windows sit in a blurry rectangle. The simplest 
method I can think of is to blur only where there is partial 
transparency. Another possibility (what Windows does) is to blur a 
fixed-sized piece inside from the edges, with knowledge if there is not 
a normal rectangular window there. A way for the client to have full 
control is for it to make a "blur window" which the compositor does not 
do an "over" to composite but it instead causes blur, and the client 
keeps it directly below the normal window.

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