[compiz] Blurring behind windows

[julio at lepi.org]

> 	That would kill the texture-fillrate. Just imagine this with a
> maximized terminal-window, at e.g. 1920x1200. You would have to do 9
> passes for a 1920x1200 texture... on only one window. Most people have a
> few more (ok maybe not all that large and transparent, but still). I
> assume that this makes most cards crawl. And not even multitexturing
> would help that much. One still would have to upload the same texture
> multiple times for each change of the "area behind the window".

Maybe you assume too much ;)
Imagine you have on current compiz 9 firefox windows open, maximized, and
each one with a very low transparency. Does that make compiz (or the whole
system) crawl? No.
A quick test I made:
http://ghanima.alarue.net/gallery/compiz/Screenshot_001
Here are 9 galeon windows opened, each one with a 9 percent transparency,
with a movie playing behind them. Xgl remains at 12% of CPU use.
http://ghanima.alarue.net/gallery/compiz/Screenshot_2_001
Here are 20 (twenty) galeon windows with animated gifs (I wanted to make
the transparent windows moving themselves). No differences. The CPU is
used mostly by galeon, not by Xgl ;)
I don't think making blur that way would mean uploading the same texture
multiple times. The texture would be uploaded once, then written in
whatever buffer (but inside the card memory) multiple times. This is the
write operation that would take time, but probably not much more time than
calling a convolution filter before a write operation.

> 	An approach like this, would it be implemented, has the potential to
> render a system nearly unresponsive (or at least very sluggish) if the
> user happens to open a few terminals (setup with transparency) and
> starts maximizing them.
>
> 	I would not recommend to follow that path, although it will likely
> achieve the intended blurred looks.

Why not give it a chance?
Anyway, nobody said that using blur on large regions of the screen would
be costless.

Regards,

-- 
Julien