Re: A possible problem with an external compositing manager

Around 22 o'clock on Nov 12, Soeren Sandmann wrote:

> It is not GetImage I am worried about. It is the fact that
> applications don't know how when their drawing reaches the screen.

That's already very true today -- the X server doesn't wait for the 
graphics engine to finish drawing before executing the next request; the 
current kludge that many applications use is to use GetImage to fetch a 
pixel from the target drawable; current rendering code doesn't try to 
figure out what that pixel will be when the rendering is finished, instead 
it simply waits for the engine to go idle before reading the frame buffer.

> Each XSync() has the effect of waiting until the server has actually
> drawn the previous frame. This:

Not true; the XSync will wait only until the X server has queued the 
rendering operations to the video card.  Then there is the delay between 
when the frame is drawn to video memory and when it is actually visible on 
the screen.  The GetImage trick reduces arbitrary graphics card latency to 
bounded retrace lacency.

Assuming our compositing manager can do screen updates within some 
reasonable time bound, the GetImage trick will do the same here, with 
perhaps a longer lag.

-keith