[cairo] delayed graphic updates under Windows

[Theo Veenker]

Hi all,

I've question about repeatedly updating window contents on Windows with DWM running.

My application tries to update windows synchronized with vblank. It works beautifully 
under Linux and it sort-of worked OK with XP. Under recent version of Windows we have to 
live with the DWM controlling when window updates make it to the screen.

Now the problem is when I try to update a window on every frame (which happens with 
animated content) and I use cairo_clip() to restrict the area to be updated then Windows 
somehow decides it's not worth redrawing every frame and skips frames (i.e. does not 
update on every frame). When I comment out the cairo_clip() it does not exhibit this 
behaviour, so does not skip (that much).

I tend to use clipping where I can to speed up drawing where I can, but under Windows in 
this case I have to paint my entire backbuffer to the window for each frame to have it 
displayed. I don't think there's anything wrong with cairo in this respect. It's just that 
I don't know how to force the DWM to update. I tried GdiFlush() and GdmFlush() with no 
effect. Calling GDI GetPixel() on the target window forces a flush, but that approach is 
much slower than repainting the entire window.

I hope someone on the list knows how to overcome this problem.

Theo


-- 
Theo Veenker  |  Beexy - Behavioral Experiment Software
+31(0)524-541531  |  +31(0)6-42525777 mobile
theo.veenker at beexy.nl  |  www.beexy.nl