[SDL] UpdateRects sketchy

Darrell Walisser walisser at mac.com
Sat Sep 14 12:44:00 PDT 2002

On Saturday, September 14, 2002, at 12:17 PM, pchans at wm.edu wrote:

> I just started playing with SDL yesterday, and pretty quickly ran into 
> a problem with screen updates.  Calls to UpdateRect() do not always 
> result in an image being drawn on the screen.  Rather, after a number 
> of calls to UpdateRect a vaguely rectangular portion of the 
> window/screen may be updated, but not necessarily the entire portion 
> that ought to have been updated.  I am reasonably confident that my 
> update rectangles are correct, as some of the time UpdateRect() 
> behaves as I would expect.  Also, in windowed mode, moving my window 
> off the screen and back on will update regions that went off the 
> screen.
> NB:  I am using SDL 1.2 Mac OS X.
> I have seen a number of questions like these in a cursory search of 
> the mailing list archives, but I have not yet seen an explanation or 
> solution.  Any suggestions?

You shouldn't be using SDL_UpdateRect() to do multiple dirty rects per 
frame, as the cost of SDL_UpdateRect() is very - dare I say *extremely* 
- high. The reason is that each time you call UpdateRect() or 
UpdateRects() or Flip(), the window server copies the region you 
specified to the display, in sync with the retrace of the monitor 
(VBL), and possibly involving compositing if the window is occluded. 
Even if there is no compositing required, just the sync with the VBL 
causes a huge hit while the CPU is idle waiting for the CRT beam to go 

Each frame, pass all of your dirty rects together to SDL_UpdateRects() 
- this minimizes the penalty associated with the VBL since all the 
rects are processed at the same time. Also, that should solve the 
problem with some areas not redrawing at the same time.

Note that SDL_UpdateRects() is the low-level function. SDL_UpdateRect() 
and SDL_Flip() call SDL_UpdateRects().

For even better performance, in the case you have a lot of rects to 
pass per frame or most of the screen area is updated, try tossing out 
dirty rects altogether and call SDL_UpdateRect (screen,0,0,0,0). 
Updating the whole screen can be faster since the window server has 
less work to do.

Benchmarking has shown that the best performance on Mac OS X is using a 
software fullscreen surface (SDL_SWSURFACE | SDL_FULLSCREEN) of the bit 
depth you desire (lower bit depths are faster).

If you are using 10.2 (Jaguar), I am aware of the flickering issue in 
windowed mode (which is actually different from your problem it seems) 
and I have a fix which should make it into SDL soon.


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