[SDL] UpdateRects sketchy
walisser at mac.com
Sun Sep 15 09:34:02 PDT 2002
On Saturday, September 14, 2002, at 05:07 PM, pchans at wm.edu wrote:
> On Saturday, September 14, 2002, at 03:43 PM, Darrell Walisser wrote:
>> 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 by.
> Yes, I am planning on using UpdateRects down the road, but I am still
> perplexed by why UpdateRect does not work. My test program catches
> mouse events and blits a sprite at the click position. I'm not
> interested in efficiency -- I just want the picture to draw, and it
> doesn't do so every time I click.
This would appear to be an error in your program. Try using the
QuartzDebug application in /Developer/Applications to see visually what
rectangles you are passing to UpdateRect().
> Is UpdateRect(screen, 0, 0, 0, 0) optimized so it will be faster than
> updating a rectangle covering the majority of the screen? That is,
> will it buy me any efficiency beyond a good algorithm for joining
> overlapping dirty rectangles?
Yes. If you are going to update the entire screen (or almost the entire
screen), the advantage of managing dirty rectangles usually becomes
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