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    <p><strong>Major Edit</strong></p> <p>It turns out I should have spent more time looking before asking. From the SDL FAQ...</p> <p><a href="http://sdl.beuc.net/sdl.wiki/FAQ_Double_Buffering_is_Tearing" rel="nofollow">http://sdl.beuc.net/sdl.wiki/FAQ_Double_Buffering_is_Tearing</a></p> <p>That seems to imply quite strongly that synchronising with the vertical retrace isn't supported in SDL windowed-mode apps.</p> <p>But...</p> <p>The basic technique <em>is</em> possible on Windows, and I'm beginning the think SDL does it, in a sense. Just not quite certain yet.</p> <p>On Windows, I said before, synchronising page-flips to vertical syncs in Windowed mode has been possible all the way back to the 16-bit days using WinG. It turns out that that's not exactly wrong, but misleading. I dug out some old source code using WinG, and there was a timer triggering the page-blits. WinG will run at ludicrous speed, just as I was surprised by SDL doing - the blit-to-screen page-flip operations don't <em>wait</em> for a vertical retrace.</p> <p>On further investigation - when you do a blit to the screen in WinG, the blit is queued for later and the call exits. The blit is executed at the next vertical retrace, so hopefully no tearing. If you do further blits to the screen (dirty rectangles) before that retrace, they are combined. If you do loads of full-screen blits before the vertical retrace, you are rendering frames that are never displayed.</p> <p>This blit-to-screen in WinG is obviously similar to the SDL_UpdateRect. SDL_UpdateRects is just an optimised way to manually combine some dirty rectangles (and be sure, perhaps, they are applied to the same frame). So maybe (on platforms where vertical retrace stuff is possible) it is being done in SDL, similarly to in WinG - no waiting, but no tearing either.</p> <p>Well, I tested using a timer to trigger the frame updates, and the result (on Windows XP) is uncertain. I could get very slight and occasional tearing on my ancient laptop, but that may be no fault of SDLs - it could be that the "raster" is outrunning the blit. This is probably my fault for using SDL_Flip instead of a direct call to SDL_UpdateRect with a minimal dirty rectangle - though I was trying to get tearing in this case, to see if I could.</p> <p>So I'm still uncertain, but it may be that windowed-mode SDL is as immune to tearing as it can be on those platforms that allow it. Results don't seem as bad as I imagined, even on my ancient laptop.</p> <p>But - can anyone offer a definitive answer?</p>
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    1. COAccepted to close down this old question, but I'm still open to switching if a definitive answer appears.
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