How to sync page-flips with vertical retrace in a windowed SDL application?

I'm currently writing a game of immense sophistication and cunning, that will fill you with awe and won- oh, OK, it's the 15 puzzle, and I'm just familiarising myself with SDL.

I'm running in windowed mode, and using SDL_Flip as the general-case page update, since it maps automatically to an SDL_UpdateRect of the full window in windowed mode. Not the optimum approach, but given that this is just the 15 puzzle...

Anyway, the tile moves are happening at ludicrous speed. IOW, SDL_Flip in windowed mode doesn't include any synchronisation with vertical retraces. I'm working in Windows XP ATM, but I assume开发者_Go百科 this is correct behaviour for SDL and will occur on other platforms too.

Switching to using SDL_UpdateRect obviously won't change anything. Presumably, I need to implement the delay logic in my own code. But a simple clock-based timer could result in updates occuring when the window is half-drawn, causing visible distortions (I forget the technical name).

EDIT This problem is known as "tearing".

So - in a windowed mode game in SDL, how do I synchronise my page-flips with the vertical retrace?

EDIT I have seen several claims, while searching for a solution, that it is impossible to synchronise page-flips to the vertical retrace in a windowed application. On Windows, at least, this is simply false - I have written games (by which I mean things on a similar level to the 15-puzzle) that do this. I once wasted some time playing with Dark Basic and the Dark GDK - both DirectX-based and both syncronising page-flips to the vertical retrace in windowed mode.

Major Edit

It turns out I should have spent more time looking before asking. From the SDL FAQ...

http://sdl.beuc.net/sdl.wiki/FAQ_Double_Buffering_is_Tearing

That seems to imply quite strongly that synchronising with the vertical retrace isn't supported in SDL windowed-mode apps.

But...

The basic technique is possible on Windows, and I'm beginning the think SDL does it, in a sense. Just not quite certain yet.

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 wait for a vertical retrace.

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.

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.

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.

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.

But - can anyone offer a definitive answer?

You can use the framerate control of SDL_gfx. Looking at the docs of library, the flow of your application will be like this:

// initialization code
FPSManager *fpsManager;
SDL_initFramerate(fpsManager);
SDL_setFramerate(fpsManager, 60 /* desired FPS */);


// in the render loop
SDL_framerateDelay(fpsManager);

Also, you may look at the source code to create your own framerate control.