[SDL] [Patch] Add OpenGL swap control

Per Inge Mathisen per.mathisen at gmail.com
Sun Jan 23 06:32:33 PST 2005

On Friday 07 January 2005 08.48, Evadream wrote:
> Until now, i blit between hardware surface and "active frame
> buffer" (set via SDL_SetVideoMode) very fast thanx to glSDL
> backend, so i wonder if it is possible to beneficiate of this
> acceleration with two hardware surface create via SDL_CreateRGB and
> blit between them. I've made some tests, and it doesn't look to
> work. Is this the normal behaviour ? Or i miss someting ?

This is a limitation of OpenGL. You can't perform accelerated 
operations between textures - at least not without relying on 
extensions that are available only on some platform/driver/hardware 

We'll try to make the best use of whatever is available in future 
versions, but we'll have to make it work correctly and reliably 
everywhere, before we move on to optimizations, and driver/hardware 
specific optimizations in particular.

Blitting to the screen is what glSDL focuses on at this point, since 
it's the only part of real time rendering that you can't avoid no 
matter what. Off-screen rendering - if you have any at all - can 
usually be done in advance, when installing the game, or when loading 

Even games that use vector objects, pixel effects, complex compound 
sprites and the like can sometimes make good use of glSDL, since 
rendering into glSDL "h/w" surfaces (actually s/w shadow surfaces 
since OpenGL cannot give you access to texture RAM) is very fast, and 
uploading textures to the video card and then rendering with OpenGL 
isn't much slower than rendering directly into VRAM. Actually, if the 
OpenGL driver uses DMA for uploading, it's about the fastest path 
there is from the CPU to VRAM - and either way, alpha blending with 
glSDL comes at very little, if any, extra cost.

//David Olofson - Programmer, Composer, Open Source Advocate

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