[SDL] SDL, OpenGL, and Drawing only part of the screen

[Ryan C. Gordon]

> I was thinking about using glViewport to only draw to part of the screen 
> but I still have to use SDL_GL_SwapBuffers();. to make the changes visible.
> 
> Is there a way I can only refresh part of the screen?

glViewport doesn't work like that...generally you redraw the whole frame 
each time and swap the buffers (which is different than how a 2D app, 
which benefits from less drawing, works).

If you have even basic hardware acceleration and can reasonably ask if 
there's a way to avoid drawing parts of the screen, OpenGL is _not_ your 
bottleneck! You might be hitting a slow path (use a display list instead 
of immediate mode, for example), or there's something you can optimize 
in the application itself (if you have 100 things to draw and you know 
that 99 of them are off screen, only draw the one instead of having 
OpenGL cull it for you).

--ryan.