Question

I use SDL for my programs graphics and I have a problem with it in flipping surfaces.When I compile following code:

int main(int argc , char* argv[])
{
SDL_Surface* scr1 = SDL_SetVideoMode(880 , 600 , 0 , SDL_HWSURFACE |SDL_DOUBLEBUF );
SDL_Surface* scr2 = SDL_SetVideoMode(880 , 600 , 0 , SDL_HWSURFACE |SDL_DOUBLEBUF );

aacircleRGBA(scr1 , 50 , 50 , 30 , 255 , 0 , 0 , 255);

SDL_Flip(scr2);

return 0;
}

It shows the circle on the screen.But I flipped only scr2.Why does it show the circle?

Answer 1

After you call SDL_SetVideoMode() a second time, the original screen buffer pointer is, in the general case, invalid. You shouldn’t be reusing it, because it doesn’t point to an allocated surface anymore.

In this case, calling SDL_SetVideoMode() twice with the same parameters gives scr2 == scr1, because there is no need for SDL to reallocate the video surface. Drawing on the surface referred to by scr1 is thus the same as drawing on that referred to by scr2.