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Drawing Into Video

You can draw on the screen directly by altering the data in the "pixels" part of an SDL_Surface:

Uint16 *bufp;
...
bufp = (Uint16 *)(scr->pixels +
                  y * (scr->pitch / 2) +
                  x);
*bufp = color;

SDL_UpdateRect(scr, x, y, 1, 1);
But most of the time you'll want to draw sprites onto the screen.

You can do this by copying all or part of another SDL_Surface onto your screen surface...