Use this function to set up a surface for directly accessing the pixels.
int SDL_LockSurface(SDL_Surface* surface)
the SDL_Surface structure to be locked
Returns 0 on success or a negative error code on failure; call SDL_GetError() for more information.
/* Make the pixels pointer valid in the surface */ SDL_LockSurface(surface); /* Surface is locked */ /* Direct pixel access on surface here */ SDL_UnlockSurface(surface); /* Surface is now unlocked */
Between calls to SDL_LockSurface() / SDL_UnlockSurface(), you can write to and read from surface->pixels, using the pixel format stored in surface->format. Once you are done accessing the surface, you should use SDL_UnlockSurface() to release it.
Not all surfaces require locking. If SDL_MUSTLOCK(surface) evaluates to 0, then you can read and write to the surface at any time, and the pixel format of the surface will not change.