How does the SDL_RenderCopy determine where to draw?

Question

I need to render something based on the map. Now SDL_RenderCopy accepts negative value for it's co-ordinate. If I use SDL_RenderCopy with a dstrect={0,-16,32,32}, it would render the bottom half of the rectangle on the Top left of the window. In my case, the map is larger than the window, and scrolling would be used. I can render something that is on map but not on the window using one of the two methods mentioned below. But I am worried about the time required to render then. If I use SDL_RenderCopy with a negative dstrect, would the SDL_RenderCopy function check and adjust the dstrect to render, or the underlying hardware pipeline would do the job? Because if the SDL_RenderCopy does it, it would be faster to check and return in case the rendering can't be done on the window, but if the underlying the protocol detects it, then it would be much later in the scenario that the rectangle can't be drawn, taking longer time to return to do the next possible rendering. Well the time difference may not be that high, probably be done within ms with a reasonable hardware, but when we are talking about a lot of rectangle (say, 10000, though even that number won't be a possible in my case) then I am worried that it may slow down the process.

The mentioned two approaches are below The scrolling is done only in the y axis

Let the SDL_RenderCopy function do the check


SDL_Rect dstrect={target.x,target.y-Map.y,target.w,target.h};
SDL_RenderCopy(renderer,texture,nullptr,&dstrect);

Let me do the check

//if within the Map, render
if(target.y<Map.y+Map.h+target.h && target.y>Map.y-target.h)
{
SDL_Rect dstrect={target.x,target.y-Map.y,target.w,target.h);
SDL_RenderCopy(renderer,texture,nullptr,&dstrect);
}

score 0 · Accepted Answer · answered May 28 '23 at 13:43

SDL_RenderCopy does perform a check:

SDL_RenderGetViewport(renderer, &real_dstrect);
real_dstrect.x = 0;
real_dstrect.y = 0;
if (dstrect) {
    if (!SDL_HasIntersection(dstrect, &real_dstrect)) {
        return 0;
    }
    real_dstrect = *dstrect;
}

This checks whether the viewport and your rect have an intersection. If they don't, then your rect is completely outside the window and the function returns early.

If your rect is neither completely outside nor completely inside the window, then what happens depends on the renderer. For example, the OpenGL renderer leaves it up to the driver, whereas the software renderer does the clipping itself using the appropriate blitting function, which calls SDL_UpperBlit.

How does the SDL_RenderCopy determine where to draw?

1 Answers1