FPS drop when detecting collision (sfml, c++)

Question

So, I have this small class with a collision detection function in it, it's called every frame.

When I run the game I get around 8000 FPS, but when the collision function gets called it drops to 2000.

This is what the function looks like (only called 1 time per frame) The second one is just for overriding.

bool DetectCollision(sf::RectangleShape other)
{
    if (entityRect.getGlobalBounds().intersects(other.getGlobalBounds()))
    {
        // Handle collision here
        return true;
    }
    else
    {
        return false;
    }
}

bool DetectCollision(sf::Sprite other)
{
    if (entityRect.getGlobalBounds().intersects(other.getGlobalBounds()))
    {
        // Handle collision here
        return true;
    }
    else
    {
        return false;
    }
}

Not sure why the FPS drops that much, is this way of detecting collisions heavy for the pc? I'm running a pretty high-end pc.

Answer 1

It can be expensive depending on what collision volumes you are using. This is why most of the geometry in games have multiple collision volumes like AABB to convex hull. Testing whether or not two AABB intersect can save a lot of time when objects don't collide as you don't have to perform that e.g. time-consuming convex hull collision detection to get a more accurate collision.

I don't think that's really the problem though, my first reaction was that you were using FPS to messure performance. You should really always use milliseconds per frame instead. 8000FPS = 0.125ms, while 2000FPS = 0.5ms. So 0.5ms - 0.125ms = 0.375ms added to each frame. Now that may or may not be a lot depending again of what collision you are doing.

Just to drive my point in further. If the game ran at 100FPS and you now added your collision calls to each frame it would only drop to 96FPS, not that much. And your probably wouldn't be so worried about it.

Answer 2

It's pretty much expected to see that kind of frame drop... 8000fps translates to 0.0000125 seconds of computation time, while 2000fps translates to 0.00002s. Subtracting those two values, the difference is as low as 0.000075s, which is way low compared to the 16 milliseconds time span you have to render at 60fps.