2D Collision: Check before or check after?

Question

I'm using velocity to move my character, I just add/subtract 0.4f and then update the players position in the update loop.

I was wondering, when would be the best place to check for collision? Do I go ahead move the player, and then if I colliding push him out of the wall until there's no collision? Or do I check collision as soon as the key is pressed, and if there is - somehow find the distance and move the player?

My problem is that my game is tile based, but the world is constantly rotating around the player - I have a function that returns a list of objects of each block that the player is colliding with, and I'm having trouble separating wall collisions with floor collisions

Answer 1

Both. As a body moves, there is a volume that it covers over a period of time. Imagine taking a sphere that moves along a line over a small period of time. The shape that it will cover (or "sweep") over this time is a capsule.

When you run your simulation over a time step, calculate these volumes and use those in your intersection testing. This will also solve the "Bullet through Paper" problem: if you have a very fast moving object move through a very thin object with a large enough time step, you won't detect the collision.

In your situation, there are lots of static objects; you can make an optimization to avoid calculating the swept volumes for these.

Interestingly, I just answered another question that is relevant, and will probably help you with other problems you're likely to encounter:

How to resolve penetration of two colliding bodies