Predictive collision detection techniques
Saturday, January 9th, 2010
In preparation for my upcoming Flash game programming training courses, I’m getting my head back into game physics, and I so I thought I’d share some useful collision detection methods I’ve discovered over the last few years.
Reactive collision detection
Collision detection in Flash games often occurs after things have moved. So you have a circle (usually a 2D representation of a ball) and you’re moving it towards a vertical line on the side of the screen (representing a wall). Every frame, you update its position and check whether it’s overlapping the wall.
Which is great, but of course if the ball is moving fast it may go from one side of the wall all the way to the other side between frames and no collision is detected. One way to get around this is to split up the movement of the ball into small segments and run this check several times between frames. This seems pretty inelegant to me so I have always strived to use predictive collision detection methods where my maths knowledge has allowed me! (This is also known as sweep testing, frame independent or continuous collision detection (CCD)).
Predicting when things will collide before you move them
So rather than just check whether the ball is intersecting with the wall between renders, we actually check the velocity of the ball to see at what point in time it would hit the wall if it continued in the direction it’s going.
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