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Direction of normal - Miglu - Oct 4, 2010 12:25 PM

I want to make a circle bounce from surfaces when a key is pressed in the way that the ball in N-Ball does, so that its velocity in the direction of the surface's normal becomes a value that has a constant magnitude and its velocity in the direction that is parallel to the surface does not change. This is not a physics engine question, only a mathematical one. How to do it? I am asking this as I can not yet do vector maths well (I know only its basics).


RE: Direction of normal - Skorche - Oct 4, 2010 01:05 PM

Simple, you just need to subtract out the normal component from the velocity.

Velocity component parallel to the normal:
vNormal = dot(velocity, n)*n

Velocity component perpendicular to the normal:
vTangent = velocity - vNormal

Your desired normal velocity:
vDesired = n*desired

Putting it all together:
vResult = vTangent + vDesired

Which simplifies to:
vResult = velocity + (desired - dot(velocity, n))*n


RE: Direction of normal - Miglu - Oct 4, 2010 01:56 PM

Thanks. Do I need to find the collision surface's normal myself, as Chipmunk does not seem to have a query for it?


RE: Direction of normal - Skorche - Oct 4, 2010 05:08 PM

You can get collision normals from the cpArbiter struct passed to your collision handler callback. (See http://files.slembcke.net/chipmunk/release/ChipmunkLatest-Docs/#cpArbiter)