## Direction of normal

Member
Posts: 131
Joined: 2010.08
Post: #1
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).
Sage
Posts: 1,482
Joined: 2002.09
Post: #2
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

Scott Lembcke - Howling Moon Software
Author of Chipmunk Physics - A fast and simple rigid body physics library in C.
Member
Posts: 131
Joined: 2010.08
Post: #3
Thanks. Do I need to find the collision surface's normal myself, as Chipmunk does not seem to have a query for it?
Sage
Posts: 1,482
Joined: 2002.09
Post: #4
You can get collision normals from the cpArbiter struct passed to your collision handler callback. (See http://files.slembcke.net/chipmunk/relea...cpArbiter)

Scott Lembcke - Howling Moon Software
Author of Chipmunk Physics - A fast and simple rigid body physics library in C.

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