   Point Reflection on a Plane
  Hey all,I'm working on a particle system and need a hand with some mathematics:I've determined that a point, moving through 3d space collides with a plane.I need help with determining its new position and velocity if it bounces off the plane.I know this much:Point starts at Pa.Has velocity Pvel.Time frame is dt.Without collisions, moves to Pb (i.e. Pb = Pa + Pvel * dt).The plane is actually a triangle, P0, P1, P2. (i.e. I know only these three points about the plane)0 <= t <= 1point collides at "time = dt * t" at "Pc = Pa + (Pb - Pa) * t"Now assuming that the angle of incidence is equal to the angle of reflection, what velocity and position would the point now haveThanks in advance   
  Just google 'vector reflection' or 'vector reflection equation'; you'll want to use the equation to reflect the velocity vector about the (unit-length) plane normal.Note that this is only for simple collision response, i.e. perfectly elastic with no friction.   ;  If the collision is elastic, the velocity before and after collision will be equal. As for the direction, that's not too hard if you take a minute to visualize it:\  {...} (particle, after collision) \  /  \/    \   /    \ /=====*========================    Plane   {...} (where particle would be if no collision occurred)Notice how the particle's position after collision looks pretty similar to where it would be if the plane didn't exist: it's still moving to the right at the same speed, and it's still the same distance away from the plane. Since the only thing that altered its course was the plane, it follows that any change in direction results entirely from the direction of the plane's surface normal.Let s be source vector (approaching the plane) and r the reflected vector (leaving the plane), and N the normal to the plane. Then r is    r = -s + 2(s·n/n·n)nwhere · is the dot operator.hope that helps,   