How can you deflect a direction/magnitude vector based on a direction/magnitude vector and a collided triangle?

So, I have a Triangle->AABB collision algorithm and I have it returning the triangle that the AABB collided with. I was hoping with the 3 vectors of t开发者_StackOverflowhe triangle and the direction/magnitude of the movement would let me determine a deflected vector so that when you run against the wall at an angle you move slower, depending on the angle of collision, but along side the wall. This would remove the sticky collision problem with only moving when there is not a collision. Any suggestions or references would be greatly appreciated! Thanks.

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First, I would convert magnitude/direction to a vector (it's much more convenient).

Then (c++):

float towards=dot(velocity,norm);  // velocity component into triangle
if(towards<0)                      // is moving into triangle
  velocity-=towards*norm;          // remove component

Then it can't move into the triangle. towards<0 might need to be reversed depending on your normal. It's also nice to have a spring force pushing it out.

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Remove the component of the velocity along the normal of the triangle.

The idea is that you can represent the movement as the part that's moving "into" the triangle and the remainder (which will be in perpendicular directions). If you then just move with the remainder, you will no longer be getting any closer to the triangle by the movement (or further, but you shouldn't be detecting a collision in that case).

In pseudo-code:

// v := velocity vector of moving object
// p[3] := points that make up the triangle

triangle_normal = cross(p[2]-p[0], p[1]-p[0])
problematic_v = project(v, onto=triangle_normal)
safe_movement = v - problematic_movement

Note that this intentionally doesn't preserve the magnitude of the movement vector, as doing so would make you slide along a wall very quickly when running straight at it.

For more details and some nice pictures, see Pool Hall Lessons: Fast, Accurate Collision Detection Between Circles or Spheres at Gamasutra. You're not using spheres, but you are essentially doing a perfectly plastic (since you don't bounce) collision.