# rigid body response behaviour

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So I have been wrapping my head around how I would perform response upon collision in my physical actors. So far I have nailed down my dynamic actors into boxes and their composites. And I have no idea what computational/data design I should use in order to have proper collision response. I have following description of my actors:

- velocity vector

- innertia tensor position

- rotation velocity quaternion

I have no idea on how to implement correct friction, movement initiation, tendence, forces from collision points. In fact I do not have even a concept of the theory.

Are responses in physical engines really procedural or they often fall back to some explicit situation describers to simulate real world AAA behaviour?

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The standard (for rigid bodies) is to form a system of linear equations and solve them. This can come down to finding an efficient way to invert a conceptually large matrix. Erin Catto's 2005 paper "Iterative Dynamics with Temporal Coherence" (found on the Box2D's google download page) describes mathematics behind this.

Usually to solve a collision a point of contact is required, along with a unit vector describing the axis of least separation, and a scalar describing overlap. Usually the narrow phase collects this information, or in other words, you can get this info during collision detection.

Box2D *Lite* (the Lite part is important) is usually the best place to get started, and can be found at Box2D's website.

Pre-scripted physics sequences are also used. For characters other forms of animation are used too, like forward and inverse kinematics.

Edited by Randy Gaul

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