Solving Collisions

Wed 23 September 2020
By Bram

I've been developing games for a long time now, but solving collisions remains a hairy subject. The Physics Engine I have used the most is OpenDE. It worked well for Little Crane. But the fact that OpenDE considers a triangle mesh as a generic 'triangle soup' as opposed to a closed surface, tends to get me into trouble. Consider the figure, below, where a wheel intersects the terrain.


Here, we expect triangles A and B to cause the wheel to be pushed out of the terrain. And we expect this correction in the direction of the triangle normals for A and B. Unfortunately, this is not always what happens. As OpenDE considers every triangle individually, it will report collisions on internal edges of the mesh, and tries to correct them with collision normals that lie in the triangle plane, as depicted below.


To make matters worse, other edges can cause collision constraints that push in the completely opposite direction, causing the wheel to get stuck in the terrain.

So what are the options here? One approach is to ignore all collision contacts with collision normals that do not align with the triangle normals. This can cause sinking into the terrain.

Another approach is to correct all reported collision normals, and override them with the triangle normals. This seems to work reasonably well, until you hit a 'thin' part of the terrain, where the wheel goes inside the terrain, and emerges from the other side, out of another triangle, as depicted below.


Blindly using the triangle normals as collision normals leads to bad things here: The wheel is simulateously restricted to go only up and to go only down, meaning the solver will freeze the wheel in place! To solve this, we need to somehow detect one collision as being the first, and ignore the other one.

At the end of the day, filtering the contact points that your physics engine gives you is a non-trivial problem. Limiting the maximum velocities, and using tiny timesteps go a long way, but even then you can get into trouble. If you can, building your terrain out of convex shapes only will save you a lot of troubles, as there is always a well defined inside and outside, making the collision resolution simpler. With generic triangle meshes, you have to be careful.