Custom Colliders for RigidBodies

While making good progress developing a new RB version of the player (which I’ll post about elsewhere), I stumbled on the issue that the Daleks solved back in the 80s – stairs!!!

After some searching through the forums I found the simplest solution (these things usually end up being quite simple) that I wanted to record here.

The issue is that when importing/using custom objects, Unity will automatically generate a mesh collider for the object using the objects’ base mesh. Thats great and makes for realistic interactions…if you’re using the character controller. Not so great if using a RB based controller as the collision will stop the player dead.

At the start of this project that was one of my reasons for going with a CC based controller, but I wanted to experiment with a RB controller and I’ve found that I prefer it – especially for physics interactions that seem much more fluid, realistic and immersive.

At this point, my RB player is using about 150 lines of code in a player manager along with a few classes ( of < 60 lines of code each) handling things like movement, power updates, raycasts, and writing some outputs to the UI. This compares to my CC main class that was > 880+ lines of code plus extra classes for raycasting etc.

In fact the new RB controller does everything the old CC based controller did, but with a better ‘feel’ , less code (therefore less bug tracking issues) and less (Ahem…..) ‘physics malfunctions‘.

  • I use the term ‘less physics malfunctions’ there as opposed to ‘no physics malfunctions……where the term ‘malfunction’ is best defined as shouting ‘What the hell is going on!!!!’ quite loudly…….

So thats all good apart from the stairs.

The solution is simply to replace the mesh collider generated by Unity with a (or series of) primitive collider(s) (in this case box colliders) as required to ‘approximate’ the overall shape of the mesh.

Colliders are added as Empty children of the mesh and adjusted using their transforms to create a RB friendly shape:

Hierarchy view of stairs and colliders
Side view: fist collider is just a box (in green) that covers the last (topmost) stair
Side View: 2nd collider is a rotated box collider (in green) that changes the ‘surface’ into a ramp
Example of RB player navigating stairs. Left stairs have mesh collider, right stairs have simplified colliders that create a ramp.

So thats it – the simple primitive based series of colliders transforms the stairs into a ramp that an RB can climb.

Some extra trickery can help achieve ‘stair-ness’ such as running a movement script/animation on the camera/player on entering the collider that takes into account current velocity (now easily accessible from the RB player!) and adds an appropriate bob/move/climb action.

Just to prove the usefulness of this approach (esp as a dev tool) here’s the custom collider for the arrow attached to the RB player that I’m using to debug and test the controller:

Hierarchy of colliders
View of colliders (in green) on arrow

The only issue with this that I immediately foresee is the depth to which the 2nd collider ‘juts’ through the ground plane. Not a problem in my sandbox scene but may become a problem when developing an environment that has multiple levels. In that case the colliders may start to act on objects that are below them and provoke some weird behaviour. One solution may be to add more, gradually smaller colliders to the stairs so the depth of ground penetration can be minimised.