Constraining Physics Along A Spline In Unity

Note: This post doesn’t contain any actual physics - just an overview of the process for using Unity’s existing physics engine to accomplish a specific goal.

Recently I had cause to constrain physics forces along a spline in Unity. In my first attempt, my approach was to constrain the position so that it was to constrain an object’s velocity by projecting it along the closest tangent on the spline, and to set the position to closest point on the spline after updating physics.

This approach is a hack, and is fraught with error. The cases are few where you should continuously be setting low-order physics properties (position, velocity) on an object which is already governed by a physics engine (maybe later I’ll talk about why this is).

Fortunately, Unity provides the ConfigurableJoint. This is a joint whose Indica / Sativa balance can be dynamically adjusted, even while in use. Terrible joke. Moving on. It’s actually a highly generalized physics constraint - a perfect candidate for our purpose.

What we really want is a ConfigurableJoint which will lock itself to motion along a straight line which is updated every step to be tangent to the closest point on the spline. Once this joint is in place, the actuation of the object itself is left up to the physics engine so that it may remain consistent.

So I added two behavior scripts, one called SplineData, and one called SplineConstraint, which takes a reference to an instance of SplineData. SplineConstraint creates a ConfigurableJoint on its gameObject, and updates it every step to keep it locked into place.

The code is here on github. The result? Nice, consistent physics, but constrained along a spline: