Realistic physics along spline like tracks

Hi all, been trying to figure out how to best approach this for a while but without much luck so far.

I'm looking to try to do realistic physics for objects being pull and pushed along a spline. I already have a spline solution in place where all I need to give it is a speed to move an object attached to it at. The problem I've been trying to figure out is how to realistically calculate those speeds on each tick along said splines as everything I've seen so far advises against trying to use Unity's built-in physics engine for this kind of thing, or at least I haven't seen a good way to use the engine for it.

The idea is that there will be a vehicle you control, like a train, that pushes cars or items along the spline and I'm trying for the most realistic possible physics in their interactions, including weight and the slack between them.

Any ideas? Thanks!

 

I'd use PathMagic, a spline tool that provides a physics joint to attach objects to a spline. Watch the 3rd video in its Asset Store page:

https://assetstore.unity.com/packages/tools/animation/pathmagic-47769

 

Thanks for the response, that actually looks very useful. The one other thing I would require though would be branching paths, like switches and controlling which route an object would take. Do you know whether it has support for that? The asset's website linked to from the store page appears to be down and I'm not finding any other documentation so far.

Thanks again!

 

I don't know if PathMagic supports it. If it doesn't, maybe you could learn from the physics joint and apply it to a different spline solution.

Anyways, I think that implementing branching and routes may be very dependent of each specific project, so I'd consider designing that part on top of an existing spline asset.

 

You need to code a spline walker by dividing the curve into many steps and storing the data in an array for reference because the 't' value in a spline is not proportional to the distance along the spline. Search Google for 'spline walker' for code samples.

 

While this an off-topic reply, I'd like to point out that a spline can be walked at constant speed without need of sampling the spline nor storing any data. I've developed a method that does that by using the spline's tangent:

https://twitter.com/VehiclePhysics/status/1214195538184290304

 

This is a super late reply but do you happen to have that available anywhere? I'm very curious to see how that works.

 

The spline components are included in Vehicle Physics Pro, also in the Community Edition:
https://assetstore.unity.com/packages/tools/physics/vehicle-physics-pro-community-edition-153556

 

Do you happen to have any information at least about how that works? I'm curious to learn about that but I see it's supplied as binaries instead of source code and I don't see any spline sources in the package contents of your other asset.

 

It's simply diving the movement rate by the tangent magnitude at the current position.

Each spline segment is interpolated from 0 to 1. Let's say that you move at a rate of 0.1, so the position t is calculated as t = t + 0.1 * deltaTime. Simply using t = t + 0.1 / tangent(t).magnitude * deltaTime results in a motion of exactly 0.1 m/s along the entire length of this spline segment.

I'm using Hermite splines, where the tangents are explicitly defined in each control point. An Hermite segment is Point0, Tangent0, Point1, Tangent1, while Bezier is defined with four control points. However, tangents in Bezier splines can also be calculated easily.

The real problem with this approach is keeping the rate constant when crossing control points. Tangents at each side of a the control point are likely different. The movement step that crosses the control point must be handled considering distances and tangents at both sides, or the motion will experience hiccups. Also, high speeds may result in several points crossed in a single movement step, which also requires special considerations. Not to mention that the tangent length may be zero at the control points in some specific configurations. Figuring out and testing all this is what took most of the time when developing my Spline component.