Rope Physics and Rope Rendering

I recently added snare turrets to Jettomero, which will attach cables to Jettomero’s arms when in range and pull them until the turret eventually gets yanked out of the ground when the tension is high enough. I’m sure there’s many different ways to handle something like this but here’s a quick tutorial for how I set it up.

Using Unity3D’s spring joints proved to be effective at running the physics side of this system. For the rendering I used a single line renderer. Since the Line Renderer component relies on specific points to draw to I needed to keep assigning new positions in my Update() function. I assign the first point in the line to the origin of the cable where the first joint lives. Then I run a for loop to iterate over each joint in order of their chain, I can assign each additional point in the line to the position of the connected rigidbody, the last of which will be Jettomero’s arm. So the cable will get rendered for the entire length of the joint system.


You’ll probably want to make sure the line renderer has the Use World Space toggle selected so that all the transform.position references are in the proper space.

As for the RigidBody and SpringJoint components, this was what my settings looked like once I got things working how I wanted.

All the joints are set up the same except for my turret base I freeze the position and rotation so it can pull against Jettomero’s arms. Then I check  joint.currentForce.magnitude against a pre-defined threshold. When the force is high enough I disable the constraints on the base and it come flying out of the ground towards Jettomero.

The joints took me a while to figure out, the Auto Configure option was throwing me off for a while. It really depends on how you’re setting up your scene so sometimes it’s best to run the game and just mess with settings until it’s working how you want. Then right click on the component in the inspector, copy the component, stop the game and then paste the runtime component settings back in. Playing with physics takes patience and lots of tuning and iteration.

That’s essentially it. If I missed something or you have any questions feels free to reach out to me on twitter @GhostTimeGames.