pulsarbird

Balls Rig Test

Published: April 3rd 2025, 11:02:22 pm

Over the past week or so I've been experimenting with different ways to improve rigging on balls on a model. I finally came up with something that I think works really well! It's easy to do, and doesn't rely on physics sims or anything heavy. This video is a quick test I did using the walk cycle of the horse model with the new balls rig. If you compare it to the previous video of the walk cycle, you can see the ball deformation is significantly improved.

If there's interest, I could maybe do a quick video tutorial showing how I did it.

Basically, there are two testicle objects inside the scrotum. These are parented to their own bones, which are then parented to the base mesh's ball bones. (You can omit this step and parent the testicle objects directly to the armature's ball bones, but it's often useful to be able to move the testicle objects separately. Parenting them to their own bones means you can use custom bone shapes to select them with and move them around while in pose mode without needing to tab out of the rig and move them separately.) Then there are 2 shrinkwrap modifiers applied to the base mesh, one for each testicle. This is done using 2 vertex groups so each side of the scrotum shrinkwraps to its respective testicle (and so the rest of the mesh ignores the shrinkwrapping obviously). The shrinkwrap is set to a small value to project to the outside of the testicle objects. The shrinkwrap modifiers then must be placed after the armature modifier, but before the subdiv modifier (if you're using one).

This works because the armature moves the mesh into its rough position, and then the shrinkwrap modifiers are just making a final adjustment to wrap the mesh around the ball shapes. It works surprisingly well! You can really stretch the balls pretty far apart, and (while looking rather painful), the scrotum retains a natural shape with the balls stretched to each side and the loose skin collapsing in across the middle. You can also clip the testicle objects into each other, which gives the scrotum a tighter more round appearance. It also allows for nice finer adjustments like having one ball sag lower than the other which is really hard to do with an armature that uses only bones.

Since I got this working, I think I'm going to apply it to all my rigs going forward. It's simple to set up, and while not perfect (well, what rig ever is?), it's so much easier to animate with and gives such a better result that there's no reason not to use it. I've been struggling with this on Pulsar's rig ever since I made him, and I'm really excited to have a solution now for so many of the problems I ran into! I need to redo Pulsar's rig anyway for a bunch of reasons, and this gives me yet another reason to work on it.