Breast Physics in Motionbuilder (Simple & Effective)

Today I wanted to share a quick tutorial on creating believable breast physics inside Motionbuilder.

Here is the result we are going to achieve:


This method is quite versatile and can be used for simulating parts of clothing, accessories and more.

In this first preview clip you can see we have our performer rotating from side to side with no physics applied. It doesn’t seem very believable, we want some movement, not over the top movement just subtle jiggle.


To correct this we must first make sure our model is up to the challenge and that it has the correct bone structure that we require.

We need to ensure the breasts have 2 joints and are skinned. The joint layout should stem from the chest/spine bone and you can call it left_breast, right_breast.

Each breast should contain a child end bone which is like:

left_breast -> left_breast_end
right_breast -> right_breast_end.

The end bones should be positioned on the nipple, for complete control.

Now that we have made sure our skeleton is ready, we can set up our constraints in Motionbuilder.

Go to the Asset Browser and Click on Relation Constraint and drag it to the left_breast_end bone and constrain it. Repeat this step for the right side.


Now we have two relation constraints setup but we need to connect them correctly for the breast physics to work.

Double click on one of the relation constraints and you should see the Relation Node Graph.
Let’s drag the breast_end bone to the graph again.

You should have two there now. An input and an output for the bone.

This is where the magic happens.

In the Relation Browser, expand Other category and find the node that is called:
"Damping (3D) (Clock Based)"

Drag this into the node graph.

We want to drag from the input translation and connect it into the P [Vector] slot. Now drag the resulting vector to the Output translation - it should look like this now.

If you playback your scene it will look the same as our preview at the start, nothing is moving.
We need to create the jiggle now.

Right-click on the Damping Factor [Number] and set a low value of 8.
This will be the playback result:


Okay, the result is super subtle you probably can’t see much difference from the original preview.
We need to raise the Damping Factor again, try a value of 16.


This is a much better result, a weighted jiggle effect when the actor moves side to side.

Finally, let’s try a value of 18. This is what I used for the final sequence here:


And we are done, ultimately it’s up to you to tweak the values to your own liking for the project’s use case.

Thanks for reading this tutorial.

If you have any questions or feedback or even suggestion please leave them below :smiley:

Also let me know if you’re interested in getting a sample motionbuilder scene to play with :+1:t3: