Tutorial - How to export stretchy bones from 3ds max to unity

Stretchy bones used to squash this chubby low-poly frog named Arnold
from our free android game JuJu Frog made in Unity

In this quick tutorial I will show you how to set up 3ds max stretchy bones (with squash and stretch) and properly export them to Unity game editor.

Setting up stretchy bones in 3ds max:

1. Start by making a simple bone and a nub bone chain
2. Create two point helpers and align their position to each bone (alt + a keyboard shortcut)
3. Open Animation > "Bone tools" dialogue and scroll down to the bottom. Select the main bone and turn off "Freeze Length", turn on "Squash" and press the "Reset Stretch" button
4. Position constrain the main bone to the first point helper. Animation > Constraints > Position Constraint and click the first point helper
5. Add "Look At" constraint to the main bone. With the main bone still selected go to Animation > Constraints > LookAt Constraint and click the second point helper.
6. Position constrain the nub bone to the second point helper. Select the nub bone and go to Animation > Constraints > Position Constraint and click the second point helper

Now you have a fully functional stretchy bone setup in 3ds max. It will not work in Unity yet because squash property of the bone is not supported. We need to convert this information to simple scale transform readable by Unity.

Preparing bones for Unity

1. Create another bone (let's call it bone_unity) and align it to the main bone. Position and orientation constrain the bone_unity to the main bone. I gave it red color so it is more visible.
2. With the bone_unity still selected, go to Motion Panel, and assign "Scale Script" controller. 
3. The Script Controller window will pop up. In it, create a new variable boneMax and click Assign Node. Navigate to the main bone and select it.
4. Now paste this into the Expression text field: 

[boneMax.stretchTM[1][1], boneMax.stretchTM[2][2], boneMax.stretchTM[3][3]]

That's it. Close the Script Controller window and you have your bone_unity matching the squash and stretch of the main bone. It is ready for export! Just export the unity_bone and your mesh, no need to export the main bone and the nub. Also, make sure you select "Bake animation" in FBX export dialogue.

Hope you liked this tutorial. If you have some questions, please share in the comments or contact me directly via e-mail.