How to convert Sim4 rig into Common rig

The Sims4 rig has a different shape than the rigs used in other common games.

Here I will write a very simple way to convert it to a common rig step by step. This entry is only about poses (body), I'm not going to write about facial expressions because I'm not familiar with it yet myself.

(I'm using Blender 3.0 here, but it's almost the same in 2.7)

(Blender 3.0 has added some useful features, especially "Local Space (Owner Orientation)" is much convinience for converting poses/animations from others, so it may be good to go get used to it.)

(However, there is a change in the save format from Blender 2.7 which supports Sims4Studio, so you need to be careful. I will write about it at the last)

Okay, here we go!

1. Use Sims4Studio to export animation file any you want. (In case of simply need a rig, a blank file is probably the good choice.) 

Then open that file in Blender 3.0.

2. Move layer only for bones you need for posing, or hide all other bones (moving layer will be more convenient later on. Moving a layer )

Bones required for the body pose are as follows:

b__Head__

b__Neck__

b__Spine0__ ~ b__Spine2__

b__Pelvis__

b__ROOT_bind__

b__L/R_Clavicle__

b__L/R_UpperArm__

b__L/R_ForeArm__

b__L/R_Hand__

b__L/R_Thumb0__ ~ b__L/R_Thumb2__

b__L/R_Index0__ ~ b__L/R_Index2__

b__L/R_Mid0__ ~ b__L/R_Mid2__

b__L/R_Ring0__ ~ b__L/R_Ring2__

b__L/R_Pinky0__ ~ b__L/R_Pinky2__

b__L/R_Thigh__

b__L/R_Calf__

b__L/R_Toe__

3. Prepare a common rig. Common rig can be anything you want. Rigify bundled with Blender is very powerful, and of course you can make your own rig from scratch. 

For the sake of Explain, I use a simple VRM rig in this time: VRM Addon 

If you enable VRMFormat in the blender addon, "VRM humanoid" will be appeared to "add" menu in object mode.

4. Move bones of Common rig so that they match location bones of Sims4 rig.

(The VRM rig not have spine bones enough, then I'm going to subdivided chest bone to increase the number. Subdivide )

(It is useful to align the rotation axis of the bones of each part after the movement is finished. Bone roll )

5. Attach bones of Common rig and bones of Sims4 rig.

There's 2 ways.

#1 Combine Common rig and Sims4 rig. -> After "Parenting of bones" (write in ch-7), then done. This is easy but for case you need to modify it after, I recommend #2.

#2 Make a duplicate of the bones of Sims4 rig, merge them into a Common rig, and put a rotation copy constraint on bones of Sims4 rig. -> I will explain this way next.

6. Create a duplicate of bones for pose listed above and separate it into a separate object. Duplicate Separate

(shortcut/ Shift+D, num0, Enter, P, Enter)

Then combine duplicated bones into Common rig. Join

7. Parenting of bones. Common rig bone is "parent" and Sims4 duplicate bones is "child".  Parenting 

(shortcut/ select child bone, Shift+select parent bone, Ctrl+P, choose "keep offset" Enter)

8. Attach a rotation copy constraint to Sims4 rig bones. In Pose mode, add a rotation constraint to each bone to a duplicate bone of the same name. Duplicated bones is "Target" and Sims4 rig bones is "Owner".  Copy Rotation

(shortcut/ select target bone, Shift+select owner bone, Ctrl+C, choose "Copy Rotation")

9. Done!

So easy to work with, but so tedious and time-consuming!

I'd like to give you the file I made for this explanation. Please take a look at it to understand how it works!

*require Blender 3.0 higher* >>>Download<<<

As a bonus for reading my poor explanation, I applied some adjustment constraint for the deform support bones. ("twist" bones.)  It make the pose look a little more natural perhaps.

Notice:

There are some things to keep in mind to use poses created with Common rig for in-game.

- Be sure to enter keyframes with "visual rotation" (visual location and rotation for b_ROOT) in order to save and reproduce the poses.

- Blender 3.0 save file cannot be read by Sims4Studio, necessary convert pose to a 2.7 file. open the blank file in 2.7 and append action from Blender 3.0 save file. Append from other .blend file
 (and I write simple How to.)

If there is anything you want to know, please ask freely. I may not be able to answer all questions as I am still learning and English is not my first language, but I will try to answer as much as I can.

I hope this helps someone else.

Thank you!