Rigging a character can be done in a million different ways, many of which involve using shape keys. In Blender 3.1, a tiny utility function was added, which I believe can benefit many of these workflows and enable the creation of entirely new ones.
While working on the upcoming Snow character rig, I wrote an add-on around this new functionality, here's a video that contains everything there is to know about it:
Click here for a link to this video only.
See the bottom of this page to download the addon.
In this article I go over a bit more of what this is and why you should care if you're a rigger!
Current Corrective Shape Key workflows
Most shape keys are a correction sculpted on top of prior deformation. For example:
The character's elbow is bent using an Armature, then a shape key is used for additional tweaking.
Two facial shape keys look beautiful on their own, but when both of them are activated, the result is terrible. So, you create a 3rd shape key which only activates when both of the previous ones are activated at the same time.
Both of these cases are Corrective Shape Keys; They are just a Delta added on top of a Deformed Mesh, in order to correct the deformation and create a Beautiful Mesh.
Here are those states of the mesh visualized with a bent cylinder:
Now, once you've sculpted your mesh into its Beautiful Mesh state, you probably don't want to lose that exact state. But currently, that state is not stored explicitly anywhere. This means that if you change any of the data used to arrive at that state, you won't arrive at the same state anymore. Here I changed the vertex weights of the cylinder:
This means you can only start working on your corrective shape keys once you are 100% sure that you will not touch any vertex weights, bone constraints, etc any more. If you find out something is broken after having made your correctives, you either fix it and re-make all your corrective shape keys, or you simply don't fix it. That's pretty bad!
So why not just store the Beautiful Mesh!?
You could do this already, but remember, at the end of the day you need to calculate a Delta, which, when added on top of an arbitrary Deformed Mesh, should result in your Beautiful Mesh. So, to calculate this Delta, you just need to do some algebra, right? Delta = Beautiful Mesh - Deformed Mesh!
But many people who tried this before had to realize that it's not so simple. I won't go into why exactly, but as of Blender 3.1, this actually IS possible, thanks to CrazySpace being exposed to the Python API.
So it works! But how can it be used?
Probably in more ways than I can even think of. Below are some workflow examples that the add-on enables.
1. Basic Workflow: Rig first, correct deformations later
Save a pose whose deformation you want to correct (eg. Elbow Bend)
Create a shape key and sculpt on it
Save the resulting Beautiful Mesh
Feel free to make modifications to the vertex weights, bone constraints, etc, to achieve better interpolation into the pose
Re-apply the saved Beautiful Mesh, so the combination of your new weights and etc. still combine into the same final shape that you had before.
2. Reverse Workflow: Sculpt first, rig later
A character artist can sculpt facial expressions to great quality and detail. This can be your Beautiful Mesh.
You pose the rig to be as close to this sculpted shape as possible, and create a rig control that blends into this pose using Action Constraints.
You can now create a shape key that will blend into the exact pose created by the character artist.
(Just make sure your shape key activates at the same time as the Action Constraints!)
As always, you can change whatever you want in the rig, and you will be able to restore the shape.
3. No bones, just shape keys
Create a facial rig based entirely on shape keys and corrective shape keys.
Realize that one of the lower level shape keys looks terrible. But there are already 20 other shapes that build on top of this!
No worries, just make your fixes where you need to.
When you're done, you can fix the other 20 shapes with the click of a button.
4. Author finger correctives 24 at a time
Create a pose where all finger bones (not the thumbs in this example) are bent by 90 degrees.
Sculpt the desired fist shape on all the fingers.
Apply the single sculpted shape to 24 shape keys with a single click.
(Although you still have to create the vertex group masks and the drivers manually)
You can now tweak and iterate on the sculpted fist shape, since you can still update all 24 shape keys with a single click.
If you've read this far, you should probably check out the video above, where I showcase many of these workflows and features.
For symmetry, you need two copies of the same Shape Key.
If it affects the center-line (eg., Neck Twist):
Enable "Mirror X" option on one of them
When you update the Shape Keys, the one with Mirror X enabled will have the corrective deformation mirrored around the X axis.
If it doesn't affect the center-line (eg. Elbow Bend):
Create vertex groups to mask left/right sides of the mesh.
Mask one Shape Key to each side.
Both Shape Keys store the deformation of both sides, each of them masked to only affect one side.
Sky's the limit!
Note though, that this add-on only contains my ideas for how this tech could be used. The core of this whole idea however, is simply the ability to deform already deformed meshes in a predictable/intuitive way. It's the same code that Blender uses to let us sculpt on deformed meshes. So, if you have ideas for what else this could be used for, and you're an add-on developer, you should try to see what you can do with it!