According to the documentation, muscle simulation has three processes: muscle pass, tissue pass, and skin pass.
By rigging the bone mesh and attaching the muscles to the bones, the basic musculoskeletal system is completed in the muscle pass.
Then, at the beginning of the tissue pass, the skin geometry(character body mesh) is imported.

I have a question here:
If the musculoskeletal(muscle pass completed) and character body(mesh without rigging) have different shapes and poses, how do I make them fit together?
(It appears that tissue pass and skin pass are possible only when the shape and pose of the musculature and the character body match.)
Actually I want to apply one musculoskeletal to various character bodies.

To do that, I think the musculoskeletal should be modified to fit the character body:
1. First, adjust the musculoskeletal pose.
2. Then, fit the skeleton by transforming the scale, rotation, and position of the rigs.
(Because muscle follows bone, and bone follows rig)
3. Finally, detailed fitting is completed by sculpting the shape and volume of the muscle. (in houdini)

Please give me advice on the right way or what I did wrong.


(A Houdini worker named Joe Raasch automated this process, but I don't know how.
Below is Joe Raasch's work. Excerpted from Art Station.)

you would need to use a combination of TopoTransfer and PointDeform like in the following diagram.
TopoTransfer will conform your source skin topology to the shape of your variant models. You would use landmark locations to ensure feature correspondence is maintained.
Once you have generated a transferred topology (probably best to stash the output as it may be expensive to regenerate), then this will serve as a target, ie Deformed Point Lattice, for the PointDeform SOP to operate on the muscle, bones, or kinefx skel geometry.
Some cleanup may still be necessary, but this approach will get you most of the way there.

This workflow is awesome and very important if you want to have a good muscle template already setup and use it for several creatures.
Thanks John for pointing us in the right direction.

I was very happy with the example you attached, it was really helpful. I really appreciate your answer johm.

I have one question when using PointDeform.

When the source (musculoskeleton) is transformed into a target, how should the rigs rigged to the bones be handled?
Considering typical rigging, the skeleton moves along the rig, not the rig moves along the skeleton, so when the musculature is transformed into the target, the rigs are expected to remain in their source positions.
In other words, the rigs deviate from the existing skeleton.
(Please understand that I am currently unable to run my PC.)

To prevent this problem, should I un-rig it before using PointDeform and re-rig it after using PointDeform?
Or, does this problem never occur in the first place?

I hope I understand your question...
If you have an animation skeleton (a joint hierarchy) refit to match your target character, and you have the anatomical skeleton (the modeled bone surfaces) also refit to match your target character, then the joint transforms should drive the bone surfaces as you'd expect. Re-weighting should not be necessary.

johm
transform

Thank you for answer!
Oh, my question was a continuation of my original post and the diagram you provided.
Simply put, “Once the transformation of the skeleton and muscles (the gifs I uploaded) is completed through a combination of TopoTransfer and PointDeform, will the rigs also be transformed correctly?”
Of course, it is assumed that these rigs were already rigged to the skeleton geometry prior to transform work.
So, I was worried that when the skeleton and muscles were transformed to the target, the rigs would remain untransformed or the skeleton and muscle geometry would be stretched. (Images below)
This is because I thought there might be a conflict between the skeleton geometry being transformed to the target and the rig holding onto this geometry.


(Consider the bodies below as skeleton and muscles.)