Questions about character rigging.
How to accurately position rigs (or joints) at the inner center of a mesh?
Setting the Joint Placement Mode to 'View Based' will cause the joints to be centered inside the mesh, but only when they are first created.
What I want to know is, after once the joints are created at a certain location(not centered), how to center them inside the mesh.
(The 'mesh inner center' is not the location estimated by my eyes, but the center of the mesh volume calculated by Houdini.)

You can extract centroids from selected parts of your mesh and then use the resulting points as snapping targets for your joints. Or even as joints themselves after piping them through Parent Joints and Rig Doctor.

https://www.sidefx.com/docs/houdini/nodes/sop/labs--straight_skeleton_3d.html [www.sidefx.com]

Another methods is to use the above node to extract a sort of center-line skeleton and snap the joints onto that.