After doing many tests trying to have a stable sim with branches, I went back to the method described on CGWiki where the root points have a Pin Constraint and their positions and orients are updated from the source geo before going into the sim. I applied this to multiple hierarchies of branches, and while this works great, I'm confused why the hair constraint only maintains stiffness if the root has a Pin constraint applied.

Does anybody have any information on this?


While it is stable, it would still be better and more realistic to have the branches able to influence the hierarchies underneath them.


I made some custom bend twist constraints and they kept the structure pretty well ( https://www.sidefx.com/forum/topic/75278/?page=1#post-322285 [www.sidefx.com] ) , but the motion suffered a bit and didn't seem to be able to deform in a natural way along the length of the branch. One thing I learned from these tests, is that since the BendTwist constraints use the “orient” channel of the points and a “restvector” on the constraint prim, we are limited in how much we can constrain an object since you can only have ONE BendTwist to keep an orientation. Right now it is either you create your own orientation like I did with the custom BendTwist, or you can maintain an orientation between points on a polyline.