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[Sidefx-houdini-list] Forward Kinematics in Houdini
Monday, 3 May 2010 Mon, 3 May '10
Pablo Gim?nez wrote:
Another point related with bones is orientation. In Maya, joints has a kind of local transformation, so you can change the joint orientation without changing the rest of the chain, I think is called Joint orientation. How can I achieve the same effect with Houdini bones.You have to be very careful here because intrinsically, joints and bones are different. In Houdini, the bones are just regular objects with some added parameters and geometry for working with the character tools. Also note that bone objects have a fixed transform and rotation order. In Maya, capture weighting is done relative to the joints themselves and there was no notion of capture regions. They've since added capture regions so I don't know how it actually works these days. The "bones" that you see in Maya are in effect these line segments that just connects the joints, ignoring their orientation. In short, there's no easy answer because there are fundamental differences between them. The closest thing is to use Null objects to represent joints and then attach bone objects as their children which will always point to the next "joint". Of course, this means that it will be hard to get this type of setup to work with the character tools because they were designed for a different setup.
Lets say I have a leg bones chain, and I want to orient the first bone so the positive X rotation will move it forward, but after drawing the whole chain my "forward" rotation is using -X. If I then rotate the bone in the Z axis 180 degrees then I got the proper rotation for the first bone but it affect the rest of the chain reversing all the children bones. So what is the best way to change bones rotation locally, not affecting the rest of the chain so I can choose what axis is going to drive the rotation i need for every bone movement?The best way is to just create the bone chain with the desired orientation in the first place. Draw the bone chain using the appropriate viewing plane (ie. one of the ortho planes). Barring that, you will need to use the "Keep Position When Moving Parent" toggle in Main Preferences > Objects and Geometry. Regards, -Edward