[Sidefx-houdini-list] Forward Kinematics in Houdini

Pablo Giménez pablogipi at gmail.com
Tue Jun 1 14:48:54 EDT 2010

2010/5/3 Edward Lam <edward at sidefx.com>

> Pablo Giménez 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.
Is there any way to enable/disable this option using Python or Hscript???

> Regards,
> -Edward
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Un saludo
Best Regards
Pablo Giménez

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