[Sidefx-houdini-list] Forward Kinematics in Houdini

Pablo Giménez pablogipi at gmail.com
Mon May 3 12:21:05 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.
>
Yep that makes the trick, thanks Edward.
I understand the houdini approvimation, and for me is better really.
the good think is that one can think about the pre transform as the joint
orient in Maya.
When you have  "Keep Position When Moving Parent" enabled then this
pretransfoprm allows to orient your bones without affecting the rest of the
chain.
I find this workflow useful because if you know that all the positive
rotations in X are pointing forward, related to the character, then just
looking a t the animation curves yo ucan take an idea of the character
motion.

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



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