[Sidefx-houdini-list] Forward Kinematics in Houdini

Pablo Giménez pablogipi at gmail.com
Fri Apr 30 12:44:12 EDT 2010

2010/4/30 Edward Lam <edward at sidefx.com>

> Pablo Giménez wrote:
> > What is the reason because FK is implemented as individuals IK per
> bone????
> > No kinematics in Houdini is like FK in Maya, why this different
> > implementation, pros and cons???
> > The only reason I can think about is to enable to animate FK using
> > translations of effectors or goal nulls rather than directly rotate the
> bone
> > itself. What are the benefits of this point?
> I think it's mainly just a terminology difference due to historical
> reasons. I'm up for better terminology suggestions. :) In robotics,
> "Forward Kinematics" (FK) is the calculation of the chain goal position
> given the joint rotation angles. And "Inverse Kinematics" (IK), is the
> calculation of the joint angles given the chain goal position.

> "Houdini FK" on the other hand is the calculation of the bone angles
> given the joint positions. From this sense, it's sort of a simpler form
> of IK. On the other hand, there is no real calculation of the goal
> position and so it's more like FK. Maybe it should be called "Per Bone IK".
I think this should be better

> As for pros, I think that "Houdini FK" is more suited to trying to fit
> bone animation to joint animation (eg. given from mocap system). For
> traditional animation rigging, I don't see strong use for it.
>  From an keyframe animation viewpoint, one usually wants to set keys in
> FK so that natural arc motion can be achieved with minimal effort.
> However, if the bone chain needs to be animated with the goal position
> constrained to something (eg. feet/ground contacts, hands opening
> doors), then IK becomes much easier to use. So in practice what often
> happens is that legs are IK and everything else is FK. Then when it
> becomes more convenient to use the other kinematics type, one
> transitions over. Since the InverseKin CHOP naturally supports both
> types with blending between them, if you ever think you need to switch,
> then it is better to create it as an IK chain initially and change the
> Blend parameter to 0 by default and remember to expose this parameter to
> the animated in the HDA. :)
The blend  parameter is great, auto FK/IK switch, nice
But for animators FK means animate rotating the joints, rather than using
So I think would be better a chzange in terminology in Houdini to adapt it
better to animation standars.
Another thing, what is the different between InverseKin CHOP and IKSolver.
The performance is the same?
When is better to use one or the other?
And finally using InverseKin it doesnt explicitelly overrides nothing in the
bones parameters, so I guess it is puting the rotation transforms in the
bone's pretransform????

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

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