Here's an interesting observation.  We've all used Poser IK with the legs to translate the hip body part so that the figure, say, sits or kneels.  With some observation, it appears that Poser treats leg IK as special and sets up a so-called 'secondary goal' which would be the Goal (foot) origin (whereas the Goal body part's endPoint is the primary goal).  So, as you translate the hip upwards the secondary goal is allowed to 'translate'.  But when you translate the hip downwards, the foot origin eventually 'coincides' with the secondary goal (the foot origin stored when IK is enabled) and the foot is 'planted' to this position.

The question arises as to why the foot origin never goes past this.  It is allowed to translate in one direction but not the other.  Is this possibly some 'Up-Vector' (+Y for the most part)?

My visual model seems to be that the local Y is used as such and determines which direction (+/-Y) determines translatability or not.  This would mean that when the delta tries to go negative (relative to the secondary goal), the secondary goal is 'limited' so as to be planted whereas when the delta is relatively positive it is allowed to translate.

What do you think?

C makes it easy to shoot yourself in the foot. C++ makes it harder, but when you do, you blow your whole leg off.

 -- Bjarne Stroustrup

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