The 
Inverse Kin CHOP provides many vector constraints. One type of IK constraint is the Twist Affector on the Inverse Kin CHOP. This constraint is used to direct where the first bone points. You can also blend between IK and FK rotations by modifying the Blend slider on the Inverse Kin CHOP.
Blend between multiple IK goals ¶
This setup lets you have multiple IK goals for a bone chain, living in different spaces, so the animator can blend between them depending on her needs at a particular moment. For example, you can set up a hand to be pose-able in character space or shoulder space.
There are advantages and disadvantages to overriding the default Houdini style of one goal per bone chain. See approaches to rigging .
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Create a bone chain with IK.
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Create two Nulls and parent each one to a different space you want to blend between. For example, parent one to the shoulder and one to the character root Null.
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Create a
Blend object and connect the two new
Null objects to its inputs. -
Parent the default goal Null created for the bone chain (e.g.
chain_goal1"to the Blend node.) -
The Blend node has separate Weight sliders for each input, letting you mix them arbitrarily. In a two-parent scenario however, you probably only want one slider control that blends between the two inputs. To do that, you need to link the second slider to the first slider so it automatically goes down/up as the other goes up/down.
Press RMB on the first slider’s value and choose Copy Parameter. Then press RMB on the second slider’s value and choose Paste Relative Reference. Houdini inserts a parameter reference expression to the first slider. Finally, to make the second slider invert the value of the first slider, insert
1-in front of the expression.
Tip
Make the first Weight slider of the Blend node into a HUD slider to make it easily available as a control for animators.