Blending the length of a bone Between other bones

#1

Hi there,

I am creating a three bone IK/FK setup for a rig. It has one for FK, one for IK and one as a master chain that blends between both.
Blending the rotations between the bones was working out okay but I am stumped trying to blend between the lengths of the IK and FK bones. (which vary a little due to stretch)

Maybe I have lost my mind but I cant think of a way to blend between these two values… If anyone knows a good way I’d love to hear it because I think I have come up against this before and it seems as though it should be simple.

Any Idea’s??
Thanks
Pete

#2

An expression on your 3rd chain’s bones (it’d be a slave btw not a master)?
If you have a blend parameter 0-1 something like:
lenghtBoneSlave = lengthBoneChainB + (lengthBoneChainB - lenghtBoneChainA)*Blend

Unless I’m mis-understanding the problem.

#3

oh yeah, it is the slave.
I had to tweak it a bit but that worked! Thanks.
I ended up with the expression -
lengthBoneChainB + (lenghtBoneChainA - lengthBoneChainB)*Blend
Just had to switch the ones in brackets around.
Thanks!

#4

Wait a sec, I’ve a feeling that would only work if a was never going to be a value lower than B… It that right?
I’ve got a cold and it’s slowing my brain down… more so than usual…

#5

it will work both ways.
If B exceeds the length of A the way you ordered it the number multiplied by the blend becomes negative, and adding it to the length of A becomes a subtraction.
The case you describe (never getting lower than) actually would require a clamping operation.

Just try it :slight_smile:

#6

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