View Full Version : weird spine IK flipping

01 January 2009, 06:48 AM
I have an IK spine set up with an extra FK joint chain to help bend the spine in an FK manner.

When i bend the controller that governs that that FK spine, the spine IK handle, when it reaches a certain point, will twist around and in turn screw up the arms and head.

I assume this is the dreaded 'arm flip' issue. Regardless, i still would like to figure out some way to fix it.

01 January 2009, 02:34 PM
Are you using the advanced twist for SplineIK? describe how you setup your spine. Take a look at something i've come up with to get past the flipping issue.

01 January 2009, 08:18 PM
i don't think i've used advanced twist. I basically followed the spine setup used in Art of Rigging volume 1.

i made a joint chain composed of the root and it leads to the head. after root is 'spine1'

from there i made an IK spline from spine 1 to lets say spine 12. The curve was set to have 5 CVs, and each one was given a cluster, so a total of 5 clusters.

What follows is a complicated network of parenting and constraints, one that i'm not really going to be able to list at the moment. The top clusters are in some way connected to the upper back control, the lower two connected to a 'hip control'. The middle cluster is parent constrained to both the top and bottom controllers.

The controllers themselves are connected (with offset nodes) to the skeleton itself, to ensure they 'follow along' when the skeleton bends.

Now there's the FK spine, which is a 4 joint chain labled spineA, spineB, spineC and spineEnd. The clusters are parent constrained to these bones in such a way as to when the FK spine is rotated, the whole upper body bends in an FK manner.

After that, an expression is written to drive the IK handle's twist attribute.... And after looking at my file to help narrate this, i realize that the twist value changes when the FK spine tilts down over 60 degrees in the X axis.

Anyway, i hope that information helps.

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01 January 2009, 08:18 PM
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