I wanted to rig and animate a worm using IK, so that if i move the tip of the tail i get the bend one would expect. Unfortunately using plain IK the tip will point towards the ground instead of staying parallel, easing into the slope of the arched spine.

This is what i end up with:

Failed rigging

Ideally it should look more like a Gaussian function.

I thought that IK limits might help, but they are all relative to the previous bone and not global. To fix this i tried using rotation constraints, however, when the limit values approach usable angles the bend just "flips" along the Z axis so it's bending underground.


Consider using a Spline IK on the armature.

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In the animation above the influence of the constraint is keyframed.

  • Animate the curve as part of your animation.
  • Animate influence of Spline IK constraint as part of your animation.
  • Your bones will flow fluidly under the control of the curve.
  • Save poses. Many options.

Attach this in pose mode.

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Bones in yellow conforming to a curve. Influence is 1.0 full strength. The image above.

No bone stretching is depicted here to have length constant. You are the artist. You can change the setting.

The target curve of the constraint beveled in blue. The image above.

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Constraint influence set to 0.0. No strength. Bone are [parallel] to XY plane [ground]. Image above.

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In the image above you see the curve in edit mode and the bevel for visibility. You need not render the curve.

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In the image above we see only the final bone positions. The last bone was selected for the IK Spline Constraint. The chain length specifies how many previous bones are included.

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Animation data in graph editor for the Spline IK Constraint Influence.

  • $\begingroup$ Thanks for your in-depth answer. Some time after posting my question i also came across this constraint, i'll have to see if this can be made to work sufficiently... $\endgroup$ – H.B. Jan 3 '16 at 19:12

Using the spline IK as suggested by atomicbezierslinger, my final rig now consists of a deform rig using many bones that have the Spline IK constraint applied:

deform rig

And a control rig, consisting of 5 bones to which the control points of the spline are hooked:

control rig

There is an IK constraint that has chain length 2 to control the bending and a rotation limit on the bone after to prevent it from rotating into the ground.


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