Spring IK

IK Spring Solver for Unity.

Normal IK bends a leg in one place. Animal legs bend in two — the knee goes forward, the hock (the backwards-pointing ankle) goes back. Spring IK does that, and lets you choose how much of the bend goes to each one.

Any 3-bone chain: dog and horse legs, bird legs, mech legs.

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Under the hood

• A custom Animation Rigging constraint. The solve runs in a Burst-compiled animation job and writes rotation deltas only, so it layers on top of existing animation instead of replacing it.
• Closed-form limits, fixed-cost solve. SpringIKMath computes the reach limits exactly and solves the bend in a fixed 8 bisection + 4 Newton steps: deterministic and allocation-free.
• One knob that keeps its promise. Spring Angle Bias sets how the bend splits between knee and hock, and that split holds as the leg compresses.
• It tells you why the foot won't reach. A live Inspector readout shows the reachable window and what the current bias costs you.
• Tested. 13 EditMode tests on the solver. Every sweep also checks that it really hit the case it claims to test.

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Install

Window → Package Manager → + → Install package from git URL… and paste:

https://github.com/Garegni/UnitySpringIK.git?path=/UnitySpringIK/Assets/SpringIK#v1.0.0

Needs Unity 6 (6000.0.23f1 or newer). Animation Rigging installs itself along with it. The #v1.0.0 at the end pins this release; leave it off to get the latest changes instead.

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Try the demo

Clone this repo, open the UnitySpringIK folder with Unity 6000.6 or newer, and open Assets/Scenes/SpringIKScene. It's the walk from the GIF above.

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Rig a leg

1. Select your character. Menu: Animation Rigging → Rig Setup. You get a Rig 1 object.
2. Right-click Rig 1 → Create Empty. Call it SpringIK.
3. On it: Add Component → Animation Rigging → Spring IK Constraint.
4. Drag your foot bone into the Tip slot. Only that one.
5. Click the ⋮ menu at the top-right of the component → Auto Setup from Tip Transform.

That's it. It walks up the chain, fills in the other three bones, and creates three objects for you: SpringIK_target, SpringIK_kneeHint, SpringIK_hockHint.

Move SpringIK_target and the leg follows. The two hints control which way each joint bends — move them if a joint bends the wrong way.

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The one knob: Spring Angle Bias

Where the bend goes.

−1	all the bend up top, at the knee
0	split evenly between both
+1	all the bend down low, at the hock

Drag it and watch the leg. You can animate it like any other value.

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If the foot won't reach

The constraint tells you. Select it and look at the Reach (live) box at the bottom of the Inspector. If the target is too close, you get:

Target is 0.600 away — inside the fold limit (1.700) at this bias. The chain holds at maximum fold.

Now drag Spring Angle Bias and watch that fold limit change.

This catches people out: 0 is not always the best setting. On a leg with a short middle bone, 0 is the worst — the readout will say something like "this bias costs 52% of travel" — while 0.75 folds all the way down. It depends on your bone lengths. Drag the slider until the number is small enough and the warning goes away.

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Every setting

Root, Knee, Hock, Tip	the four bones, top to bottom. Auto Setup fills these in.
Target	where the foot should go. Move this to pose the leg.
Knee Hint / Hock Hint	which side each joint bends toward.
Spring Angle Bias	where the bend goes. See above.
Target Position Weight	how much the foot follows the target. 0 = ignore it.
Target Rotation Weight	how much the foot copies the target's rotation.
Knee / Hock Hint Weight	how strongly each hint steers.
Weight	overall strength. 0 turns the constraint off completely.

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Made by José Ignacio Alonso Kuri · PixelCyto · LinkedIn · GitHub · Sketchfab · ArtStation

MIT licensed.