Why does my 2D character pinch at the elbow when bent?

Vertex weights at the seam are too binary. Vertices belong 100% to either upper or lower arm bone, so the seam crease shows. Use the Skinning Editor's Smooth Weights tool to blend the seam vertices.

What does Auto weight do?

It assigns each vertex weights based on distance to the bone segment, normalized so they sum to 1. It's a good starting point but always needs refinement at high-deformation joints (elbow, knee, shoulder).

How many bones can a vertex be weighted to?

Up to 4 by default in Unity 2D Animation. The Sprite asset's 'Bones Per Vertex' setting controls the limit. Two bones is enough for most rigging; four allows finer blending at complex seams.

Fix: Unity 2D Animation Rig Bone Deformation Twist

Quick answer: Open the Sprite’s Skinning Editor (Sprite Editor → Skinning Editor). Use Auto Weights for a baseline, then Smooth Weights along the seam vertices at every bend. Increase mesh density at deforming joints to give weights room to interpolate.

2D Animation rig with a single arm bone. Bend the elbow — the upper arm pinches at the seam, the forearm crumples at the wrist. The vertices snap from one bone’s influence to the next without smoothing. Weights need a few minutes of paint to look right.

The Symptom

Sharp creases or pinches appear at joints during deformation. The mesh visibly tears or buckles where bones meet. Auto Weights ran but the result is rough.

What Causes This

Auto weights assign by proximity. Vertices near the bone boundary get sharp transitions: 95% upper / 5% lower one vertex over from 5% upper / 95% lower. The 1-vertex transition is too short to look organic.

The Fix

Step 1: Open the Skinning Editor. Select the Sprite asset. Sprite Editor → dropdown → Skinning Editor.

Step 2: Increase mesh density at joints. Geometry → Edit Geometry. Add vertices around bend regions (elbow, knee, neck, hip). More vertices = smoother weight transition.

Step 3: Auto Weights baseline. Weights → Auto Weights with Generate Weights set to Use Selection or All. This gives every vertex a per-bone weight set normalized to 1.

Step 4: Smooth the seam. Weights → Smooth Weights tool. Brush along the joint several times. Smooth Weights averages weights with neighbors, broadening sharp transitions into a few-vertex blend.

Workflow:
  1. Add seam vertices (2-3 rows around each bend)
  2. Auto Weights (default)
  3. Bend the bone with Pose tool, observe the pinch
  4. Smooth Weights brush along the seam
  5. Repeat 3-4 until the bend looks organic

Step 5: Limit influence count. If weights look noisy, reduce Bones Per Vertex (Sprite Importer → Sprite). 2 is fine for limbs; 4 only when you have overlapping bones (cape over body, hair over neck).

Sprite Library Trap

If the rig uses Sprite Library swaps (different art for the same bone hierarchy), the weights are stored per-sprite. Re-bake weights for every sprite category that gets swapped, otherwise some swaps look fine and others pinch.

IK Constraints Help

For limbs that need natural bending, add a 2D IK Constraint with a Limb solver. The IK chain determines bone rotations; well-painted weights then deform smoothly.

Verifying

In the Skinning Editor, drag the bone with the Pose tool through full motion range. The mesh should bend smoothly at every angle without pinching. Save; preview in Scene view.

“Mesh density. Auto. Smooth. Limit influences. The character bends naturally.”

Related Issues

For 2D IK chain length, see 2D IK chain. For Sprite Library swap issues, see sprite library.

Density. Auto. Smooth. Pinches go away.