Introduction

You know what a 3D mesh is. You know what a skeleton is. There is a thing in the middle that connects them. That thing is skinning. And it makes or breaks how a character looks when it moves.

What Skinning Is

A mesh is vertices in 3D space connected by triangles. A skeleton is a tree of bones. Without skinning these two are separate. Rotate the bones. The mesh sits there frozen.

Skinning is the link. It tells each vertex which bones to follow.

That is the whole job.

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Vertex Weights

Skinning works through numbers called weights. Every vertex gets a weight for every bone that influences it. A value between 0 and 1.

Weight of 1. The bone fully controls that vertex. Weight of 0. The bone has zero influence. Anything between. Partial influence.

A vertex can be influenced by multiple bones at once. The weights from all bones on a vertex add up to 1.

Example. A vertex on the upper arm.

• Upper arm bone. 0.8

• Shoulder bone. 0.2

That vertex follows the upper arm mostly. The shoulder still pulls on it a little.

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Rigid vs Smooth Skinning

Rigid skinning. Each vertex bound to exactly one bone with weight 1. No blending. Good for robots. Bad for skin.

Smooth skinning. Vertices near a joint get partial weights from bones on both sides. Influence blends across the seam. Good for anything organic.

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What This Looks Like At The Elbow

Bend an elbow with rigid skinning. Upper arm vertices follow the upper arm bone. Forearm vertices follow the forearm bone. The two halves rotate independently. The mesh creases hard at the seam. Looks like folded paper. Sometimes the mesh pokes through itself.

Bend an elbow with smooth skinning. Vertices at the joint have weights split between both bones. Maybe 0.5 and 0.5. Move further from the joint and the weights shift toward whichever bone is closer.

When the elbow bends those middle vertices follow both bones partially. The result is a smooth curve. Skin appears to stretch and fold like real skin.

That is the entire point.