Skinning and Bone Weights: How a Character Bends without Creases
The skeleton gives the character the ability to move, but on its own it does not change the surface. For an arm to bend together with its sleeve and a belly to stretch when leaning, you need skinning — the process in which each point of the model is bound to one or more bones.
In Animatic this stage runs automatically. This article explains how it works, how to read the result and what to do if the character deforms incorrectly.
What Skinning Is
Skinning is binding the model's geometry to the skeleton. For each vertex of the surface it is defined which bones influence it and how strongly. For example, a point on the elbow responds more to the forearm, and a point on the biceps — more to the upper arm.
The degree of influence is set by weights: a number from zero to one for each bone. The sum of a vertex's weights is usually one. This is what makes the surface bend smoothly at the joints, without tears or sharp creases.
Why Weights Matter
The same skeleton can produce completely different results depending on the weights. If the weights are distributed poorly, the character will look like this:
- the sleeve breaks or the skin "collapses" when the elbow bends;
- the neck or shoulder stretches when the head turns;
- separate body parts move along with their neighbors, as if glued together.
Good weights make motion natural and require no extra tweaking.
How Automatic Skinning Works
After the skeleton is built, the program computes weights for all vertices of the model. The algorithm takes the bone positions and the shape of the surface into account: points near a joint receive influence from both adjacent bones, while points on rigid areas mostly get a single bone. You don't have to paint weights by hand, as many 3D editors require.
How to Check the Result with the Heatmap
The program can display a weight heatmap. The color on the model shows which bone influences an area: warm colors mean strong influence, cold ones — weak or none.
The heatmap makes it easy to spot typical errors:
- an area of one body part is colored with another bone's color — the weights are mixed up;
- there is no smooth transition at a joint bend — a crease is likely during motion;
- part of the model is not colored at all — it is not bound to the skeleton and will stay in place.
What to Do if the Deformation Is Wrong
- Check the skeleton. Incorrectly placed joints are the most common cause of errors. Fix the rig and repeat the skinning.
- Check the model. Holes, detached pieces and overlapping surfaces prevent weights from being distributed correctly. Clean up the geometry.
- Repeat the skinning. After the fixes, run the computation again and compare the result.
Checking in Motion
The heatmap shows the weights, but the final verdict is only possible in motion. Play a simple pose: bend the arms, lean the torso, turn the head. If there are no sharp creases at the joints and the surface moves smoothly, the character is ready for animation.
What Next
A character with a finished skeleton and weights can be animated. Your options:
- motion transfer from video — an accurate reproduction of real human movement;
- animation from a text description — the motion is defined by words.