Skinning and blend shapes

A skinned mesh is drawn by a SkinnedMeshRenderer (class 137) that lists the bone Transforms (m_Bones); the mesh stores, per vertex, which bones influence it and by how much, and per bone a bind pose. Sourced · Unity scripting reference

Influences

A mesh can hold its influences in two forms, and the engine draws one of them:

• Blend channels. BlendWeight (channel 12) and BlendIndices (13) hold up to four influences per vertex. Weights are Float (0), Float16 (1), UNorm8 (2) or UNorm16 (4); indices UInt8 (6), UInt16 (8) or UInt32 (10). An indices channel without a weights channel is one influence of weight 1: a rigid bind of each vertex to one bone. A weights channel without indices is not a valid skin. Observed · SkateBIRD, Sledding
• Variable bone counts. m_VariableBoneCountWeights.m_Data is a list of u32 words. The first nv + 1 words (for nv vertices) are start offsets, as word indices into m_Data: offset[0] = nv + 1 and offset[nv] is the length. Vertex v’s influences are the words offset[v] to offset[v+1], each a bone index in the low 16 bits and a weight in the high 16 bits as a normalised unsigned 16-bit value (value / 65535), sorted by descending weight and summing to 1. Observed · Sledding

A mesh with variable bone counts also fills the blend channels with only its first four influences, renormalised. For Sledding Game’s toad body mesh, vertex 0 has eight influences (.4731, .2608, .1681, .0373, .0331, .0234, .0038, .0005) and the channel holds .5036, .2776, .1789, .0397. Observed · Sledding

Which form is drawn

QualitySettings.skinWeights limits the influences the engine uses: 1, 2, 4, or 255 for unlimited. Unity draws the variable data only at unlimited weights, and the four-influence channels (cut and renormalised) at four or fewer. Sourced · Unity scripting reference

Game	skinWeights per quality level	Which form is drawn	Confidence
Sledding Game	255 (unlimited) at all three levels	the variable data (21 meshes of the 129 SkinnedMeshRenderers have it)	Observed · Sledding
SkateBIRD	4, and 2 in the lowest two levels	the channels; its meshes never have variable data	Observed · SkateBIRD

Bind poses

m_BindPose is one Matrix4x4 per bone, stored row-major, taking the renderer’s local space to the bone’s local space at bind time. A bone slot in the influences indexes the renderer’s m_Bones. Observed A zero-weight influence can name a slot beyond the bind poses; its weight is zero, so it does not matter. Observed

Blend shapes

m_Shapes holds a mesh’s blend shapes in four arrays: channels (a name, the CRC-32 of the name that an animation curve refers to it by, a first frame and a frame count), shapes (one per frame: its first vertex entry, entry count, and whether it carries normal and tangent deltas), vertices (sparse deltas: the vertex index and the position, normal and tangent deltas) and fullWeights (the weight each frame is at). Weights run from 0 to 100 for a full shape. Observed · Sledding The deltas apply to the mesh as stored, before skinning. Sourced · Unity scripting reference

An animation clip drives a blend shape with a curve on the renderer, bound by the renderer’s path hash and the shape’s name hash (animation clips). Observed · Sledding

SkateBIRD’s bird

SkateBIRD’s bird rig lives in sharedassets0.assets; the controller, avatar and clips of the bird are described in animator controllers. Skinned renderers account for 386 of the 86,103 objects of the playground. Observed · SkateBIRD