UV Layers and Mesh Attributes

Trigger

Use this skill when the user:

• Creates or edits UVs from a script (mesh.uv_layers, bm.loops.layers.uv, calc_uvs=True)
• Builds a second UV channel for baked lighting, or runs uv.lightmap_pack / uv.smart_project on it
• Computes tangents for a normal-mapped game asset (calc_tangents, bitangent_sign)
• Writes vertex colors (color_attributes, the deprecated vertex_colors)
• Authors shape keys or reads a blended shape
• Caps skin weights at four influences per vertex for a game engine
• Reports a textured mesh that renders as one flat color, a UV op that wrecked channel 0, a crash or garbage values after calc_tangents, or weights that sum short of one

High-level shape

Every one of these lives in the mesh's CustomData. Three rules cover most bugs: create the layer before anything that fills it, address layers by name, and re-fetch a layer by name after any call that can reallocate CustomData.