Primvar

Primvar stands for primitive variable, an attribute attached to a geometric primitive in a scene description framework. It is used to store data that can vary across a surface, such as per-vertex colors or texture coordinates, and is consumed by shading and geometry processing pipelines.

In USD, primvars are exposed as UsdGeomPrimvar objects authored on a prim. Each primvar has a name, a data array, and an interpolation rule that describes how the values are interpolated across the primitive’s surface. Common interpolations are constant, uniform, vertex, varying, and faceVarying; the latter is typically used for per-face-vertex data like UVs. Primvars may also reference an index array to map data to geometry.

Data types for primvars include scalar values and vectors, such as float, int, GfVec2f, and GfVec3f, as well as colors and other composite types. They are commonly named with the primvars: prefix (for example primvars:displayColor or primvars:st). Shaders read these attributes to determine material appearance, geometry shading, or procedural effects.

Usage and scope: Primvars carry attributes from assets and formats into USD scenes and are a central mechanism for conveying per-vertex, per-face-vertex, or constant data to renderers. They enable flexible attribute conveyance, such as colors, normals, texture coordinates, and custom data, across a variety of geometric primitives within a scene.