mesh-xform-positioning#

Code

VG.023

Validator

Compatibility

core usd

Tags

✅

Summary#

Meshes should be positioned using transformations, not by embedding positions into point positions.

Description#

Transformations should be applied through USD’s transformation stack (xform ops) rather than being baked into the mesh point positions. This maintains flexibility for transformation operations and enables memory de-duplication for re-occuring meshes.

Why is it required?#

  • Preserves mesh data integrity and reusability

  • Allows for proper transformation manipulation in applications

  • Enables efficient memory de-duplication of identical meshes

  • Maintains separation between geometry and transformation data

Note

In OpenUSD, gPrims (such as Meshes, Curves, Xforms, etc.) are all transformable. It is not required to use an explicit Xform prim to apply transformations to a mesh.

In some cases / workflows there might be a conventional choice to use an explicit Xform prim to apply transformations to a mesh. This is not a technical requirement and may lead to performance overheads (see “Leverage transformable gprims” here).

Examples#

#usda 1.0

def Xform "MyObject" (
) {
    # Recommended: Mesh positioned using xform ops
    def Mesh "Geometry" {
        float3 xformOp:translate = (5, 0, 0)
        float3 xformOp:rotateXYZ = (0, 45, 0)
        uniform token[] xformOpOrder = ["xformOp:translate", "xformOp:rotateXYZ"]


        point3f[] points = [(-1, 0, -1), (1, 0, -1), (1, 0, 1), (-1, 0, 1)]
        # Mesh points are in local space, transformations applied via xform ops
    }

    # Not Recommended: Transformation baked into mesh points
    def Mesh "BadGeometry" {
        point3f[] points = [(2.54, 0, -2.54), (7.54, 0, -2.54), (7.54, 0, 2.54), (2.54, 0, 2.54)]
        # Mesh points have transformation baked in, making reuse difficult
    }
}

How to comply#

  • Keep mesh points in local coordinate space

  • Apply transformations using xform ops on xformable prims (xforms, meshes, cubes, etc.)

  • Use UsdGeomXformCommonAPI for consistent transformation handling

  • Avoid pre-transforming mesh vertices during asset creation

  • Separate geometry data from transformation data in the USD hierarchy

For more information#