usdgeom-mesh-normals-must-be-valid#

Code

VG.028

Validator

Compatibility

core usd

Tags

✅

Summary#

Mesh normals values must be valid to produce correct shading.

Description#

Mesh normals need to follow a set of conventions to produce correct shading results in most renderers:

  • Sizes of normal arrays shall be consistent with the interpolation mode (faceVarying, vertex, uniform)

  • Normals shall have a length of 1 within a tolerance of 1e-4 (including zero length, nan, inf etc)

  • Normals must align with the front side of their contained face (ie, the normal should point outwards from the mesh as per the winding order)

Why is it required?#

  • Incorrect normals may lead to incorrect shading results

  • Incorrect normals may lead to incorrect simulation results

  • Avoid confusion about the front/back side of the mesh

Examples#

#usda 1.0
(
    metersPerUnit = 1.0
    upAxis = "Z"
)

# Invalid: Triangle with inconsistent winding order and normals
def Mesh "InconsistentWindingOrder"
{
    point3f[] points = [(0, 0, 0), (1, 0, 0), (0, 1, 0)]
    int[] faceVertexCounts = [3]
    int[] faceVertexIndices = [0, 1, 2]  # CCW, so normal points +Z

    # Normal is set explicitly to point in the -Z direction
    normal3f[] normals = [(0, 0, -1)]
    uniform token normalsInterpolation = "faceVarying"
}

def Mesh "ConsistentWindingOrder"
{
    point3f[] points = [(0, 0, 0), (1, 0, 0), (0, 1, 0)]
    int[] faceVertexCounts = [3]
    int[] faceVertexIndices = [0, 1, 2]  # CCW, so normal points +Z

    # Normal is set explicitly to point in the -Z direction
    normal3f[] normals = [(0, 0, 1)]
    uniform token normalsInterpolation = "faceVarying"
}

# TODO: Example with 0 length normals, nan no

How to comply#

  • Ensure that the normals are authored cleanly and correctly.

  • As a developer of converters, ensure that the normals are correctly converted from the source format to the USD format or calculate normals correctly.

For more information#