usdgeom-mesh-normals-must-be-valid#
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#
Valid: Unit normals with each interpolation#
#usda 1.0
def Mesh "VertexNormals"
{
int[] faceVertexCounts = [4]
int[] faceVertexIndices = [0, 1, 2, 3]
point3f[] points = [(0, 0, 0), (1, 0, 0), (1, 1, 0), (0, 1, 0)]
normal3f[] normals = [(0, 0, 1), (0, 0, 1), (0, 0, 1), (0, 0, 1)] (
interpolation = "vertex"
)
uniform token subdivisionScheme = "none"
}
def Mesh "IndexedFaceVaryingNormals"
{
int[] faceVertexCounts = [4]
int[] faceVertexIndices = [0, 1, 2, 3]
point3f[] points = [(0, 0, 0), (1, 0, 0), (1, 1, 0), (0, 1, 0)]
normal3f[] primvars:normals = [(0, 0, 1), (1, 0, 0)] (
interpolation = "faceVarying"
)
int[] primvars:normals:indices = [0, 0, 1, 1]
uniform token subdivisionScheme = "none"
}
def Mesh "UniformNormals"
{
int[] faceVertexCounts = [4]
int[] faceVertexIndices = [0, 1, 2, 3]
point3f[] points = [(0, 0, 0), (1, 0, 0), (1, 1, 0), (0, 1, 0)]
normal3f[] normals = [(0, 0, 1)] (
interpolation = "uniform"
)
uniform token subdivisionScheme = "none"
}
def Mesh "ConstantNormals"
{
int[] faceVertexCounts = [4]
int[] faceVertexIndices = [0, 1, 2, 3]
point3f[] points = [(0, 0, 0), (1, 0, 0), (1, 1, 0), (0, 1, 0)]
normal3f[] primvars:normals = [(0, 0, 1)] (
interpolation = "constant"
)
uniform token subdivisionScheme = "none"
}
Valid: Mesh without normals#
#usda 1.0
def Mesh "QuadWithoutNormals"
{
int[] faceVertexCounts = [4]
int[] faceVertexIndices = [0, 1, 2, 3]
point3f[] points = [(0, 0, 0), (1, 0, 0), (1, 1, 0), (0, 1, 0)]
uniform token subdivisionScheme = "none"
}
Valid: Subdivision mesh without normals#
#usda 1.0
def Mesh "SubdivisionQuad"
{
int[] faceVertexCounts = [4]
int[] faceVertexIndices = [0, 1, 2, 3]
point3f[] points = [(0, 0, 0), (1, 0, 0), (1, 1, 0), (0, 1, 0)]
uniform token subdivisionScheme = "catmullClark"
}
Invalid: Triangle with inconsistent winding order and normals#
#usda 1.0
(
metersPerUnit = 1.0
upAxis = "Z"
)
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"
}
Invalid: Normals with non-finite components#
#usda 1.0
def Mesh "QuadWithNonFiniteNormals"
{
int[] faceVertexCounts = [4]
int[] faceVertexIndices = [0, 1, 2, 3]
point3f[] points = [(0, 0, 0), (1, 0, 0), (1, 1, 0), (0, 1, 0)]
normal3f[] normals = [(0, 0, 1), (inf, 0, 1), (0, nan, nan), (0, 0, inf)] (
interpolation = "vertex"
)
uniform token subdivisionScheme = "none"
}
Invalid: Normals that are not unit length#
#usda 1.0
def Mesh "QuadWithNonUnitNormals"
{
int[] faceVertexCounts = [4]
int[] faceVertexIndices = [0, 1, 2, 3]
point3f[] points = [(0, 0, 0), (1, 0, 0), (1, 1, 0), (0, 1, 0)]
normal3f[] normals = [(0, 0, 1), (1, 0, 1), (0, 1, 2), (0, 0, 177)] (
interpolation = "vertex"
)
uniform token subdivisionScheme = "none"
}
Invalid: Number of normals that does not match the interpolation#
#usda 1.0
def Mesh "QuadWithMissingNormals"
{
int[] faceVertexCounts = [4]
int[] faceVertexIndices = [0, 1, 2, 3]
point3f[] points = [(0, 0, 0), (1, 0, 0), (1, 1, 0), (0, 1, 0)]
# Face-varying interpolation requires one normal per face corner
normal3f[] normals = [(0, 0, 1), (0, 0, 1), (0, 0, 1)] (
interpolation = "faceVarying"
)
uniform token subdivisionScheme = "none"
}
Invalid: Normals on a subdivision mesh#
#usda 1.0
def Mesh "SubdivisionQuadWithNormals"
{
int[] faceVertexCounts = [4]
int[] faceVertexIndices = [0, 1, 2, 3]
point3f[] points = [(0, 0, 0), (1, 0, 0), (1, 1, 0), (0, 1, 0)]
normal3f[] normals = [(0, 0, 1), (0, 0, 1), (0, 0, 1), (0, 0, 1)] (
interpolation = "vertex"
)
uniform token subdivisionScheme = "catmullClark"
}
Invalid: Normals on a mesh without a subdivision scheme#
#usda 1.0
def Mesh "QuadWithNormalsAndNoScheme"
{
int[] faceVertexCounts = [4]
int[] faceVertexIndices = [0, 1, 2, 3]
point3f[] points = [(0, 0, 0), (1, 0, 0), (1, 1, 0), (0, 1, 0)]
# The subdivision scheme is not authored, so it is catmullClark
normal3f[] normals = [(0, 0, 1), (0, 0, 1), (0, 0, 1), (0, 0, 1)] (
interpolation = "vertex"
)
}
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.