usdgeom-mesh-winding-order

Code	VG.029
Validator
Compatibility	core usd
Tags	✅

Summary

The winding order of faces in a mesh must correctly represent the orientation (front/back) of the face.

Description

In order to correctly represent the orientation (front/back) of a face, the winding order of faces in a mesh must be correct. In OpenUSD, the winding order is defined by the faceVertexIndices array of a mesh and is running in counter-clockwise (CCW) order.

Why is it required?

Visualization, culling and other algorithms rely on correct winding order to determine the front/back sides of a mesh. For example, when calculating shadows. Surface normals, which are used primarily for shading, should be consistent with the winding order to avoid potential artifacts.

Examples

Inconsistent winding order

Visual artifacts when the winding order is incorrect in Storm.

# Invalid: Cube with flipped winding order for the top face
#usda 1.0
(
    metersPerUnit = 1.0
    upAxis = "Z"
)

def Xform "World"
{
    def Mesh "Cube"
    {
        point3f[] points = [(-0.5, -0.5, -0.5), (0.5, -0.5, -0.5), (0.5, 0.5, -0.5), (-0.5, 0.5, -0.5), (-0.5, -0.5, 0.5), (0.5, -0.5, 0.5), (0.5, 0.5, 0.5), (-0.5, 0.5, 0.5)]
        int[] faceVertexCounts = [4, 4, 4, 4, 4, 4]

        # Indices for each quad face, CCW by default
        # Let's flip the winding order for the top face (face 5)
        int[] faceVertexIndices = [
            0, 1, 2, 3,      # Bottom (-Z)
            3, 2, 6, 7,      # Front (+Y)
            2, 1, 5, 6,      # Right (+X)
            1, 0, 4, 5,      # Back (-Y)
            0, 3, 7, 4,      # Left (-X)
            4, 7, 6, 5       # Top (+Z) -- FLIPPED WINDING
        ]
    }
}