identical-mesh-consistency#

Code	VG.024
Validator	Manual
Compatibility	core usd
Tags	✅

Summary#

Repeated occurrences of identically shaped objects should have identical mesh connectivity

Description#

When identically shaped and parameterized objects appear multiple times in an asset, all occurrences should have identical mesh connectivity - point positions and connections / edges between the vertices. This enables proper memory de-duplication optimizations and primvar usage (texture coordinates, normals, etc.)

../../../_images/identical-meshes.jpg — The locks on this toolbox are identical and should have identical mesh connectivity.#

Why is it required?#

Enables efficient memory de-duplication of identical meshes
Ensures consistent behavior across different instances of the same mesh, for example when applying a textured material or applying mesh decimation operations
Reduces file size and memory usage

Performance Impact Example:

Consider a .usdc file containing 100 identical screw meshes, each with 1000 polygons:

Compliant approach (identical meshes): 1MB
Non-compliant approach (unique meshes with different vertex connectivity): 11MB

This represents an 11x increase in file size when identical geometry is not properly deduplicated.

Examples#

#usda 1.0

# Optimal: Identical meshes with same point positions and vertex connectivity
def Mesh "pPlane1"
{
    float3[] extent = [(-0.5, 0, -0.5), (0.5, 0, 0.5)]
    int[] faceVertexCounts = [3, 3]
    int[] faceVertexIndices = [0, 1, 2, 2, 1, 3]
    point3f[] points = [(-0.5, 0, 0.5), (0.5, 0, 0.5), (-0.5, 0, -0.5), (0.5, 0, -0.5)]
}

def Mesh "pPlane2"
{
    float3[] extent = [(-0.5, 0, -0.5), (0.5, 0, 0.5)]
    int[] faceVertexCounts = [3, 3]
    int[] faceVertexIndices = [0, 1, 2, 2, 1, 3]
    point3f[] points = [(-0.5, 0, 0.5), (0.5, 0, 0.5), (-0.5, 0, -0.5), (0.5, 0, -0.5)]
}

# Suboptimal: Identical meshes with same point positions and different vertex connectivity
def Mesh "pPlane1"
{
    float3[] extent = [(-0.5, 0, -0.5), (0.5, 0, 0.5)]
    int[] faceVertexCounts = [3, 3]
    int[] faceVertexIndices = [0, 1, 2, 2, 1, 3]
    point3f[] points = [(-0.5, 0, 0.5), (0.5, 0, 0.5), (-0.5, 0, -0.5), (0.5, 0, -0.5)]
}

def Mesh "pPlane2"
{
    float3[] extent = [(-0.5, 0, -0.5), (0.5, 0, 0.5)]
    int[] faceVertexCounts = [3, 3]
    int[] faceVertexIndices = [0, 1, 3, 0, 3, 2]
    point3f[] points = [(-0.5, 0, 0.5), (0.5, 0, 0.5), (-0.5, 0, -0.5), (0.5, 0, -0.5)]
}

How to comply#

Use Scene Optimizer Fuzzy Deduplicate to deduplicate meshes with identical shape but different connectivity
When splitting a previously merged mesh into multiple separate meshes, de-duplicate/replace any resulting geometry that should be identical with other geometry. For example bolts and screws or wheels in a vehicle.
When positioning duplicated geometry, don’t do so by moving the points / vertices directly. Instead, apply transformation (xform ops) to the geometry to position it.
Use identical mesh data for geometrically identical objects
Consider using USD’s instancing capabilities for repeated geometry