Getting Started with Universal Scene Description (USD)

USD (Universal Scene Description) is an open-source framework developed by Pixar for describing, composing, and collaborating on 3D scenes. It is widely used in animation, VFX, and industrial applications for its scalability and interoperability.

Key Features

• High-performance 3D scene interchange
• Extensible schemas for geometry, shading, lighting, and physics (CAE datasets)
• Powerful composition and layering system
• Python and C++ APIs
• Open-source and industry-supported

Reference: USD Documentation

USD File Types

• .usda: ASCII format (human-readable)
• .usdc: Binary format (optimized for speed and size)
• .usd: Can be ASCII or binary
• .usdz: Compressed archive format (zip)

Basic Concepts

• Stage: The root of a USD scene, manages the scene graph and composition.
• Prim: A node in the scene graph (e.g., Mesh, Xform, Scope).
• Attribute: Data associated with a prim (e.g., points, normals).
• Relationship: Links between prims (e.g., material binding).
• Schema: Defines the structure and behavior of prims (e.g., UsdGeom, UsdShade).

Example: Minimal USDA File Structure

#usda 1.0
(
    defaultPrim = "MyScene"
    upAxis = "Y"
)

def Xform "Root" {
    def Mesh "Plane" {
        point3f[] points = [(0,0,0), (1,0,0), (1,1,0), (0,1,0)]
        int[] faceVertexCounts = [4]
        int[] faceVertexIndices = [0,1,2,3]
    }
}

Python Example: Creating a Simple Mesh

This example shows how to create a USD file with a single plane mesh using Python.

from pxr import Usd, UsdGeom, Sdf, Gf

# Create a new USD stage
stage = Usd.Stage.CreateNew("plane_example.usda")

# Define a plane mesh
mesh = UsdGeom.Mesh.Define(stage, "/World/Plane")
mesh.CreatePointsAttr([
    (0, 0, 0), (1, 0, 0), (1, 1, 0), (0, 1, 0)
])
mesh.CreateFaceVertexCountsAttr([4])
mesh.CreateFaceVertexIndicesAttr([0, 1, 2, 3])

# Save the file
stage.GetRootLayer().Save()

Next Steps

• Explore USD schemas for materials, lights, and cameras
• Learn about USD composition and layering
• Try visualizing your USD files in a compatible viewer (e.g., NVIDIA Omniverse, usdview)
  • Setup instructions: Omniverse Kit App Template
  • Follow README for installation and usage
  • Setup USD Composer App to visualize and edit USD files.
  • Load your created USD files into the USD Composer App.
• Experiment with more complex geometry and attributes

References

• USD Glossary
• Pixar USD GitHub