UsdGeom meshes and cameras: Worked Example — Visualization (NVIDIA-Certified Professional: OpenUSD Development)

Introduction to UsdGeom Meshes and Cameras in OpenUSD

In the context of the NVIDIA-Certified Professional: OpenUSD Development certification, understanding how to work with UsdGeom meshes and cameras is essential for building and optimizing 3D content creation pipelines. This worked example demonstrates a practical approach to creating and manipulating a simple 3D scene using these components.

Scenario

You are tasked with creating a USD stage that contains a geometric mesh representing a simple cube and a camera positioned to view the cube. The goal is to define the mesh geometry, assign it to the stage, and set up a camera with appropriate parameters to capture the scene.

Step 1: Setting Up the USD Stage

Begin by creating a new USD stage, which acts as the container for all scene elements.

• Use Usd.Stage.CreateNew("example.usda") to create a new stage file.

Step 2: Defining the UsdGeom Mesh

The UsdGeomMesh schema is used to define polygonal geometry. For a cube, specify the vertices, face vertex counts, and face vertex indices.

1. Define the points (vertices): A cube has 8 vertices. These are specified as a list of 3D points.
2. Specify face vertex counts: Each face of the cube is a quad, so each face has 4 vertices.
3. Specify face vertex indices: These indices reference the points list to define each face.

Code Snippet

from pxr import Usd, UsdGeom, Gf

stage = Usd.Stage.CreateNew("example.usda")

Define the mesh prim

mesh = UsdGeom.Mesh.Define(stage, "/Cube")

Define cube vertices

points = [ Gf.Vec3f(-1, -1, -1), Gf.Vec3f(1, -1, -1), Gf.Vec3f(1, 1, -1), Gf.Vec3f(-1, 1, -1), Gf.Vec3f(-1, -1, 1), Gf.Vec3f(1, -1, 1), Gf.Vec3f(1, 1, 1), Gf.Vec3f(-1, 1, 1) ]

mesh.CreatePointsAttr(points)

Each face has 4 vertices

faceVertexCounts = [4, 4, 4, 4, 4, 4] mesh.CreateFaceVertexCountsAttr(faceVertexCounts)

Indices for each face

faceVertexIndices = [ 0, 1, 2, 3, # Bottom face 4, 5, 6, 7, # Top face 0, 4, 7, 3, # Left face 1, 5, 6, 2, # Right face 3, 2, 6, 7, # Front face 0, 1, 5, 4 # Back face ] mesh.CreateFaceVertexIndicesAttr(faceVertexIndices)

Step 3: Defining the UsdGeom Camera

The UsdGeomCamera schema allows you to define camera parameters such as position, orientation, focal length, and clipping planes.

1. Create the camera prim: Define a camera under the root of the stage.
2. Set transform: Position the camera at (0, 0, 5) looking towards the origin.
3. Set camera parameters: Define focal length, horizontal aperture, and clipping planes.

Code Snippet

from pxr import UsdGeom, Gf

camera = UsdGeom.Camera.Define(stage, "/Camera")

Set camera transform (translate 5 units on Z axis)

camera.AddTranslateOp().Set(Gf.Vec3d(0, 0, 5))

Set focal length in mm

camera.CreateFocalLengthAttr(50.0)

Set horizontal aperture in mm

camera.CreateHorizontalApertureAttr(20.955)

Set clipping planes

camera.CreateClippingRangeAttr((0.1, 1000.0))

Step 4: Saving the Stage

Once the mesh and camera are defined, save the USD stage to persist the scene.

• Call stage.GetRootLayer().Save() to write changes to the file.

Summary

This example demonstrated how to create a simple USD scene with a UsdGeomMesh representing a cube and a UsdGeomCamera positioned to view the mesh. Understanding these steps is critical for visualization tasks in OpenUSD pipelines, enabling developers to build and optimize 3D content efficiently.

For further study, explore how to integrate usdShade materials and usdLux lights to enhance scene realism.

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