Rendering and Materials¶

📖 USD Rendering - USD shading documentation

Materials in USD¶

USD Preview Surface¶

The standard material in USD for cross-platform compatibility:

Properties: - diffuseColor - Base color (RGB) - metallic - Metallic factor (0-1) - roughness - Surface roughness (0-1) - opacity - Transparency (0-1) - emissiveColor - Self-illumination color - ior - Index of refraction - normal - Normal map input - occlusion - Ambient occlusion

def Material "SimpleMaterial"
{
    token outputs:surface.connect = </SimpleMaterial/Shader.outputs:surface>

    def Shader "Shader"
    {
        uniform token info:id = "UsdPreviewSurface"
        color3f inputs:diffuseColor = (0.8, 0.2, 0.2)
        float inputs:metallic = 0.0
        float inputs:roughness = 0.5
        token outputs:surface
    }
}

MDL Materials¶

NVIDIA Material Definition Language: - Advanced physically-based material system - Procedural textures and noise functions - Layered materials and complex shading - GPU-optimized for RTX rendering - Extensive material library - 📖 MDL SDK - Material language reference

Key MDL Features: - Mathematical functions for procedural generation - Layer blending (clear coat, subsurface) - Measured material data support - Texture coordinate manipulation - Custom BSDF definitions

Material Binding¶

from pxr import UsdShade

# Create material
material = UsdShade.Material.Define(stage, '/Materials/Red')

# Create shader
shader = UsdShade.Shader.Define(stage, '/Materials/Red/Shader')
shader.CreateIdAttr('UsdPreviewSurface')
shader.CreateInput('diffuseColor', Sdf.ValueTypeNames.Color3f).Set((1, 0, 0))
shader.CreateInput('roughness', Sdf.ValueTypeNames.Float).Set(0.4)

# Connect shader to material output
material.CreateSurfaceOutput().ConnectToSource(shader.ConnectableAPI(), 'surface')

# Bind material to geometry
UsdShade.MaterialBindingAPI(mesh_prim).Bind(material)

Texture Mapping¶

def Shader "DiffuseTexture"
{
    uniform token info:id = "UsdUVTexture"
    asset inputs:file = @./textures/diffuse.png@
    float2 inputs:st.connect = </Material/UVReader.outputs:result>
    color3f outputs:rgb
}

def Shader "UVReader"
{
    uniform token info:id = "UsdPrimvarReader_float2"
    string inputs:varname = "st"
    float2 outputs:result
}

Lighting¶

USD Light Types¶

UsdLuxDistantLight: - Infinitely far away (sun-like) - Parallel rays - Properties: angle, color, intensity

UsdLuxSphereLight: - Point or sphere-shaped light source - Properties: radius, color, intensity - Soft shadows based on radius

UsdLuxRectLight: - Rectangular area light - Properties: width, height, color, intensity - Good for window and panel lighting

UsdLuxDomeLight: - Environment/sky light - Image-based lighting (IBL) with HDR textures - Properties: texture file, intensity - Provides ambient illumination

UsdLuxCylinderLight: - Tube-shaped light source - Good for fluorescent lighting simulation

Light Properties¶

• intensity - Light brightness
• color - Light color (RGB)
• exposure - Photographic exposure adjustment
• normalize - Normalize intensity by area
• enableColorTemperature - Use Kelvin color temperature

Light Linking¶

• Control which geometry a light affects
• Include/exclude lists for selective lighting
• Per-light shadow enable/disable
• Light group management

Cameras¶

USD Camera Properties¶

• focalLength - Lens focal length (mm)
• horizontalAperture - Sensor width (mm)
• verticalAperture - Sensor height (mm)
• clippingRange - Near/far clip planes
• fStop - Aperture for depth of field
• focusDistance - Focus distance for DoF

Hydra Rendering Architecture¶

Overview¶

• USD's pluggable rendering system
• Decouples scene description from rendering
• Multiple render delegates can render the same scene
• Scene index translates USD to render-ready data

Render Delegates¶

Storm (Default): - OpenGL/Vulkan rasterizer - Fast interactive viewport rendering - Good for scene navigation and layout - Lower quality than ray tracing

RTX (NVIDIA): - Real-time ray tracing - Path tracing for photorealistic output - GPU-accelerated (requires NVIDIA RTX GPU) - Global illumination, reflections, caustics - Progressive refinement for final quality

Third-Party Delegates: - RenderMan (Pixar) - Arnold (Autodesk) - Karma (SideFX) - Blender Cycles

Render Settings¶

• Resolution and aspect ratio
• Samples per pixel (quality vs speed)
• Ray depth for reflections/refractions
• Denoising options
• AOV (Arbitrary Output Variable) channels
• Motion blur settings

Key Exam Concepts¶

• USD Preview Surface properties and PBR workflow
• MDL vs USD Preview Surface (capabilities and use cases)
• Material binding API for connecting materials to geometry
• USD light types and their properties
• Hydra architecture and render delegate concept
• Storm (rasterizer) vs RTX (ray tracer) render delegates