MaterialX support

MaterialX support in TinyUSDZ

• Support Blender v5.x exported MaterialX(OpenPBR)
  • MaterialXConfigAPI inline description of MaterialX material.
  • No support for shader network in Tydra RenderMaterial

NOTE: MaterialX Parser Logic Summary in OpenUSD

v25.11

This document summarizes the node graph parsing logic in pxr/usd/usdMtlx/parser.cpp.

Overview

The parser converts MaterialX node definitions into USD's Sdr (Shader Definition Registry) shader nodes. It implements SdrParserPlugin to integrate with USD's shader discovery system.

Key Components

1. ShaderBuilder (lines 78-136)

A builder pattern class that accumulates data for constructing SdrShaderNode:

• Fields: discoveryResult, valid, definitionURI, implementationURI, context, properties, metadata
• AddPropertyNameRemapping(): Maps MaterialX property names to USD names
• AddProperty(): Converts MaterialX typed elements to SdrShaderProperty
• Build(): Creates the final SdrShaderNodeUniquePtr

2. Property Parsing (AddProperty, lines 224-391)

Converts MaterialX inputs/outputs to Sdr properties:

1. Type Resolution: Maps MaterialX types to USD Sdf types via UsdMtlxGetUsdType()
2. Default Values: Extracts via UsdMtlxGetUsdValue()
3. Metadata Extraction:
   • defaultinput for outputs
   • target for inputs
   • colorspace for inputs/outputs
   • UI metadata: uiname, doc, uifolder, uimin, uimax, uisoftmin, uisoftmax, uistep
   • Unit info: unit, unittype
4. Primvar Handling: Tracks defaultgeomprop references, replacing UV0 with the configured primary UV set name

3. ParseElement (lines 436-551)

Main NodeDef parsing function:

1. Context Determination (lines 444-454):

   • Checks type's semantic for "shader" to get context
   • Falls back to standard typedefs
   • Defaults to SdrNodeContext->Pattern

2. Node Metadata (lines 462-467):

   • Label = nodeDef's node string
   • Category = nodeDef's type
   • Help from doc attribute
   • Target from target attribute
   • Role from nodegroup attribute

3. Primvar Collection (lines 471-535):

   • ND_geompropvalue* nodes: add $geomprop
   • ND_texcoord_vector2: add primary UV set name
   • Implementation NodeGraph traversal:
     • geompropvalue nodes: extract primvar from geomprop input
     • texcoord nodes: add primary UV set
     • image/tiledimage nodes: add primary UV set if no texcoord already added
   • internalgeomprops attribute: parse and add primvars

4. Property Collection (lines 538-544):

   • Iterates nodeDef->getActiveInputs() for input properties
   • Iterates nodeDef->getActiveOutputs() for output properties

4. UsdMtlxParserPlugin (lines 556-643)

The main parser plugin:

ParseShaderNode() (lines 567-622):

1. Load MaterialX document from resolvedUri or sourceCode
2. Look up NodeDef by identifier or subIdentifier
3. Create ShaderBuilder and call ParseElement()
4. Return built shader node

GetDiscoveryTypes(): Returns ["mtlx"]

GetSourceType(): Returns empty string (default source type)

Data Flow

MaterialX Document
       │
       ▼
   NodeDef lookup
       │
       ▼
   ShaderBuilder
       │
       ├─── ParseElement() ───┬─── Extract context
       │                      ├─── Extract metadata
       │                      ├─── Collect primvars
       │                      └─── Parse inputs/outputs
       │
       ▼
   AddProperty() for each input/output
       │
       ├─── Type conversion (UsdMtlxGetUsdType)
       ├─── Default value (UsdMtlxGetUsdValue)
       ├─── Metadata extraction
       └─── Primvar tracking
       │
       ▼
   ShaderBuilder.Build()
       │
       ▼
   SdrShaderNode

Key Tokens (lines 32-56)

Private tokens for attribute names:

• mtlx, defaultgeomprop, defaultinput, doc, enum, enumvalues
• nodecategory, nodegroup, target, uifolder, uimax, uimin
• uiname, uisoftmax, uisoftmin, uistep, unit, unittype, UV0

Environment Settings

• USDMTLX_PRIMARY_UV_NAME: Override the primary UV set name (defaults to USD's UsdUtilsGetPrimaryUVSetName(), typically "st")

Role Normalization (lines 405-414)

For stdlib texture nodes, texture2d and texture3d roles are normalized to texture for Sdr compatibility.