tinyusdz/C++_MATERIALX_IMPORT.md

C++ MaterialX Import Support

Overview

TinyUSDZ now includes built-in C++ support for loading MaterialX (.mtlx) files without external dependencies. The implementation uses a secure, dependency-free XML parser specifically designed for MaterialX documents.

Architecture

Components

1. MaterialX Parser (src/mtlx-*.hh/cc)

   • mtlx-xml-tokenizer: Low-level XML tokenization with security limits
   • mtlx-simple-parser: Lightweight DOM tree builder
   • mtlx-dom: MaterialX-specific document object model
   • mtlx-usd-adapter: pugixml-compatible interface

2. USD Integration (src/usdMtlx.cc/hh)

   • MaterialX to USD conversion
   • Support for multiple shader types
   • PrimSpec generation

Supported Shaders

• ✅ OpenPBR Surface (open_pbr_surface) - Full support
• ✅ Autodesk Standard Surface (standard_surface) - Full support
• ✅ USD Preview Surface (UsdPreviewSurface) - Full support

API Usage

Basic Import

#include "usdMtlx.hh"

// Load from string
std::string xml_content = "...";
tinyusdz::MtlxModel mtlx;
std::string warn, err;

bool success = tinyusdz::ReadMaterialXFromString(
    xml_content,
    "material.mtlx",  // asset name
    &mtlx,
    &warn,
    &err
);

if (success) {
    std::cout << "Loaded MaterialX version: " << mtlx.version << std::endl;
}

Load from File

#include "usdMtlx.hh"

tinyusdz::AssetResolutionResolver resolver;
tinyusdz::MtlxModel mtlx;
std::string warn, err;

bool success = tinyusdz::ReadMaterialXFromFile(
    resolver,
    "path/to/material.mtlx",
    &mtlx,
    &warn,
    &err
);

Convert to USD PrimSpec

// Convert MaterialX model to USD PrimSpec
tinyusdz::PrimSpec ps;
std::string err;

bool success = tinyusdz::ToPrimSpec(mtlx, ps, &err);

if (success) {
    // Use PrimSpec in USD Stage
    // ...
}

Load as USD Asset Reference

#include "usdMtlx.hh"

tinyusdz::Asset asset;
tinyusdz::PrimSpec ps;
std::string warn, err;

bool success = tinyusdz::LoadMaterialXFromAsset(
    asset,
    "material.mtlx",
    ps,  // inout parameter
    &warn,
    &err
);

OpenPBR Surface Support

The MtlxOpenPBRSurface shader supports all OpenPBR specification parameters:

Base Layer

• base_weight (float)
• base_color (color3)
• base_metalness (float)
• base_diffuse_roughness (float)

Specular Layer

• specular_weight (float)
• specular_color (color3)
• specular_roughness (float)
• specular_ior (float)
• specular_anisotropy (float)
• specular_rotation (float)

Transmission

• transmission_weight (float)
• transmission_color (color3)
• transmission_depth (float)
• transmission_scatter (color3)
• transmission_scatter_anisotropy (float)
• transmission_dispersion (float)

Subsurface

• subsurface_weight (float)
• subsurface_color (color3)
• subsurface_radius (color3)
• subsurface_scale (float)
• subsurface_anisotropy (float)

Coat (Clearcoat)

• coat_weight (float)
• coat_color (color3)
• coat_roughness (float)
• coat_anisotropy (float)
• coat_rotation (float)
• coat_ior (float)
• coat_affect_color (float)
• coat_affect_roughness (float)

Thin Film

• thin_film_thickness (float)
• thin_film_ior (float)

Emission

• emission_luminance (float)
• emission_color (color3)

Geometry

• geometry_opacity (float)
• geometry_thin_walled (bool)
• geometry_normal (normal3)
• geometry_tangent (vector3)

Example MaterialX File

<?xml version="1.0"?>
<materialx version="1.38">
  <surfacematerial name="RedMetal" type="material">
    <input name="surfaceshader" type="surfaceshader" nodename="RedMetal_shader" />
  </surfacematerial>

  <open_pbr_surface name="RedMetal_shader" type="surfaceshader">
    <input name="base_color" type="color3" value="0.8, 0.2, 0.2" />
    <input name="base_weight" type="float" value="1.0" />
    <input name="base_metalness" type="float" value="0.8" />
    <input name="specular_roughness" type="float" value="0.3" />
    <input name="specular_ior" type="float" value="1.5" />
  </open_pbr_surface>
</materialx>

Security Features

The built-in parser includes multiple security safeguards:

• Maximum name length: 256 characters
• Maximum string length: 64KB
• Maximum text content: 1MB
• Maximum nesting depth: 1000 levels
• Safe entity handling: HTML entities only (no external entity expansion)
• No external file access: Prevents XXE attacks
• Memory limits: Prevents denial-of-service attacks

Build Configuration

CMake

# Enable MaterialX support (enabled by default)
set(TINYUSDZ_USE_USDMTLX ON CACHE BOOL "Enable MaterialX support")

Compile Flags

# Enable MaterialX in your build
-DTINYUSDZ_USE_USDMTLX

Testing

Run Tests

cd tests/feat/mtlx
make clean
make
./test_mtlx_import

Test with Custom File

./test_mtlx_import path/to/your/material.mtlx

Expected Output

=== TinyUSDZ MaterialX Import Test ===

Test 1: Parsing MaterialX XML from string...
✓ Successfully parsed MaterialX

Parsed MaterialX information:
  Asset name: test.mtlx
  Version: 1.38
  Shader name: TestMaterial_shader
  Surface materials: 1
  Shaders: 1

Test 2: Converting MaterialX to USD PrimSpec...
✓ Successfully converted to PrimSpec
  PrimSpec name: TestMaterial
  PrimSpec type: Material

=== All tests passed! ===

Comparison: pugixml vs Built-in Parser

Feature	pugixml	Built-in Parser
External Dependency	Yes	No
Size	~200KB	Integrated
Security	Basic	Enhanced
Memory Limits	Manual	Automatic
MaterialX Specific	No	Yes
XXE Protection	Manual	Built-in
Performance	Fast	Fast

Migration from pugixml

The migration is automatic - the built-in parser provides a pugixml-compatible adapter:

Before (with external pugixml):

#include "external/pugixml.hpp"
pugi::xml_document doc;
pugi::xml_parse_result result = doc.load_string(xml);

After (with built-in parser):

#include "mtlx-usd-adapter.hh"
tinyusdz::mtlx::pugi::xml_document doc;
tinyusdz::mtlx::pugi::xml_parse_result result = doc.load_string(xml);

The API is compatible, so existing code continues to work.

Limitations

• MaterialX versions: Supports 1.36, 1.37, and 1.38
• XML namespaces: Basic support (MaterialX doesn't use them heavily)
• XPath: Not supported (not needed for MaterialX)
• DOM manipulation: Read-only parsing

Error Handling

std::string warn, err;
bool success = tinyusdz::ReadMaterialXFromString(xml, name, &mtlx, &warn, &err);

if (!success) {
    std::cerr << "Error: " << err << std::endl;
    return 1;
}

if (!warn.empty()) {
    std::cout << "Warnings: " << warn << std::endl;
}

Future Enhancements

• MaterialX node graph support beyond surface shaders
• MaterialX standard library includes
• Write support (currently read-only)
• XPath queries for advanced filtering
• Texture node parsing and loading
• MaterialX validation against schema

References

• MaterialX Specification
• OpenPBR Specification
• Autodesk Standard Surface
• USD Specification

License

Apache 2.0 - Same as TinyUSDZ project

Contact

• Issues: https://github.com/lighttransport/tinyusdz/issues
• Discussions: https://github.com/lighttransport/tinyusdz/discussions