Arnold USD

The Hydra render delegate provides deep integration for rendering USD scenes via Arnold.

Key Capabilities:

• RPrim Support: Mesh (including primvars, normal translation, and subdivision), Volume, and Points.
• SPrim Support: Arnold shaders (via info:id), UsdPreviewSurface translation, Lights (Distant, Sphere, Disk, Rect, Cylinder, Dome), and Light/Shadow linking.
• Advanced Rendering: Motion Blur (Deformation, Transformation, and motion vectors), Point Instancers, and Deep render products.
• AOVs: Support for Color, Depth, and primId (for selection).
• Render Settings: Sampling, threading, CPU/GPU switching, and configurable defaults via environment variables.

Limitations:

• No support for periodic or pinned curves.
• No field3d volume grids.
• No normal maps on UsdPreviewSurface.
• Only converging renders are supported (viewport blocking is not possible).
• No coordsys support.
• Face-varying primvars may not interpolate correctly.

Arnold USD follows USD's naming conventions for C++. When writing code, adhere to these rules for identifiers:

• Classes: Use CamelCase and include a module-specific prefix (e.g., HdArnoldLight).
• Structs: Use CamelCase. Structs used for compact data storage without visibility or member functions only need a module prefix if exposed in a header.
• Functions: Use CamelCase.
  • Private or protected class functions must be prefixed with an underscore (e.g., _MyFunction).
  • Functions defined within an anonymous namespace in a source file must be prefixed with an underscore (e.g., _MyFunction).
• Variables: Use camelCase.
  • Class variables must be prefixed with an underscore (e.g., _myVariable).
  • Struct variables should not be prefixed with an underscore.
• Macros and Enums: Use ALL_UPPER_CASE.

// Class with prefix
class HdArnoldRenderDelegate;

// Private function with underscore
void _MyPrivateFunction();

// Class variable with underscore
int _myVariable;

// Struct variable without underscore
struct MyStruct {
    int myVariable;
};

// Macro/Enum
#define UPDATE_CAMERA 1
enum PartitionType { POINTS = 0 };

Arnold USD uses Doxygen for automated documentation generation. When documenting C++ code, follow these conventions:

• Use triple slashes (///) for Doxygen comments.
• Use ///< for documenting member variables/members.
• The first line (ending with a dot . or a blank line) is treated as the brief one-line description.
• Use the @ prefix for Doxygen commands and tags (e.g., @param, @return).
• For classes, describe their responsibilities and dependencies.
• For methods and functions, describe the purpose, every individual parameter, and the return value.
• Every header file must include a short description after the license using the @file tag.

/// Short description in one phrase.
///
/// Optionally, here goes a much more detailed description of the function.
/// This longer description can spawn multiple lines, include code samples, etc.
///
/// @param a This is the first floating point number to be added
/// @param b And this is the second number
/// @return  The sum a + b
float myAddFunction(float a, float b)
{
   // ...
}

To build against Houdini libraries (18.0+), you must account for Houdini's specific naming conventions and library locations.

Key Configuration Requirements

• USD Library Prefix: Houdini prefixes standard USD library names with libpxr_. Set USD_LIB_PREFIX='libpxr_'.
• Boost Naming:
  • On Linux/macOS: Boost libraries are prefixed with h. Set BOOST_LIB_NAME='hboost_%s'.
  • On Windows: Boost libraries are prefixed with h and suffixed with -mt. Set BOOST_LIB_NAME='hboost_%s-mt'.
• Schemas: As of Houdini 18.0.287, Houdini does not ship usdGenSchema, so BUILD_SCHEMAS should be set to False.

Example: Houdini 18.0.287 on macOS (custom.py)

ARNOLD_PATH='/opt/solidAngle/arnold'

WARN_LEVEL='warn-only'

USD_PATH='./'
USD_LIB='/Applications/Houdini/Houdini18.0.287/Frameworks/Houdini.framework/Versions/Current/Libraries'
USD_INCLUDE='/Applications/Houdini/Houdini18.0.287/Frameworks/Houdini.framework/Versions/Current/Resources/toolkit/include'
USD_BIN='/Applications/Houdini/Houdini18.0.287/Frameworks/Houdini.framework/Versions/Current/Resources/bin'
USD_BUILD_MODE='shared_libs'
USD_LIB_PREFIX='libpxr_'

BOOST_INCLUDE='/Applications/Houdini/Houdini18.0.287/Frameworks/Houdini.framework/Versions/Current/Resources/toolkit/include/hboost'
BOOST_LIB='/Applications/Houdini/Houdini18.0.287/Frameworks/Houdini.framework/Versions/Current/Libraries'
BOOST_LIB_NAME='hboost_%s'

PYTHON_INCLUDE='/Applications/Houdini/Houdini18.0.287/Frameworks/Python.framework/Versions/2.7/include/python2.7'
PYTHON_LIB='/Applications/Houdini/Houdini18.0.287/Frameworks/Python.framework/Versions/2.7/lib'
PYTHON_LIB_NAME='python2.7'

BUILD_SCHEMAS=False
BUILD_RENDER_DELEGATE=True
BUILD_PROCEDURAL=True
BUILD_TESTSUITE=True
BUILD_DOCS=True

PREFIX='/opt/solidAngle/arnold-usd'

The project uses C++11 and follows the C++ Core Guidelines. Key style requirements include:

• Formatting: Use the supplied .clang-format with clang-format 9.0.1. Indent with four (4) spaces; do not use tabs.
• Line Width: Maximum column width is 120, though ~80 is preferred to match USD conventions.
• Function Calls: No spaces between the function name and parenthesis, and no spaces inside the parenthesis (e.g., func(a, b);).
• Modern C++:
  • Use const and constexpr whenever possible.
  • Use nullptr instead of NULL or 0 for pointers.
  • Use the override keyword for class functions.
  • Use C++ style casting (static_cast, reinterpret_cast) instead of C-style casts. const_cast is forbidden unless required by external library design.
  • Use C++ notation for includes (e.g., #include <cstring>).
• Comments: Use C++ style comments // (with a following space). Do not use /* ... */.
• Control Flow: Brackets {} are always required for every block of code, even for single-line statements.

// Correct usage examples
const float b = 5.0f;
constexpr float c = 5.0f;
A* a = nullptr;

class Child : public Parent {
public:
    void MyFunc() override;
};

if (statement) {
    OneLineFunction();
}

// Correct casting
constexpr float d = static_cast<int>(a);