Path Tracer

Various implementations for the final scene from Ray Tracing in One Weekend:

• GPU Path Tracer
  • Apple's Metal API through metal-cpp
• CPU Path Tracer
  • Built-in RTIOW C++ shading backend
  • OSL shading backend

Benchmarks

Measurements used a 1200 x 675 image at 500 samples per pixel.

On an M1 MacBook Pro (8-core CPU, 8-core GPU):

Renderer	Shading	Parallelism	Traversal	Time	Speedup
CPU	Built-in C++	Single-threaded	Linear	6h 06m 44s	1x
CPU	Built-in C++	Multithreaded	Linear	53m 42s	6.8x
CPU	Built-in C++	Multithreaded	BVH	1m 57s	188x
CPU	OSL	Multithreaded	BVH	2m 27s	150x
GPU	Metal	GPU-parallel	Linear	2m 28s	149x

On an M5 Pro MacBook Pro (18-core CPU, 20-core GPU):

Renderer	Shading	Parallelism	Traversal	Time
CPU	Built-in C++	Multithreaded	BVH	40s
CPU	OSL	Multithreaded	BVH	42s
GPU	Metal	GPU-parallel	Linear	24s

See OSL_Integration.md for details.

CPU · Built-in C++	CPU · OSL	GPU · Metal

GPU Path Tracer

Requirements

The GPU renderer requires macOS and the Xcode command-line tools. metal-cpp is vendored in this repository.

xcode-select --install

Build

From the project root:

cmake -S . -B build/gpu -DBUILD_GPU_PT=ON -DBUILD_CPU_PT=OFF
cmake --build build/gpu

BUILD_CPU_PT defaults to ON, so leaving it out here would also configure cpu_pt and require oneTBB.

CMake compiles src/gpu/Shaders/*.metal into build/gpu/default.metallib, beside the executable, where the renderer looks for it.

Run

./build/gpu/gpu_pt > output.ppm

CPU Path Tracer

The CPU renderer shares its geometry, BVH, camera, and threading between two material backends:

• --builtin: C++ materials modeled on the book.
• --osl: OSL shaders compiled to .oso files at build time.

OSL support is optional and adds no OSL dependencies to a built-in-only build.

Requirements

The CPU renderer requires oneTBB:

brew install tbb

The OSL backend also requires compatible OpenShadingLanguage and OpenImageIO installations.

Build

For built-in shading only:

cmake -S . -B build/cpu -DBUILD_CPU_PT=ON
cmake --build build/cpu

To include OSL shading, point CMake at the OSL and OpenImageIO installations if they are not already on its search path:

OSL_ROOT=$HOME/projects/OpenShadingLanguage/dist
OIIO_ROOT=$HOME/projects/OpenImageIO/dist

cmake -S . -B build/cpu -DBUILD_CPU_PT=ON -DENABLE_OSL=ON \
    -DCMAKE_PREFIX_PATH="$OSL_ROOT;$OIIO_ROOT"
cmake --build build/cpu

CMake compiles src/cpu/osl/shaders/*.osl into build/cpu/shaders/*.oso. If the executable fails to start with a Library not loaded error, add the missing library's directory with -DPT_OSL_EXTRA_RPATHS=/path/to/lib.

Run

./build/cpu/cpu_pt > output.ppm

An OSL-enabled build defaults to --osl. Select a backend explicitly when comparing the two:

./build/cpu/cpu_pt --osl     > osl.ppm
./build/cpu/cpu_pt --builtin > builtin.ppm

References

• Ray Tracing in One Weekend
• GPU Programming with the Metal Shading Language
• Accelerated Ray Tracing in One Weekend in CUDA
• testrender, OSL's reference renderer integration