pathtracer

Physical pathtracer / raycaster

This project implements a pathtracer to demonstrate the capabilities of the proposed OpenMaterial_reference_link, OpenMaterial_material_parameters and OpenMaterial_ior_data extensions for the Khronos Group glTF 2.0 file format.

The pathtracer loads an object (a 3D geometry) from a glTF file. Using the proposed OpenMaterial_reference_link extension, the material section of glTF files is extended by references to files providing physical material parameters (specified by OpenMaterial_material_parameters). These files can in turn contain references to look-up tables with measurement data (e.g. OpenMaterial_ior_data). This way, material parameters and measurement data can be used by different geometries and vice versa.

The pathtracer renders a single picture from a glTF file and referenced material data. Examples for geometry glTF files can be found in the folder ../objects/. The physical parameters provided in the material files in ../materials/ are used to compute the reflection of rays at the geometry. Corresponding light sources for this reflection are provided by a HDR (high definition range) background image. The implemented camera model is a thin lens camera. The rendered output image is saved as a PPM (Portable Pixmap format) file.

Besides its major use case as a pathtracer, the implementation provides the functionality of a raycaster. In this case, reflections and secondary rays are neglected. Instead, material coefficients from the first ray-object intersection are rendered to a monochromatic output image.

Compilation

The project source code can be compiled using CMake. On Linux systems, this requires the following commands:

$ mkdir bin
$ cd bin
$ cmake ..
$ make

The above will generate the executable pathtracer in the directory bin/. The code is written in C++11 and dependant on OpenMP for parallelization.

The project was tested on Ubuntu 18.04, Ubuntu 20.04 and Windows 10 (CMake & Visual Studio 2015). In the latter case, the project can be compiled using:

$ mkdir bin
$ cd bin
$ cmake .. -G "Visual Studio 14 2015 Win64"
$ MSBuild PathTracer.sln /p:Configuration=Release /p:Platform=x64

As the code is pure C++11, the code should also compile and run on other platforms with compilers that support OpenMP.

Usage

The pathtracer is executed using its command line interface. Running pathtracer --help will print usage instructions. Mandatory parameters are --input or -i for the specification of the input glTF file, and --hdr for the specification of a background HDR file.

The following example renders a golden teapot at a resolution of 1200x1200 pixels on a Linux machine:

$ bin/pathtracer --resolution 1200 --hdr ../hdr/green_point_park_4k.hdr --input ../objects/teapot_gold.gltf --output teapot_gold.ppm
Loading ../objects/teapot_gold.gltf
Loading material: gold:
gold:
    user_preferences:
        geometricalOptics: true
        includeDiffraction: false
        includeNumericalSimulation: false
        materialScheme: surface
        materialClassification: "solid-metal"
        materialType:
            isotropic: true
            homogeneous: true
            magnetic: false
        temperature: 300
        surfaceDisplacementUri: ""
        surfaceRoughness:
            surfaceHeight: 0
            surfaceCorrelationLength: 0
        lambertEmission: 0
        subsurface:
            subsurface: false
            subsurfaceThickness: 0
    physical_properties:
        refractiveIndexUri: "data/gold_ior.gltf"
        meanFreePath: 0
        particleDensity: 0
        particleCrossSection: 0
        emissivityCoefficientUri: ""
        applicableSensors:
            camera: true
            lidar: true
            radar: true

Loaded blender teapot
Rendering the scene...
Saving image to teapot_gold.ppm

The output image is saved as teapot_gold.ppm:

On Windows, the above example can be executed using

$ bin\Release\pathtracer.exe --resolution 1200 --hdr ..\hdr\green_point_park_4k.hdr --input ..\objects\teapot_gold.gltf --output teapot_gold.ppm

Raycaster

The raycaster (command line option -R) uses the same executable and parameters as the pathtracer. The raycaster renders material coefficients from the first ray-object intersection to a monochromatic output image:

While specification of an HDR image is not required in this case, adding command line option -v provides verbose debug information.

Texture based displacement

Texture based mesh displacement is activated by using command line option -d and specification of corresponding textures (e.g. from ../materials/).

$ bin/pathtracer -i <yourgltfmodel> -H <yourbackground> -d ../materials/paper_displacement.bmp

UV mapping is specified by adding the command line option -U:

$ bin/pathtracer -i <yourgltfmodel> -H <yourbackground> -d ../materials/paper_displacement.bmp -U <yourUVmapping>

Available options for UV mapping comprise random (default), sphere, cube and cylinder. Thus, reference examples for texture based displacement can be created using the following commands:

Sphere

$ bin/pathtracer -H <yourfile.hdr> -i ../objects/sphereEnriched_aluminium.gltf -d <yourmap> -U sphere  

Cube

$ bin/pathtracer -H <yourfile.hdr> -i ../objects/cubeEnriched_aluminium.gltf -d <yourmap> -U cube  

Cylinder

$ bin/pathtracer -H <yourfile.hdr> -i ../objects/cylinderEnriched_aluminium.gltf -d <yourmap> -U cylinder  

Testing

Unit tests are executed using the command line option -t.

Documentation

Source code documentation is generated using doxygen:

doxygen doxygen.conf

Documentation will be written to the docs/ directory.

Further details

• Camera model: Pinhole camera
• Available materials: Metals
• Illumination: Given by a background HDR image
• Rendering:
  • Each ray consists of three wavelengths (red 650nm, green 510nm, blue 440nm)
  • Light is assumed to be unpolarized.
  • The reflection coefficients (the Fresnel coefficients) are computed as the mean of s- and p-polarization.

Third party software

This project depends on third party software. The corresponding code can be found in the directory ../external/.