goad-py 1.1.2

Physical optics light scattering computation

GOAD - Geometric Optics with Aperture Diffraction

GOAD is a Rust-based physical-optics hybrid light scattering model based on geometric optics with aperture diffraction. It computes the 2D Mueller matrix by using geometric optics and a polygon clipping algorithm to compute the electric field on the particle surface. The surface field is then mapped to the far-field on the basis of the electromagnetic equivalence theorem, which takes the form of a vector surface integral diffraction equation. Green's theorem is used to reduce the surface integral to a line integral around the contours of outgoing beam cross sections, which leads to fast computations compared to some other methods. Compared to the PBT method, GOAD uses a beam clipping algorithm instead of ray backtracing on a meshed geometry, which makes the computation more accurate and faster if the particle has smooth planar surfaces.

📖 Reference Paper If you use this code in your work, please cite: A Light Scattering Model for Large Particles with Surface Roughness H. Ballington, E. Hesse JQSRT, 2024

---

📚 Contents

• GOAD - Geometric Optics with Aperture Diffraction
  • 📚 Contents
  • 🚀 Quickstart
  • ✨ Features
  • 🛠️ Installation
  • ⚙️ Usage
    • Configuration
    • Command-Line Arguments
  • ▶️ Running the Simulation
  • 🛠️ Testing
  • 🤝 Contributing
  • 📜 License

---

🚀 Quickstart

1. Run the setup.sh script from the project root to compile the code and initialise settings:

   ./setup.sh
   

2. Execute the binary located at ./target/release/goad:

   ./target/release/goad [OPTIONS]
   

For more information, see the quickstart guide in the docs.

---

✨ Features

• Full Mueller Matrix Output: Rigorous vector diffraction theory for computation of all Mueller matrix elements.
• Extensive Geometry Possibilities: GOAD is built with the flexibility to extend beyond simple convex polyhedral geometries, such as concavities, inclusions, layered media, negative refractive indices, and surrounding mediums other than air.
• Fixed and Multiple Orientation Scattering: Rapid computation of 2D scattering patterns in fixed orientation at arbitrary scattering angles, as well as fast orientation-averaged scattering computations for radiative transfer and remote sensing applications.

---

🛠️ Installation

Before building the project, ensure you have Rust's package manager, Cargo, installed. You can install Rust and Cargo by following the instructions on the official Rust website.

On Linux and macOS:

curl https://sh.rustup.rs -sSf | sh

Clone the repository and build the project:

git clone [e-mail]:hballington12/goad.git
cd goad
cargo build --release

After building, the binary will be in the target/release directory.

---

⚙️ Usage

Configuration

The application uses a default configuration file (config/default.toml). To customise:

• Copy it to config/local.toml and edit as needed.
• Options in config files are overridden by command line arguments, which are in turn overridden by environment variables.

Example ways to set the wavelength:

1. Edit config/local.toml:

   wavelength = 0.532
   

2. Use a command line argument:

   goad -- -wavelength 0.532
   

3. Use an environment variable:

   export GOAD_wavelength=0.532
   goad
   

Command-Line Arguments

GOAD - Geometric Optics with Aperture Diffraction
Harry Ballington

Usage: goad [OPTIONS]

Options:
  -w, --w <W>
          Wavelength in units of the geometry. Should be larger than the smallest feature in the geometry
      --bp <BP>
          Minimum beam power threshold for propagation. Beams with less power than this will be truncated
      --baf <BAF>
          Minimum area factor threshold for beam propagation. The actual area threshold is wavelength² × factor. Prevents geometric optics from modeling sub-wavelength beams
      --cop <COP>
          Total power cutoff fraction (0.0-1.0). Simulation stops when this fraction of input power is accounted for. Set to 1.0 to disable and trace all beams to completion
      --rec <REC>
          Maximum recursion depth for beam tracing. Typical values: 8-15. Higher values rarely improve results when reasonable beam power thresholds are set
      --tir <TIR>
          Maximum allowed total internal reflections. Prevents infinite TIR loops by truncating beams after this many TIR events
  -g, --geo <GEO>
          Path to geometry file (.obj format). Contains all input shapes for the simulation
      --ri0 <RI0>
          Surrounding medium refractive index. Format: "re+im" (e.g., "1.3117+0.0001i")
  -r, --ri <RI>...
          Particle refractive indices, space-separated. Each shape in the geometry is assigned a refractive index. If fewer values than shapes are provided, the first value is reused
      --distortion <DISTORTION>
          Distortion factor for the geometry. Applies distortion sampled from a Gaussian distribution. Default: sigma = 0.0 (no distortion). Sigma is the standard deviation of the facet theta tilt (in radians)
      --geom-scale <GEOM_SCALE>...
          Geometry scale factors for each axis (x, y, z). Format: "x y z" (e.g., "1.0 1.0 1.0"). Default: "1.0 1.0 1.0" (no scaling)
      --uniform <UNIFORM>
          Use uniform random orientation scheme. The value specifies the number of random orientations
      --discrete <DISCRETE>...
          Use discrete orientation scheme with specified Euler angles (degrees). Format: alpha1,beta1,gamma1 alpha2,beta2,gamma2 ...
      --euler <EULER>
          Specify Euler angle convention for orientation. Valid values: XYZ, XZY, YXZ, YZX, ZXY, ZYX, etc. Default: ZYZ
      --simple <SIMPLE> <SIMPLE>
          Use simple equal-spacing binning scheme. Format: <num_theta_bins> <num_phi_bins>
      --interval
          Enable interval binning scheme with variable spacing. Allows fine binning in regions of interest like forward/backward scattering
      --theta <THETA> <THETA> <THETA>...
          Theta angle bins for interval binning (degrees). Format: start step1 mid1 step2 mid2 ... stepN end Example: 0 1 10 2 180 = 0° to 10° in 1° steps, then 10° to 180° in 2° steps
      --phi <PHI> <PHI> <PHI>...
          Phi angle bins for interval binning (degrees). Format: start step1 mid1 step2 mid2 ... stepN end Example: 0 2 180 = 0° to 180° in 2° steps
      --custom <CUSTOM>
          Path to custom binning scheme file. Contains a list of (theta, phi) bin pairs in TOML format. Overrides other binning parameters
  -s, --seed <SEED>
          Random seed for reproducibility. Omit for a randomized seed
      --dir <DIR>
          Output directory for simulation results. If not specified, a directory in the format 'run00001' will be created automatically
  -h, --help
          Print help
  -V, --version
          Print version

EXAMPLES:
    # Run with a specific wavelength and geometry file
    goad -w 0.5 --geo geometry.obj

    # Run with a specific refractive index and random orientations
    goad --ri 1.31+0.01i --uniform 100

    # Run over discrete orientations with an interval binning scheme
    goad --discrete="-30.0,20.0,1.0 -40.0,13.0,12.1" --interval \
         --theta 0 1 10 2 180 --phi 0 2 180

    # Run inside a medium other than air
    goad --ri0 1.5+0.0i

    # Run with multiple shapes with different refractive indices
    goad --ri 1.31+0.0i 1.5+0.1i --geo geometries.obj

    # Save output to a specific directory
    goad --dir /path/to/output

---

▶️ Running the Simulation

cargo run --release -- [OPTIONS]

or

./target/release/goad -w 0.532 --geo ./examples/data/cube.obj

---

🛠️ Testing

To run the tests:

cargo test

---

🤝 Contributing

Contributions are welcome! Please open an issue or submit a pull request on GitHub.

---

📜 License

This project is licensed under the GNU General Public License. See the LICENSE file for details.