Allow SceneObject to be scaled by different amounts in each dimension

[James-Rhodes]

Description

I have extended the capability of scaling a SceneObject to allow for scaling by different amounts in each dimension. If the user sets the scaling parameter to a scalar then it will still behave as expected, scaling all dimensions equally. However, if a mitsuba.Vector3f is passed in, then the object will be scaled by the x, y and z dimensions in the coordinate system of the object. This does make the setter for scaling more expensive, however it also adds additional functionality so I think the trade off is worth while personally (open to suggestions though).

I have additionally added tests to ensure that no other behaviour is broken. Please note that I do not have a GPU so I have only been able to run the tests on the CPU using the --cpu flag for pytest.

This is my first time making a pull request so I am very open to criticism and fixing things up as needed. Thanks!

• Fixes a bug?
  NA.

• Adds a new Feature?
  This feature allows a user to scale SceneObjects by different amounts in each dimension.

• Introduces API changes?
  It changes the value returned by the "getter" of scaling from a scalar float to a mitsuba.Vector3f to show the scaling in each direction. The setter signature is also changed to additionally support mitsuba.Vector3f as well as mitsuba.Float. The setter should not be a breaking change though, given scalar values are still supported.

• Other contributions
  NA

I have supplied a short program that highlights this feature in use below:

import matplotlib.pyplot as plt
import mitsuba as mi

mi.set_variant("llvm_ad_mono_polarized")  # Use CPU
import sionna
from sionna.rt import (
    load_scene,
    Camera,
    ITURadioMaterial,
    SceneObject,
    Receiver,
)


def main():
    scene = load_scene()  # Objects are merged by default
    cam = Camera(position=mi.Point3f(0.0, 0.0, 100), look_at=mi.Point3f(0, 0, 0))
    car_material = ITURadioMaterial(
        "car-material", "metal", thickness=0.01, color=(0.8, 0.1, 0.1)
    )

    car = SceneObject(fname=sionna.rt.scene.low_poly_car, name="unscaled_car", radio_material=car_material)
    scene.edit(add=car)
    car.position = mi.Point3f(0., 5., 0.)

    scaled_car = SceneObject(fname=sionna.rt.scene.low_poly_car, name="scaled_car", radio_material=car_material)
    scene.edit(add=scaled_car)
    scaled_car.position = mi.Point3f(0., -5., 0.)
    scaled_car.scaling = mi.Vector3f(10, 2, 1) # Scale the car by some amount

    # Only here so that a green circle is drawn in the positive x direction
    scene.add(
        Receiver(
            "rx-1",
            position=mi.Point3f(
                10,
                0,
                0,
            ),
            display_radius=2,
        )
    )

    scene.render(camera=cam)
    plt.show()


if __name__ == "__main__":
    main()

Checklist

• Detailed description
• Added references to issues and discussions
• Added / modified documentation as needed
• Added / modified unit tests as needed
• Passes all tests
• Lint the code
• Performed a self review
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[faycalaa]

Hi @James-Rhodes,

Thank you for this pull request! We will review it and get back to you.