world material u_s leads to infinite time IPP.

[PyMarc2]

Problem
IPP (Interaction per Photon) Estimation is infinite if the world medium does not have a minimal mu_a when there is a mu_s.
Either an error message should be displayed to add a mu_aif there is an error, or a timeout should be applied.

Reproduction

from pytissueoptics import *

def exampleCode():
    N = 100000 if hardwareAccelerationIsAvailable() else 1000
    absorptionlayer = Cuboid(1,1,0.1, position=Vector(0,0,0.2), material=ScatteringMaterial(mu_s=0, mu_a=0.1, g=1, n=1.01))
    scene = ScatteringScene([absorptionlayer], worldMaterial=ScatteringMaterial(mu_s=100, mu_a=0, g=0.9, n=1.0))
    logger = EnergyLogger(scene)
    source = PencilPointSource(position=Vector(0, 0, 0), direction=Vector(0, 0, 1), N=N)

    scene.show(source=source)
    source.propagate(scene, logger=logger)

    viewer = Viewer(scene, source, logger)
    viewer.reportStats()

    viewer.show2D(View2DProjectionX(binSize=0.001))
    viewer.show2D(View2DSurfaceZ(solidLabel="cuboid", surfaceLabel="cuboid_front"))
    viewer.show2D(View2DSurfaceZ(solidLabel="cuboid", surfaceLabel="cuboid_front"))
    viewer.show3D()

if __name__ == "__main__":
    exampleCode()

Expected behavior

2 potential solutions:

• Have boundaries set around the scene by default
• Tell the user that mu_a must not be 0 if the world medium has mu_s != 0.

** System Information**

• OS: Windows 11
• Python 3.11