Curved edges when using MaterialGrid

[ramji28]

Hello Meep community,

I'm attempting to generate a square grid with two materials using MaterialGrid. However, when visualizing the structure using Plot2D the material doesn't appear as a perfect square. I've set $\beta=\infty$. Can anyone help me identify what might be the issue?
Thank you for the help

import meep as mp
import numpy as np
import matplotlib
matplotlib.use('agg')
import matplotlib.pyplot as plt

resolution = 70

cell_size = mp.Vector3(2,2,0)
weights = np.ones((11,11))
weights[0,0]=0
Nx = len(weights)
Ny = len(weights)
mat_interp = mp.MaterialGrid(mp.Vector3(Nx,Ny,0),
                             mp.Medium(index=3.5),
                             mp.Medium(index=1.5),
                             weights=weights,
                             beta=mp.inf,)

geometry = [mp.Block(material=mat_interp,
                     center=mp.Vector3(),
                     size=mp.Vector3(2,2,0))]

sim = mp.Simulation(cell_size=cell_size,
                    resolution=resolution,
                    geometry=geometry)
fig, ax = plt.subplots()
sim.plot2D(ax=ax)
fig.suptitle('resolution = {}, (Nx,Ny) = ({},{})'.format(resolution,Nx,Ny))
fig.savefig('matgrid_graded_res{}_{}_{}.png'.format(resolution,Nx,Ny),
            bbox_inches='tight',
            dpi=300)

[peterropac]

This might happen because of subpixel smoothing. You can try eps_averaging=False in the Simulation class. Have you tried increasing the resolution of the MaterialGrid?

[smartalecH]

Why is your MaterialGrid so course? Typically you should match or exceed the resolution as your simulation.

When you specify a beta, meep runs a linear interpolation routine to map the MaterialGrid to the yee lattice. This acts like a spatial low pass filter.

Try a much finer MaterialGrid.

[ramji28]

Thank you so much. I used a coarse grid to generate a random grid for different material shapes to train a neural network. However, as you mentioned, I got the square using a finer grid.