Draw perfect metals (ε = -∞) in dark red in plot2D

[oskooi]

Context

mp.metal and mp.perfect_electric_conductor are defined as Medium(epsilon=-inf) (python/meep.i:1825-1826), and that -inf survives as a large finite -1e20 all the way through geom_epsilon::chi1p1 → Simulation.get_epsilon_grid.

plot_eps — the function plot2D calls to draw the geometry (python/visualization.py:599-732) — passes that array straight to ax.imshow and, when colorbar=True, straight into mpl.colors.Normalize(np.amin(eps_data), np.amax(eps_data)). The consequences of this are:

• Colorbar: vmin = -inf, so the colorbar is meaningless and every finite dielectric collapses onto one end of the scale.
• Image: matplotlib's internal safe_masked_invalid masks the -inf pixels and draws them with the colormap's "bad" color, which defaults to fully transparent — metal renders as blank background rather than as a material (see first image below).
• Contour path: levels=np.unique(eps_data) includes -inf as a contour level.

Changes

python/visualization.py

• new pec_color: "darkred" key in default_eps_parameters (dropped by filter_dict before reaching imshow, like the other non-imshow dicts).
• plot_eps now masks metallic pixels (~np.isfinite(eps) | (eps <= -mp.inf)) and paints them via cmap.set_bad(pec_color) on a copy.copy of the colormap, so imshow autoscales over the finite permittivities only.
• Colorbar vmin/vmax now come from the unmasked values, with a guard for an all-metal plane.
• Contour path uses np.unique(eps_data.compressed()) so -1e20 is no longer emitted as a contour level.
• Fixed a latent aliasing bug: eps_parameters = default_eps_parameters handed out the module-level dict itself, which the new code would have mutated — now copy.deepcopy.
• Added _get_colormap() (handles str or Colormap, matplotlib ≥3.5 or older) and used it in _add_colorbar; required because a Colormap instance is now passed there, and mpl.cm.get_cmap is removed in matplotlib 3.11. This also clears the deprecation warning the old cell emitted.

Verification

test_plot2D_pec (clim finite and equal to (1, 12), bad color = darkred, mask non-empty, defaults unmutated, user override honored) and test_plot2D_pec_contour.

Here is an example involving a cleaved multilayer stack in which the substrate is a perfect metal. Metallic/PEC regions render as a distinct dark red, and the color scale (image and colorbar) is driven only by the finite permittivity values.