update interpolation weight in get_array_slice

[oskooi]

Addresses #1456 (comment).

[stevengj]

I assume it passes the tests locally?

[oskooi]

python/tests/array_metadata.py is failing with and without including the weights array W in computing the integral:

meep/python/tests/array_metadata.py

Lines 75 to 81 in 27d4512

	Ez = sim.get_dft_array(dft_obj, mp.Ez, 0)
	(X,Y,Z,W) = sim.get_array_metadata(dft_cell=dft_obj)
	Eps = sim.get_array(vol=nonpml_vol, component=mp.Dielectric)
	EpsE2 = np.real(Eps*np.conj(Ez)*Ez)
	xm, ym = np.meshgrid(X, Y)
	vec_func_sum = np.sum(W*(xm**2 + 2*ym**2))
	pulse_modal_volume = np.sum(W*EpsE2)/np.max(EpsE2) * vec_func_sum

The error from this branch:

======================================================================
FAIL: test_array_metadata (__main__.TestArrayMetadata)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "python/tests/array_metadata.py", line 83, in test_array_metadata
    self.assertAlmostEqual(cw_modal_volume/pulse_modal_volume, 1.00, places=2)
AssertionError: (0.9768618126950315+0j) != 1.0 within 2 places

[oskooi]

Not including the weights from get_array_metadata in computing the test integral np.sum(EpsE2)/np.max(EpsE2) * np.sum(xm**2 + 2*ym**2) from array_metadata.py makes the result much larger than the reference value computing using the field function. Thus, even with the change to the weights in this PR, the weights must still be included in computing the integral which is not what we expected.

Fixing the failing test just requires reducing the tolerance slightly.

[stevengj]

Actually, it looks like the collapsed arrays do not include the edge weights, so we don't need to remove them:

meep/src/dft.cpp

Lines 786 to 800 in 4d4ccb7

	/*****************************************************************/
	/* For collapsing empty dimensions, we want to retain interpolation
	weights for empty dimensions, but not interpolation weights for
	integration of edge pixels (for retain_interp_weights == true).
	All of the weights are stored in (s0, s1, e0, e1), so we make
	a copy of these with the weights for non-empty dimensions set to 1. */
	vec s0i(s0), s1i(s1), e0i(e0), e1i(e1);
	LOOP_OVER_DIRECTIONS(fc->gv.dim, d) {
	if (!empty_dim[d]) {
	s0i.set_direction(d, 1.0);
	s1i.set_direction(d, 1.0);
	e0i.set_direction(d, 1.0);
	e1i.set_direction(d, 1.0);
	}
	}