add a test for the depth argument in off axis projs for grids

yt/visualization/tests/test_offaxisprojection_pytestonly.py

@@ -1,10 +1,12 @@
 import numpy as np
 import pytest
 import unyt
+from numpy.testing import assert_allclose
 
 from yt.testing import (
     assert_rel_equal,
     cubicspline_python,
+    fake_amr_ds,
     fake_sph_flexible_grid_ds,
     integrate_kernel,
 )
@@ -165,3 +167,40 @@ def makemasses(i, j, k):
     # print("expected:\n", expected_out)
     # print("recovered:\n", img.v)
     assert_rel_equal(expected_out, img.v, 4)
+
+
+def test_offaxisprojection_depth():
+    # this checks that the depth keyword argument works as expected.
+    # it sets up an off axis projection center at the left edge
+    # of the domain, looking towards the right edge, and then integrates
+    # the (index, ones) field at increasing depth.
+    # The result should scale linearly with the depth value and should
+    # max out at the diagonal distance of the cube. Note that because
+    # the depth is actually a full-width centered on the projection
+    # center, the actual scaling is: integrated distance = depth / 2.0
+
+    ds = fake_amr_ds()
+
+    n = [1.0, 1.0, 1.0]
+    c = ds.domain_left_edge
+    field = ("index", "ones")
+
+    diag_dist = np.linalg.norm(ds.domain_width.in_units("code_length"))
+
+    # pick a point less than the max, exactly at the max and past it
+    depths = np.array([diag_dist / 3.0, diag_dist * 2.0, diag_dist * 4.0])
+
+    # expected values of the integrations
+    expected = depths / 2.0
+    expected[depths > 2 * diag_dist] = diag_dist
+
+    maxvals = []
+    for depth in depths:
+        p = ProjectionPlot(
+            ds, n, field, depth=depth, weight_field=None, center=c, width=diag_dist
+        )
+
+        maxvals.append(p.frb[field].max())
+
+    maxvals = np.array(maxvals)
+    assert_allclose(expected, maxvals, atol=1e-2)