Add test of masked surface

Author: tomneep

lib/mpl_toolkits/tests/baseline_images/test_mplot3d/surface3d_masked.png

@@ -420,6 +420,33 @@ def test_surface3d_shaded():
     ax.set_zlim(-1.01, 1.01)
 
 
+@mpl3d_image_comparison(['surface3d_masked.png'])
+def test_surface3d_masked():
+    fig = plt.figure()
+    ax = fig.add_subplot(projection='3d')
+    x = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11]
+    y = [1, 2, 3, 4, 5, 6, 7, 8]
+
+    x, y = np.meshgrid(x, y)
+    matrix = np.array(
+        [
+            [-1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0],
+            [-1, 1, 2, 3, 4, 4, 4, 3, 2, 1, 1],
+            [-1, -1., 4, 5, 6, 8, 6, 5, 4, 3, -1.],
+            [-1, -1., 7, 8, 11, 12, 11, 8, 7, -1., -1.],
+            [-1, -1., 8, 9, 10, 16, 10, 9, 10, 7, -1.],
+            [-1, -1., -1., 12, 16, 20, 16, 12, 11, -1., -1.],
+            [-1, -1., -1., -1., 22, 24, 22, 20, 18, -1., -1.],
+            [-1, -1., -1., -1., -1., 28, 26, 25, -1., -1., -1.],
+        ]
+    )
+    z = np.ma.masked_less(matrix, 0)
+    norm = mcolors.Normalize(vmax=z.max(), vmin=z.min())
+    colors = plt.get_cmap("plasma")(norm(z))
+    ax.plot_surface(x, y, z, facecolors=colors)
+    ax.view_init(30, -80)
+
+
 @mpl3d_image_comparison(['text3d.png'], remove_text=False)
 def test_text3d():
     fig = plt.figure()