BUG: prevent NaNs in layered overlay compositing when resulting alpha is zero (#13714)

Co-authored-by: Marijn van Vliet <w.m.vanvliet@gmail.com>

Author: PragnyaKhandelwal

.mailmap

@@ -291,6 +291,8 @@ Pierre-Antoine Bannier <pierreantoine.bannier@gmail.com> Pierre-Antoine Bannier
 Pierre-Antoine Bannier <pierreantoine.bannier@gmail.com> Pierre-Antoine Bannier <pierre-antoine@MacBook-Pro-de-Pierre-Antoine.local>
 Pierre-Antoine Bannier <pierreantoine.bannier@gmail.com> Pierre-Antoine Bannier <pierre-antoine@mbpdepieantoine.home>
 Ping-Keng Jao <ping-keng.jao@alumni.epfl.ch> nafraw <ping-keng.jao@alumni.epfl.ch>
+Pragnya Khandelwal <prag1704@gmail.com> Pragnya <prag1704@gmail.com>
+Pragnya Khandelwal <prag1704@gmail.com> PragnyaKhandelwal <prag1704@gmail.com>
 Praveen Sripad <pravsripad@gmail.com> prav <prav@prav-dell.(none)>
 Praveen Sripad <pravsripad@gmail.com> prav <pravsripad@gmail.com>
 Proloy Das <pdas6@mgh.harvard.edu> pdas6 <pdas6@mgh.harvard.edu>

doc/changes/dev/13714.bugfix.rst

@@ -0,0 +1 @@
+Fix bug in 3D overlay compositing that could produce NaN RGBA values when the resulting alpha is zero, by `Pragnya Khandelwal`_.

mne/viz/_3d_overlay.py

@@ -119,7 +119,9 @@ def _compute_over(self, B, A):
         C[:, :3] *= A_w
         C[:, :3] += B[:, :3] * B_w
         C[:, 3:] += B_w
-        C[:, :3] /= C[:, 3:]
+        C_alpha_zero = C[:, 3] == 0
+        C[~C_alpha_zero, :3] /= C[~C_alpha_zero, 3:]
+        C[C_alpha_zero, :3] = 0
         return np.clip(C, 0, 1, out=C)
 
     def _compose_overlays(self):

mne/viz/_brain/tests/test_brain.py

@@ -138,23 +138,27 @@ def test_layered_mesh(renderer_interactive_pyvistaqt):
     assert len(mesh._overlays) == 0
     mesh.add_overlay(
         scalars=np.array([0, 1, 1, 0]),
-        colormap=np.array([(1, 1, 1, 1), (0, 0, 0, 0)]),
+        colormap=np.array([(255, 255, 255, 255), (0, 0, 0, 0)]),
         rng=[0, 1],
         opacity=None,
         name="test1",
     )
-    assert mesh._current_colors is not None
+    assert_array_equal(
+        mesh._current_colors, [[1, 1, 1, 1], [0, 0, 0, 0], [0, 0, 0, 0], [1, 1, 1, 1]]
+    )
     assert mesh._cached_colors is None
     assert len(mesh._overlays) == 1
     assert "test1" in mesh._overlays
     mesh.add_overlay(
-        scalars=np.array([1, 0, 0, 1]),
-        colormap=np.array([(1, 1, 1, 1), (0, 0, 0, 0)]),
+        scalars=np.array([1, 1, 0, 0]),
+        colormap=np.array([(255, 255, 255, 255), (0, 0, 0, 0)]),
         rng=[0, 1],
         opacity=None,
         name="test2",
     )
-    assert mesh._current_colors is not None
+    assert_array_equal(
+        mesh._current_colors, [[1, 1, 1, 1], [0, 0, 0, 0], [1, 1, 1, 1], [1, 1, 1, 1]]
+    )
     assert mesh._cached_colors is not None
     assert len(mesh._overlays) == 2
     assert "test2" in mesh._overlays