add plot face normals

pyvista/core/filters/data_set.py

@@ -4418,8 +4418,10 @@ def reflect(self, normal, point=None, inplace=False,
 
         """
         t = transformations.reflection(normal, point=point)
-        return self.transform(t, transform_all_input_vectors=transform_all_input_vectors,
-                                 inplace=inplace, progress_bar=progress_bar)
+        return self.transform(
+            t, transform_all_input_vectors=transform_all_input_vectors,
+            inplace=inplace, progress_bar=progress_bar
+        )
 
     def reconstruct_surface(self, nbr_sz=None, sample_spacing=None,
                             progress_bar=False):

pyvista/core/filters/poly_data.py

@@ -1925,7 +1925,7 @@ def plot_boundaries(self, edge_color="red", line_width=None,
         return plotter.show()
 
     def plot_normals(self, show_mesh=True, mag=1.0, flip=False,
-                     use_every=1, color=None, **kwargs):
+                     use_every=1, faces=False, color=None, **kwargs):
         """Plot the point normals of a mesh.
 
         Parameters
@@ -1945,6 +1945,9 @@ def plot_normals(self, show_mesh=True, mag=1.0, flip=False,
             displayed.  Display every 10th normal by setting this
             parameter to 10.
 
+        faces : bool, optional
+            Plot face normals instead of the default point normals.
+
         color : str, optional
             Color of the arrows.  Defaults to
             :attr:`pyvista.themes.DefaultTheme.edge_color`.
@@ -1961,11 +1964,17 @@ def plot_normals(self, show_mesh=True, mag=1.0, flip=False,
 
         Examples
         --------
-        Plot the normals of a sphere.
+        Plot the point normals of a sphere.
 
         >>> import pyvista as pv
-        >>> sphere = pv.Sphere()
-        >>> sphere.plot_normals(mag=0.1)
+        >>> sphere = pv.Sphere(phi_resolution=10, theta_resolution=10)
+        >>> sphere.plot_normals(mag=0.1, show_edges=True)
+
+        Plot the face normals of a sphere.
+
+        >>> import pyvista as pv
+        >>> sphere = pv.Sphere(phi_resolution=10, theta_resolution=10)
+        >>> sphere.plot_normals(mag=0.1, faces=True, show_edges=True)
 
         """
         plotter = pyvista.Plotter(off_screen=kwargs.pop('off_screen', None),
@@ -1976,11 +1985,19 @@ def plot_normals(self, show_mesh=True, mag=1.0, flip=False,
         if color is None:
             color = pyvista.global_theme.edge_color
 
-        normals = self.point_normals
+        if faces:
+            centers = self.cell_centers().points[::use_every]
+            normals = self.cell_normals
+        else:
+            centers = self.points[::use_every]
+            normals = self.point_normals
+
         if flip:
             normals *= -1
-        plotter.add_arrows(self.points[::use_every], normals[::use_every],
+
+        plotter.add_arrows(centers, normals[::use_every],
                            mag=mag, color=color, show_scalar_bar=False)
+
         return plotter.show()
 
     def remove_points(self, remove, mode='any', keep_scalars=True, inplace=False):

tests/test_polydata.py

@@ -12,6 +12,11 @@
 
 radius = 0.5
 
+skip_plotting = pytest.mark.skipif(
+    not system_supports_plotting(), reason="Requires system to support plotting"
+)
+
+
 @pytest.fixture
 def sphere():
     # this shadows the main sphere fixture from conftest!
@@ -245,7 +250,7 @@ def test_ray_trace(sphere):
     assert np.any(ind)
 
 
-@pytest.mark.skipif(not system_supports_plotting(), reason="Requires system to support plotting")
+@skip_plotting
 def test_ray_trace_plot(sphere):
     points, ind = sphere.ray_trace([0, 0, 0], [1, 1, 1], plot=True, first_point=True,
                                    off_screen=True)
@@ -265,7 +270,7 @@ def test_multi_ray_trace(sphere):
     assert np.any(ind_t)
 
 
-@pytest.mark.skipif(not system_supports_plotting(), reason="Requires system to support plotting")
+@skip_plotting
 def test_plot_curvature(sphere):
     sphere.plot_curvature(off_screen=True)
 
@@ -563,16 +568,17 @@ def test_volume(sphere_dense):
     assert np.isclose(sphere_dense.volume, ideal_volume, rtol=1E-3)
 
 
-@pytest.mark.skipif(not system_supports_plotting(), reason="Requires system to support plotting")
+@skip_plotting
 def test_plot_boundaries():
     # make sure to plot an object that has boundaries
     pyvista.Cube().plot_boundaries(off_screen=True)
 
 
-@pytest.mark.skipif(not system_supports_plotting(), reason="Requires system to support plotting")
+@skip_plotting
 @pytest.mark.parametrize('flip', [True, False])
-def test_plot_normals(sphere, flip):
-    sphere.plot_normals(off_screen=True, flip=flip)
+@pytest.mark.parametrize('faces', [True, False])
+def test_plot_normals(sphere, flip, faces):
+    sphere.plot_normals(off_screen=True, flip=flip, faces=faces)
 
 
 def test_remove_points_any(sphere):