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"""
.. _surface_normal_example:
Computing Surface Normals
~~~~~~~~~~~~~~~~~~~~~~~~~
Compute normals on a surface.
"""
from __future__ import annotations
import numpy as np
# sphinx_gallery_thumbnail_number = 2
from pyvista import examples
# %%
# Computing the normals of a surface is quite easy using
# :class:`pyvista.PolyData`'s :func:`pyvista.PolyDataFilters.compute_normals`
# method.
mesh = examples.download_topo_global()
mesh.plot(cmap="gist_earth", show_scalar_bar=False)
# %%
# Now we have a surface dataset of the globe loaded - unfortunately, the
# dataset shows the globe with a uniform radius which hides topographic relief.
# Using :func:`pyvista.PolyDataFilters.compute_normals`, we can compute the normal
# vectors on the globe at all points in the dataset, then use the values given
# in the dataset to warp the surface in the normals direction to create some
# exaggerated topographic relief.
# Compute the normals in-place and use them to warp the globe
mesh.compute_normals(inplace=True) # this activates the normals as well
# Now use those normals to warp the surface
warp = mesh.warp_by_scalar(factor=0.5e-5)
# And let's see it
warp.plot(cmap="gist_earth", show_scalar_bar=False)
# %%
# We could also use face/cell normals to extract all the faces of a mesh
# facing a general direction. In the following snippet, we take a mesh, compute
# the normals along its cell faces, and extract the faces that face upward.
mesh = examples.download_nefertiti()
# Compute normals
mesh.compute_normals(cell_normals=True, point_normals=False, inplace=True)
# Get list of cell IDs that meet condition
ids = np.arange(mesh.n_cells)[mesh['Normals'][:, 2] > 0.0]
# Extract those cells
top = mesh.extract_cells(ids)
cpos = [
(-834.3184529757553, -918.4677714398535, 236.5468795300025),
(11.03829376004883, -13.642289291587957, -35.91218884207208),
(0.19212361465657216, 0.11401076390090074, 0.9747256344254143),
]
top.plot(cpos=cpos, color=True)
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