"""
.. _terrain_following_mesh_example:

Terrain Following Mesh
~~~~~~~~~~~~~~~~~~~~~~

Use a topographic surface to create a 3D terrain-following mesh.

Terrain following meshes are common in the environmental sciences, for instance
in hydrological modelling (see
`Maxwell 2013 <https://www.sciencedirect.com/science/article/abs/pii/S0309170812002564>`_
and
`ParFlow <https://parflow.org>`_).

In this example, we demonstrate a simple way to make a 3D grid/mesh that
follows a given topographic surface. In this example, it is important to note
that the given digital elevation model (DEM) is structured (gridded and not
triangulated): this is common for DEMs.
"""

import numpy as np

# sphinx_gallery_thumbnail_number = 3
import pyvista as pv
from pyvista import examples

# %%
# Download a gridded topography surface (DEM)
dem = examples.download_crater_topo()
dem

# %%
# Now let's subsample and extract an area of interest to make this example
# simple (also the DEM we just load is pretty big).
# Since the DEM we loaded is a :class:`pyvista.ImageData` mesh, we can use
# the :func:`pyvista.ImageDataFilters.extract_subset` filter:
subset = dem.extract_subset((500, 900, 400, 800, 0, 0), (5, 5, 1))
subset.plot(cpos='xy')


# %%
# Now that we have a region of interest for our terrain following mesh, lets
# make a 3D surface of that DEM:
terrain = subset.warp_by_scalar()
terrain

# %%
terrain.plot()


# %%
# And now we have a 3D structured surface of the terrain. We can now extend
# that structured surface into a 3D mesh to form a terrain following grid.
# To do this, we first our cell spacings in the z-direction (these start
# from the terrain surface). Then we repeat the XYZ structured coordinates
# of the terrain mesh and decrease each Z level by our Z cell spacing.
# Once we have those structured coordinates, we can create a
# :class:`pyvista.StructuredGrid`.

z_cells = np.array([25] * 5 + [35] * 3 + [50] * 2 + [75, 100])

xx = np.repeat(terrain.x, len(z_cells), axis=-1)
yy = np.repeat(terrain.y, len(z_cells), axis=-1)
zz = np.repeat(terrain.z, len(z_cells), axis=-1) - np.cumsum(z_cells).reshape((1, 1, -1))

mesh = pv.StructuredGrid(xx, yy, zz)
mesh['Elevation'] = zz.ravel(order='F')
mesh

# %%
cpos = pv.CameraPosition(
    position=(1826736.796308761, 5655837.275274233, 4676.8405505181745),
    focal_point=(1821066.1790519988, 5649248.765538796, 943.0995128226014),
    viewup=(-0.2797856225380979, -0.27966946337594883, 0.9184252809434081),
)

mesh.plot(show_edges=True, lighting=False, cpos=cpos)
# %%
# .. tags:: load
