Basic API Usage
===============

PyVista provides tools to get started with just about any VTK dataset
and wrap that object into an easily accessible data object.
Whether you are new to the VTK library or a power user, the best place to
get started is with PyVista's :func:`pyvista.wrap` and :func:`pyvista.read`
functions to either wrap a VTK data object in memory or read a VTK or
VTK-friendly file format.


Wrapping a VTK Data Object
~~~~~~~~~~~~~~~~~~~~~~~~~~

The wrapping function :func:`pyvista.wrap` is usable from the top level of
PyVista. This allows users to quickly wrap any VTK dataset they have as a
PyVista object:

.. code:: python

    import vtk
    import pyvista as pv

    stuff = vtk.vtkPolyData()
    better = pv.wrap(stuff)


Reading a VTK File
~~~~~~~~~~~~~~~~~~

PyVista provides a convenience function to read VTK file formats into their
respective PyVista data objects. Simply call the :func:`pyvista.read` function
passing the filename:

.. code-block:: python

    import pyvista as pv
    mesh = pv.read('my_strange_vtk_file.vtk')


Accessing the Wrapped Data Object
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Now that you have a wrapped VTK data object, you can start accessing
and modifying the dataset. Some of the most common properties to
access include the points and point/cell data (the data attributes
assigned to the nodes or cells of the mesh respectively).

First, check out some common meta-properties:

.. code:: python

    >>> import pyvista as pv
    >>> from pyvista import examples
    >>> import numpy as np

.. code:: python

    >>> mesh = examples.load_airplane()

    Inspect how many cells are in this mesh

    >>> mesh.n_cells
    2452

    Inspect how many points are in this mesh
    >>> mesh.n_points
    1335

    What about scalar arrays? Are there any?

    >>> mesh.n_arrays
    0

    What are the mesh bounds?
    >>> mesh.bounds
    [139.061, 1654.93, 32.09, 1319.95, -17.74, 282.13]

    Where is the center of this mesh?

    >>> mesh.center
    [897.0, 676.02, 132.19]


Access the points by fetching the :attr:`points
<pyvista.DataSet.points>` attribute on any PyVista mesh
as a :class:`numpy.ndarray`:

.. code:: python

    >>> the_pts = mesh.points
    >>> isinstance(the_pts, np.ndarray)
    True

    >>> the_pts[0:5, :]
    array([[896.994 ,  48.7601,  82.2656],
           [906.593 ,  48.7601,  80.7452],
           [907.539 ,  55.4902,  83.6581],
           [896.994 ,  55.4902,  85.3283],
           [896.994 ,  42.8477,  77.825 ]], dtype=float32)


Accessing the different data attributes on the nodes and cells of the mesh
is interfaced via dictionaries with callbacks to the VTK object.
These dictionaries of the different point and cell arrays can be directly
accessed and modified as NumPy arrays. In the example below, we load a dataset,
access an array on that dataset, then add some more data:

.. code-block:: python

    >>> mesh = examples.load_uniform()

    Fetch a data array from the point data dictionary

    >>> arr = mesh.point_data['Spatial Point Data']

    Assign a new array to the cell data:

    >>> mesh.cell_data['foo'] = np.random.rand(mesh.n_cells)

    Don't remember if your array is point or cell data? You can
    directly query the mesh object and access the array from the
    dataset.

    >>> foo = mesh['foo']
    >>> isinstance(foo, np.ndarray)
    True

    Or maybe you just want to add an array where it fits.

    >>> mesh['new-array'] = np.random.rand(mesh.n_points)


Plotting
~~~~~~~~

PyVista includes numerous plotting routines that are intended to be intuitive
and highly controllable with ``matplotlib`` similar syntax and keyword
arguments.

To get started, try out the :func:`pyvista.plot` convenience method
that is bound to each PyVista data object.

.. pyvista-plot::
   :include-source: false
   :nofigs:
   :context:

   # must have this here as our global backend may not be static
   import pyvista
   pyvista.set_plot_theme('document')
   pyvista.set_jupyter_backend('static')
   pyvista.global_theme.window_size = [600, 400]
   pyvista.global_theme.axes.show = False
   pyvista.global_theme.smooth_shading = True
   pyvista.global_theme.anti_aliasing = 'fxaa'
   pyvista.global_theme.show_scalar_bar = False


.. pyvista-plot::
    :context:

    import pyvista as pv
    from pyvista import examples

    mesh = examples.load_airplane()
    mesh.plot()


You can also create a plotter object to fine tune the scene. First,
instantiate a plotter such as :class:`pyvista.Plotter` or
:class:`pyvistaqt.BackgroundPlotter`.  The :class:`pyvista.Plotter`
will create a rendering window that will pause the execution of the
code after calling :func:`show() <pyvista.Plotter.show>`.

.. pyvista-plot::
    :context:

    mesh = examples.load_airplane()

    plotter = pv.Plotter()    # instantiate the plotter
    plotter.add_mesh(mesh)    # add a mesh to the scene
    plotter.camera.zoom(2)    # Note how we can now access underlying attributes
    plotter.show()            # show the rendering window


Optionally :func:`show() <pyvista.Plotter.show>` can return
the last used camera position of the rendering window in case you want
to choose a camera position and use it again later. The camera
position is also available as the :attr:`camera_position
<pyvista.Plotter.camera_position>` attribute of the plotter (even
after it's closed).

You can then use this cached camera position for additional plotting
without having to manually interact with the plotting window:

.. code:: python

    # reuse the camera position from the previous plotter
    cpos = plotter.camera_position
    plotter = pv.Plotter(off_screen=True)
    plotter.add_mesh(mesh, color='lightblue')
    plotter.camera_position = cpos
    plotter.show(screenshot='airplane.png')


Be sure to check out all the available plotters and their options for
your use case:

* :class:`pyvista.Plotter`: The standard plotter that pauses the code
  until closed.
* :class:`pyvistaqt.BackgroundPlotter`: Creates a rendering window that
  is interactive and does not pause the code execution (for more
  information see the `pyvistaqt`_ library)

.. _pyvistaqt: https://qtdocs.pyvista.org/



Exporting
~~~~~~~~~

Any PyVista mesh object can be saved to a VTK file format using
:func:`save() <pyvista.DataObject.save>`. For example, the mesh in the
code block above could be saved like:

.. code:: python

    mesh.save("mesh.vtk")

Or since that mesh is :class:`pyvista.PolyData`, we could use the ``.vtp``,
``.stl``, or ``.ply`` formats as well.
For more details on which formats are supported in the ``.save()`` method,
please refer to the docs for that method on each mesh type.

Also note that we can export any PyVista mesh to any file format supported by
`meshio <https://github.com/nschloe/meshio>`_. Meshio supports many formats
including: Abaqus, Ansys msh, AVS-UCD, CGNS, DOLFIN XML, Exodus, FLAC3D, H5M,
Kratos/MDPA, Medit, MED/Salome, Gmsh (versions 2 and 4), OBJ, OFF, PERMAS,
PLY, STL, TetGen .node/.ele, SVG (2D only, output only), UGRID, WKT (TIN),
XDMF, and more.

To save a PyVista mesh using ``meshio``, use :func:`pyvista.save_meshio`:

.. code-block:: python

    pv.save_meshio("mesh.obj", mesh)
