.. only:: html

    .. note::
        :class: sphx-glr-download-link-note

        Click :ref:`here <sphx_glr_download_gallery_images_contours_and_fields_pcolormesh_grids.py>`     to download the full example code
    .. rst-class:: sphx-glr-example-title

    .. _sphx_glr_gallery_images_contours_and_fields_pcolormesh_grids.py:


============================
pcolormesh grids and shading
============================

`.axes.Axes.pcolormesh` and `~.axes.Axes.pcolor` have a few options for
how grids are laid out and the shading between the grid points.

Generally, if *Z* has shape *(M, N)* then the grid *X* and *Y* can be
specified with either shape *(M+1, N+1)* or *(M, N)*, depending on the
argument for the ``shading`` keyword argument.  Note that below we specify
vectors *x* as either length N or N+1 and *y* as length M or M+1, and
`~.axes.Axes.pcolormesh` internally makes the mesh matrices *X* and *Y* from
the input vectors.


.. code-block:: default


    import matplotlib
    import matplotlib.pyplot as plt
    import numpy as np








Flat Shading
------------

The grid specification with the least assumptions is ``shading='flat'``
and if the grid is one larger than the data in each dimension, i.e. has shape
*(M+1, N+1)*.  In that case *X* and *Y* specify the corners of quadrilaterals
that are colored with the values in *Z*. Here we specify the edges of the
*(3, 5)* quadrilaterals with *X* and *Y* that are *(4, 6)*.


.. code-block:: default


    nrows = 3
    ncols = 5
    Z = np.arange(nrows * ncols).reshape(nrows, ncols)
    x = np.arange(ncols + 1)
    y = np.arange(nrows + 1)

    fig, ax = plt.subplots()
    ax.pcolormesh(x, y, Z, shading='flat', vmin=Z.min(), vmax=Z.max())


    def _annotate(ax, x, y, title):
        # this all gets repeated below:
        X, Y = np.meshgrid(x, y)
        ax.plot(X.flat, Y.flat, 'o', color='m')
        ax.set_xlim(-0.7, 5.2)
        ax.set_ylim(-0.7, 3.2)
        ax.set_title(title)

    _annotate(ax, x, y, "shading='flat'")





.. image:: /gallery/images_contours_and_fields/images/sphx_glr_pcolormesh_grids_001.png
    :alt: shading='flat'
    :class: sphx-glr-single-img





Flat Shading, same shape grid
-----------------------------

Often, however, data is provided where *X* and *Y* match the shape of *Z*.
As of Matplotlib v3.3, ``shading='flat'`` is deprecated when this is the
case, a warning is raised, and the last row and column of *Z* are dropped.
This dropping of the last row and column is what Matplotlib did silently
previous to v3.3, and is compatible with what Matlab does.


.. code-block:: default


    x = np.arange(ncols)  # note *not* ncols + 1 as before
    y = np.arange(nrows)
    fig, ax = plt.subplots()
    ax.pcolormesh(x, y, Z, shading='flat', vmin=Z.min(), vmax=Z.max())
    _annotate(ax, x, y, "shading='flat': X, Y, C same shape")




.. image:: /gallery/images_contours_and_fields/images/sphx_glr_pcolormesh_grids_002.png
    :alt: shading='flat': X, Y, C same shape
    :class: sphx-glr-single-img


.. rst-class:: sphx-glr-script-out

 Out:

 .. code-block:: none

    /build/matplotlib-FmqznT/matplotlib-3.3.2/examples/images_contours_and_fields/pcolormesh_grids.py:66: MatplotlibDeprecationWarning: shading='flat' when X and Y have the same dimensions as C is deprecated since 3.3.  Either specify the corners of the quadrilaterals with X and Y, or pass shading='auto', 'nearest' or 'gouraud', or set rcParams['pcolor.shading'].  This will become an error two minor releases later.
      ax.pcolormesh(x, y, Z, shading='flat', vmin=Z.min(), vmax=Z.max())




Nearest Shading, same shape grid
--------------------------------

Usually, dropping a row and column of data is not what the user means when
they make *X*, *Y* and *Z* all the same shape.  For this case, Matplotlib
allows ``shading='nearest'`` and centers the colored quadrilaterals on the
grid points.

If a grid that is not the correct shape is passed with ``shading='nearest'``
an error is raised.


.. code-block:: default


    fig, ax = plt.subplots()
    ax.pcolormesh(x, y, Z, shading='nearest', vmin=Z.min(), vmax=Z.max())
    _annotate(ax, x, y, "shading='nearest'")




.. image:: /gallery/images_contours_and_fields/images/sphx_glr_pcolormesh_grids_003.png
    :alt: shading='nearest'
    :class: sphx-glr-single-img





Auto Shading
------------

It's possible that the user would like the code to automatically choose which
to use, in this case ``shading='auto'`` will decide whether to use 'flat' or
'nearest' shading based on the shapes of *X*, *Y* and *Z*.


.. code-block:: default


    fig, axs = plt.subplots(2, 1, constrained_layout=True)
    ax = axs[0]
    x = np.arange(ncols)
    y = np.arange(nrows)
    ax.pcolormesh(x, y, Z, shading='auto', vmin=Z.min(), vmax=Z.max())
    _annotate(ax, x, y, "shading='auto'; X, Y, Z: same shape (nearest)")

    ax = axs[1]
    x = np.arange(ncols + 1)
    y = np.arange(nrows + 1)
    ax.pcolormesh(x, y, Z, shading='auto', vmin=Z.min(), vmax=Z.max())
    _annotate(ax, x, y, "shading='auto'; X, Y one larger than Z (flat)")




.. image:: /gallery/images_contours_and_fields/images/sphx_glr_pcolormesh_grids_004.png
    :alt: shading='auto'; X, Y, Z: same shape (nearest), shading='auto'; X, Y one larger than Z (flat)
    :class: sphx-glr-single-img





Gouraud Shading
---------------

`Gouraud shading <https://en.wikipedia.org/wiki/Gouraud_shading>`_ can also
be specified, where the color in the quadrilaterals is linearly interpolated
between the grid points.  The shapes of *X*, *Y*, *Z* must be the same.


.. code-block:: default


    fig, ax = plt.subplots(constrained_layout=True)
    x = np.arange(ncols)
    y = np.arange(nrows)
    ax.pcolormesh(x, y, Z, shading='gouraud', vmin=Z.min(), vmax=Z.max())
    _annotate(ax, x, y, "shading='gouraud'; X, Y same shape as Z")

    plt.show()



.. image:: /gallery/images_contours_and_fields/images/sphx_glr_pcolormesh_grids_005.png
    :alt: shading='gouraud'; X, Y same shape as Z
    :class: sphx-glr-single-img





------------

References
""""""""""

The use of the following functions and methods is shown in this example:


.. code-block:: default


    matplotlib.axes.Axes.pcolormesh
    matplotlib.pyplot.pcolormesh




.. rst-class:: sphx-glr-script-out

 Out:

 .. code-block:: none


    <function pcolormesh at 0x7efe0403e4c0>




.. rst-class:: sphx-glr-timing

   **Total running time of the script:** ( 0 minutes  2.019 seconds)


.. _sphx_glr_download_gallery_images_contours_and_fields_pcolormesh_grids.py:


.. only :: html

 .. container:: sphx-glr-footer
    :class: sphx-glr-footer-example



  .. container:: sphx-glr-download sphx-glr-download-python

     :download:`Download Python source code: pcolormesh_grids.py <pcolormesh_grids.py>`



  .. container:: sphx-glr-download sphx-glr-download-jupyter

     :download:`Download Jupyter notebook: pcolormesh_grids.ipynb <pcolormesh_grids.ipynb>`


.. only:: html

 .. rst-class:: sphx-glr-signature

    Keywords: matplotlib code example, codex, python plot, pyplot
    `Gallery generated by Sphinx-Gallery
    <https://sphinx-gallery.readthedocs.io>`_
