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

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

.. _sphx_glr_gallery_color_custom_cmap.py:


=========================================
Creating a colormap from a list of colors
=========================================

For more detail on creating and manipulating colormaps see
:doc:`/tutorials/colors/colormap-manipulation`.

Creating a :doc:`colormap </tutorials/colors/colormaps>`
from a list of colors can be done with the
:meth:`~.colors.LinearSegmentedColormap.from_list` method of
`LinearSegmentedColormap`. You must pass a list of RGB tuples that define the
mixture of colors from 0 to 1.


Creating custom colormaps
-------------------------
It is also possible to create a custom mapping for a colormap. This is
accomplished by creating dictionary that specifies how the RGB channels
change from one end of the cmap to the other.

Example: suppose you want red to increase from 0 to 1 over the bottom
half, green to do the same over the middle half, and blue over the top
half.  Then you would use::

  cdict = {'red':   ((0.0,  0.0, 0.0),
                     (0.5,  1.0, 1.0),
                     (1.0,  1.0, 1.0)),

           'green': ((0.0,  0.0, 0.0),
                     (0.25, 0.0, 0.0),
                     (0.75, 1.0, 1.0),
                     (1.0,  1.0, 1.0)),

           'blue':  ((0.0,  0.0, 0.0),
                     (0.5,  0.0, 0.0),
                     (1.0,  1.0, 1.0))}

If, as in this example, there are no discontinuities in the r, g, and b
components, then it is quite simple: the second and third element of
each tuple, above, is the same--call it "y".  The first element ("x")
defines interpolation intervals over the full range of 0 to 1, and it
must span that whole range.  In other words, the values of x divide the
0-to-1 range into a set of segments, and y gives the end-point color
values for each segment.

Now consider the green. cdict['green'] is saying that for
0 <= x <= 0.25, y is zero; no green.
0.25 < x <= 0.75, y varies linearly from 0 to 1.
x > 0.75, y remains at 1, full green.

If there are discontinuities, then it is a little more complicated.
Label the 3 elements in each row in the cdict entry for a given color as
(x, y0, y1).  Then for values of x between x[i] and x[i+1] the color
value is interpolated between y1[i] and y0[i+1].

Going back to the cookbook example, look at cdict['red']; because y0 !=
y1, it is saying that for x from 0 to 0.5, red increases from 0 to 1,
but then it jumps down, so that for x from 0.5 to 1, red increases from
0.7 to 1.  Green ramps from 0 to 1 as x goes from 0 to 0.5, then jumps
back to 0, and ramps back to 1 as x goes from 0.5 to 1.::

  row i:   x  y0  y1
                  /
                 /
  row i+1: x  y0  y1

Above is an attempt to show that for x in the range x[i] to x[i+1], the
interpolation is between y1[i] and y0[i+1].  So, y0[0] and y1[-1] are
never used.




.. code-block:: python

    import numpy as np
    import matplotlib.pyplot as plt
    from matplotlib.colors import LinearSegmentedColormap

    # Make some illustrative fake data:

    x = np.arange(0, np.pi, 0.1)
    y = np.arange(0, 2 * np.pi, 0.1)
    X, Y = np.meshgrid(x, y)
    Z = np.cos(X) * np.sin(Y) * 10








--- Colormaps from a list ---



.. code-block:: python


    colors = [(1, 0, 0), (0, 1, 0), (0, 0, 1)]  # R -> G -> B
    n_bins = [3, 6, 10, 100]  # Discretizes the interpolation into bins
    cmap_name = 'my_list'
    fig, axs = plt.subplots(2, 2, figsize=(6, 9))
    fig.subplots_adjust(left=0.02, bottom=0.06, right=0.95, top=0.94, wspace=0.05)
    for n_bin, ax in zip(n_bins, axs.ravel()):
        # Create the colormap
        cm = LinearSegmentedColormap.from_list(
            cmap_name, colors, N=n_bin)
        # Fewer bins will result in "coarser" colomap interpolation
        im = ax.imshow(Z, interpolation='nearest', origin='lower', cmap=cm)
        ax.set_title("N bins: %s" % n_bin)
        fig.colorbar(im, ax=ax)





.. image:: /gallery/color/images/sphx_glr_custom_cmap_001.png
    :class: sphx-glr-single-img




--- Custom colormaps ---



.. code-block:: python


    cdict1 = {'red':   ((0.0, 0.0, 0.0),
                       (0.5, 0.0, 0.1),
                       (1.0, 1.0, 1.0)),

             'green': ((0.0, 0.0, 0.0),
                       (1.0, 0.0, 0.0)),

             'blue':  ((0.0, 0.0, 1.0),
                       (0.5, 0.1, 0.0),
                       (1.0, 0.0, 0.0))
            }

    cdict2 = {'red':   ((0.0, 0.0, 0.0),
                       (0.5, 0.0, 1.0),
                       (1.0, 0.1, 1.0)),

             'green': ((0.0, 0.0, 0.0),
                       (1.0, 0.0, 0.0)),

             'blue':  ((0.0, 0.0, 0.1),
                       (0.5, 1.0, 0.0),
                       (1.0, 0.0, 0.0))
            }

    cdict3 = {'red':  ((0.0, 0.0, 0.0),
                       (0.25, 0.0, 0.0),
                       (0.5, 0.8, 1.0),
                       (0.75, 1.0, 1.0),
                       (1.0, 0.4, 1.0)),

             'green': ((0.0, 0.0, 0.0),
                       (0.25, 0.0, 0.0),
                       (0.5, 0.9, 0.9),
                       (0.75, 0.0, 0.0),
                       (1.0, 0.0, 0.0)),

             'blue':  ((0.0, 0.0, 0.4),
                       (0.25, 1.0, 1.0),
                       (0.5, 1.0, 0.8),
                       (0.75, 0.0, 0.0),
                       (1.0, 0.0, 0.0))
            }

    # Make a modified version of cdict3 with some transparency
    # in the middle of the range.
    cdict4 = {**cdict3,
              'alpha': ((0.0, 1.0, 1.0),
                    #   (0.25,1.0, 1.0),
                        (0.5, 0.3, 0.3),
                    #   (0.75,1.0, 1.0),
                        (1.0, 1.0, 1.0)),
              }








Now we will use this example to illustrate 3 ways of
handling custom colormaps.
First, the most direct and explicit:



.. code-block:: python


    blue_red1 = LinearSegmentedColormap('BlueRed1', cdict1)







Second, create the map explicitly and register it.
Like the first method, this method works with any kind
of Colormap, not just
a LinearSegmentedColormap:



.. code-block:: python


    blue_red2 = LinearSegmentedColormap('BlueRed2', cdict2)
    plt.register_cmap(cmap=blue_red2)







Third, for LinearSegmentedColormap only,
leave everything to register_cmap:



.. code-block:: python


    plt.register_cmap(name='BlueRed3', data=cdict3)  # optional lut kwarg
    plt.register_cmap(name='BlueRedAlpha', data=cdict4)







Make the figure:



.. code-block:: python


    fig, axs = plt.subplots(2, 2, figsize=(6, 9))
    fig.subplots_adjust(left=0.02, bottom=0.06, right=0.95, top=0.94, wspace=0.05)

    # Make 4 subplots:

    im1 = axs[0, 0].imshow(Z, interpolation='nearest', cmap=blue_red1)
    fig.colorbar(im1, ax=axs[0, 0])

    cmap = plt.get_cmap('BlueRed2')
    im2 = axs[1, 0].imshow(Z, interpolation='nearest', cmap=cmap)
    fig.colorbar(im2, ax=axs[1, 0])

    # Now we will set the third cmap as the default.  One would
    # not normally do this in the middle of a script like this;
    # it is done here just to illustrate the method.

    plt.rcParams['image.cmap'] = 'BlueRed3'

    im3 = axs[0, 1].imshow(Z, interpolation='nearest')
    fig.colorbar(im3, ax=axs[0, 1])
    axs[0, 1].set_title("Alpha = 1")

    # Or as yet another variation, we can replace the rcParams
    # specification *before* the imshow with the following *after*
    # imshow.
    # This sets the new default *and* sets the colormap of the last
    # image-like item plotted via pyplot, if any.
    #

    # Draw a line with low zorder so it will be behind the image.
    axs[1, 1].plot([0, 10 * np.pi], [0, 20 * np.pi], color='c', lw=20, zorder=-1)

    im4 = axs[1, 1].imshow(Z, interpolation='nearest')
    fig.colorbar(im4, ax=axs[1, 1])

    # Here it is: changing the colormap for the current image and its
    # colorbar after they have been plotted.
    im4.set_cmap('BlueRedAlpha')
    axs[1, 1].set_title("Varying alpha")
    #

    fig.suptitle('Custom Blue-Red colormaps', fontsize=16)
    fig.subplots_adjust(top=0.9)

    plt.show()




.. image:: /gallery/color/images/sphx_glr_custom_cmap_002.png
    :class: sphx-glr-single-img




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

References
""""""""""

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



.. code-block:: python


    import matplotlib
    matplotlib.axes.Axes.imshow
    matplotlib.pyplot.imshow
    matplotlib.figure.Figure.colorbar
    matplotlib.pyplot.colorbar
    matplotlib.colors
    matplotlib.colors.LinearSegmentedColormap
    matplotlib.colors.LinearSegmentedColormap.from_list
    matplotlib.cm
    matplotlib.cm.ScalarMappable.set_cmap
    matplotlib.pyplot.register_cmap
    matplotlib.cm.register_cmap







.. _sphx_glr_download_gallery_color_custom_cmap.py:


.. only :: html

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



  .. container:: sphx-glr-download

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



  .. container:: sphx-glr-download

     :download:`Download Jupyter notebook: custom_cmap.ipynb <custom_cmap.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>`_
