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# Creating a colormap from a list of colors

Creating a colormap from a list of colors can be done with the `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.

    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 ---

    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)

    # --- Custom colormaps ---

    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.copy()
    cdict4['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:

    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:

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

    # Third, for LinearSegmentedColormap only,
    # leave everything to register_cmap:

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

    # Make the figure:

    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)

    plt.show()

Total running time of the script: ( 0 minutes 0.468 seconds)

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