Python create colormap from named colors#

Software requirements:
Python 3
numpy
matplotlib
Example script#

generate_new_colormap_from_named_colors_rgb.py
#!/usr/bin/env python
'''
DKRZ example 

Generate colormaps from given named colors

This script generates a new Matplotlib color object of n-colors from a given
list of named colors. Furthermore we write the RGB tuples of the color object
into a text file with two header lines needed for the use with NCL. And of
course we check the results in each case.

-------------------------------------------------------------------------------
2023 copyright DKRZ licensed under CC BY-NC-SA 4.0
               (https://creativecommons.org/licenses/by-nc-sa/4.0/deed.en)
-------------------------------------------------------------------------------
'''
import os
import numpy as np
import matplotlib.colors as mcolors
import matplotlib.pyplot as plt

def main():
    #-- Choose named colors
    #
    # We want to create a colormap that goes from white to red, yellow, light cyan,
    # lightblue to dark blue. Therefore we create a list of the appropriate named
    # colors.
    color_list = ['white', 'red', 'yellow', 'lightcyan', 'lightblue', 'navy']
    
    # Let's have a look at the RGB values of the color_list.
    for color in color_list:
        print(mcolors.to_rgba(color))
    
    #-- Create colormap from linear mapping segments (color_list)
    #
    # In the next step we want to generate the a colormap with 100 colors named
    # cmap_wryb. The color_list can be converted into an matplotlib.colors object
    # with `matplotlib.colors.LinearSegmentedColormap.from_list`.
    colormap_name = 'cmap_wryb'
    ncolors = 100
    
    color_obj = mcolors.LinearSegmentedColormap.from_list(colormap_name,
                                                          color_list,
                                                          N=ncolors)
    
    # Within the notebook, we can call the colormap object directly and it will
    # automatically display the corresponding colorbar. In a script nothing happens.
    color_obj
    
    for i in range(5):
        print(color_obj(i)[0:3])
    
    #-- Check colormap
    #
    # Let's see how the colormap looks in detail now, therefore we create a
    # 2-dimensional array of range 0 to ncolors and plot the array with
    # Matplotlib's `pcolormesh`.
    data = np.arange(0,ncolors).reshape((10,10))
    
    plt.switch_backend('agg')

    fig, ax = plt.subplots()
    
    plot = ax.pcolormesh(data, cmap=color_obj)
    plt.colorbar(plot)
    
    plt.savefig(colormap_name+'_display.png', bbox_inches='tight', dpi=100)
    
    #-- Save the RGB values of the color object to an NCL colormap file
    #
    # Now we can write the RGB color tuples of the color object into an RGB text
    # file if we want to use them again elsewhere.
    colormap_file = 'cmap_wryb.rgb'
    
    # delete colormap file
    os.system('rm -f '+colormap_file)
    
    # Write the two NCL header lines to tzhe colormap file and the RGB color
    # tuples.
    with open(colormap_file, 'w') as f:
        # write NCL colormap header lines
        f.write(f'ncolors = {ncolors}\n')
        f.write('# r  g  b\n')
    
        # write the RGB color tuples
        for j in range(ncolors):
            #color_tuple = color_obj(j)
            #f.write(' '.join([str(x) for x in color_tuple[0:3]])+'\n')
            f.write(' '.join([str(x) for x in color_obj(j)[0:3]])+'\n')
    
    os.system('head -5 cmap_wryb.rgb')
    
    #-- Read an NCL colormap file with Matplotlib
    #
    # You can read an NCL colormap file with the `numpy.loadtxt` function by skipping
    # the first two lines. The colormap can only be used by its name when it was
    # registered before. The next steps shows how to do that.
    ncl_wryb = np.loadtxt('cmap_wryb.rgb', skiprows=2, delimiter=' ')
    
    # Now, we can convert the RGB array to a Matplotlib color object with
    # matplotlib.colors.ListedColormap function.
    CMAP = mcolors.ListedColormap(ncl_wryb, name='NCL_WRYB')
    CMAP
    
    # To be able to use the colormap by its name, here NCL_WRYB, we have to use
    # `matplotlib.pyplot.register_cmap` to register the color object with this name.
    #
    # Use the try/except method to get rid of the warning when you re-run the cell
    # again.
    try:
        plt.register_cmap('NCL_WRYB', CMAP)
    except ValueError:
        pass
    
    # Check if the colormap is correct and registered.
    xi = np.arange(-0.75, 1.75, 0.05)
    yi = np.arange(-0.75, 1.75, 0.05)
    xmg, ymg = np.meshgrid(xi, yi)
    
    x1 = np.exp(-xmg**2 - ymg**2)
    x2 = np.exp(-(xmg-1)**2 - (ymg-1)**2)
    data  = (x1-x2)*2
    
    fig, ax = plt.subplots(figsize=(8,4))
    plot = ax.pcolormesh(data, cmap='NCL_WRYB')
    cbar = plt.colorbar(plot, shrink=0.7)
    plt.savefig('test_plot_'+colormap_name+'.png', bbox_inches='tight', dpi=100)


if __name__ == '__main__':
    main()
Python create colormap from named colors#

Example script#

Plot results#
