Extract matplotlib colormap in hex-format

Question

I am trying to extract discrete colors from a matplotlib colormap by manipulating this example. However, I cannot find the N discrete colors that are extracted from the colormap.

In the code below I've used cmap._segmentdata, but I've found that it is the definition of the entire colormap. Given a colormap and an integer N, how do I extract N discrete colors from the colormap and export them in hex-format?

from pylab import *

delta = 0.01
x = arange(-3.0, 3.0, delta)
y = arange(-3.0, 3.0, delta)
X,Y = meshgrid(x, y)
Z1 = bivariate_normal(X, Y, 1.0, 1.0, 0.0, 0.0)
Z2 = bivariate_normal(X, Y, 1.5, 0.5, 1, 1)
Z = Z2 - Z1 # difference of Gaussians

cmap = cm.get_cmap('seismic', 5)    # PiYG
cmap_colors = cmap._segmentdata

def print_hex(r,b,g):
               if not(0 <= r <= 255 or 0 <= b <= 255 or 0 <= g <= 255):
                              raise ValueError('rgb not in range(256)')
               print '#%02x%02x%02x' % (r, b, g)


for i in range(len(cmap_colors['blue'])):
               r = int(cmap_colors['red'][i][2]*255)
               b = int(cmap_colors['blue'][i][2]*255)
               g = int(cmap_colors['green'][i][2]*255)
               print_hex(r, g, b)



im = imshow(Z, cmap=cmap, interpolation='bilinear',
            vmax=abs(Z).max(), vmin=-abs(Z).max())
axis('off')
colorbar()

show()

Solution 1

You can get a tuple of rgba values for the segment with index i by calling cmap(i). There is also already a function that turns rgb values into hex. As Joe Kington wrote in the comments, you can use matplotlib.colors.rgb2hex. Therefore, a possible solution would be:

from pylab import *

cmap = cm.get_cmap('seismic', 5)    # PiYG

for i in range(cmap.N):
    rgba = cmap(i)
    # rgb2hex accepts rgb or rgba
    print(matplotlib.colors.rgb2hex(rgba))

The output is:

#00004c
#0000ff
#ffffff
#ff0000
#7f0000

Solution 2

For future reference: My CMasher package provides a function called take_cmap_colors() (https://cmasher.readthedocs.io/user/usage.html#taking-colormap-colors), which allows one to take any number of discrete colors from a given colormap and return them in any format (8-bit, normalized or HEX) they want.

So, if you for example wanted to take 5 colors in HEX from the viridis colormap, you could do this with:

import cmasher as cmr

colors = cmr.take_cmap_colors('viridis', 5, return_fmt='hex')

or if you want all colors in HEX from a colormap in a specific value range, you can do that with:

colors = cmr.take_cmap_colors('viridis', None, cmap_range=(0.2, 0.8), return_fmt='hex')

Comments

[alexmloveless]

or using list comprehension [matplotlib.colors.rgb2hex(c) for c in cmap.colors]