[Python] Custom colormap in matplotlib using LinearSegmentedColormap not working as expected

I have a question about creating my custom colormap in Matplotlib. I start by initializing three colors to represent three points (nodes). Here is the minimal working example code:

import numpy as np
import matplotlib as mpl
from matplotlib.colors import LinearSegmentedColormap

#define a custom colormap
nodes = np.array([0, 0.5, 1.0])
colors = ['black', 'green', 'blue']
cmap = LinearSegmentedColormap.from_list('', list(zip(nodes, colors)))

#get the rgba codes for the colors array
arr_rgba = np.array([mpl.colors.to_rgba(colors[idx]) for idx in range(len(colors))])

print(cmap(nodes))
print(arr_rgba)



I expect cmap(nodes) and arr_rgba to give me the same RGBA codes, since the nodes are where my anchor points are having colors black, green and blue. However, on running this code, the output is:

>>> print(cmap(nodes))
array([[0. , 0. , 0. , 1. ],
[0. , 0.49999231, 0.00392157, 1. ],
[0. , 0. , 1. , 1. ]])


and

>>> print(arr_rgba)
array([[0. , 0. , 0. , 1. ],
[0. , 0.50196078, 0. , 1. ],
[0. , 0. , 1. , 1. ]])


I know that intermediate points are being interpolated linearly.

But why at the nodes, do I not get the exact color that I have set up the colormap? Is this the expected behaviour? Any idea?

Continue reading...