pir pir - 1 month ago 19
Python Question

Extract matplotlib colormap in hex-format

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()

Answer

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):
    rgb = cmap(i)[:3] # will return rgba, we take only first 3 so we get rgb
    print(matplotlib.colors.rgb2hex(rgb))

The output is:

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