pir - 1 year ago 220
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()
``````

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
``````
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