convert colormap to RGB color array

Community matplotlib-users
1

This is related to a previous question I had about colormaps;
Im looking for a method to evenly split up a colormap into an RGB colors array.
something like:

def cmap_to_array(cmap,N):
...

mycolors=cmap_to_array(cm.jet,20)
lev=np.arange(1,20,1)
cs=contourf(Z,lev,colors=mycolors)

...

where 'mycolors' would be a 20x3 array with RGB values for 20 colors that represent the cm.jet spectrum broken up evenly into 20 colors...

I believe colors array can contain RGB tuples, something like [[0.2,0.3,1],[0.3,0.5,1], ... ,[1,1,0]] should work.

However, I dont know how to extract the tuples from an existing colormap.
How can this be done?

Please help...
Thanks,

P.Romero

···

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2

Please disregard this question, as a solution was found to this problem using the 'BoundaryNorm' function.

P.Romero

···

----------------------------------------

From: romero619@...32...
To: matplotlib-users@lists.sourceforge.net
Date: Sun, 15 Mar 2009 13:37:04 -0700
Subject: [Matplotlib-users] convert colormap to RGB color array

This is related to a previous question I had about colormaps;
Im looking for a method to evenly split up a colormap into an RGB colors array.
something like:

def cmap_to_array(cmap,N):
...

mycolors=cmap_to_array(cm.jet,20)
lev=np.arange(1,20,1)
cs=contourf(Z,lev,colors=mycolors)

...

where 'mycolors' would be a 20x3 array with RGB values for 20 colors that represent the cm.jet spectrum broken up evenly into 20 colors...

I believe colors array can contain RGB tuples, something like [[0.2,0.3,1],[0.3,0.5,1], ... ,[1,1,0]] should work.

However, I dont know how to extract the tuples from an existing colormap.
How can this be done?

Please help...
Thanks,

P.Romero
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3

Pablo Romero wrote:

This is related to a previous question I had about colormaps;
Im looking for a method to evenly split up a colormap into an RGB colors array.
something like:
def cmap_to_array(cmap,N):
...
mycolors=cmap_to_array(cm.jet,20)
lev=np.arange(1,20,1)
cs=contourf(Z,lev,colors=mycolors)
...

where 'mycolors' would be a 20x3 array with RGB values for 20 colors that represent the cm.jet spectrum broken up evenly into 20 colors...
I believe colors array can contain RGB tuples, something like [[0.2,0.3,1],[0.3,0.5,1], ... ,[1,1,0]] should work.

However, I dont know how to extract the tuples from an existing colormap.
How can this be done?

Pablo,

I think that what you want can be handled very simply using the BoundaryNorm as I have suggested earlier. However, it is not hard to generate any number of evenly spaced colors (for use in the "colors" kwarg of contourf). Here is one way to do it:

import numpy as np
import matplotlib.cm as cm
def make_N_colors(cmap_name, N):
     cmap = cm.get_cmap(cmap_name, N)
     return cmap(np.arange(N))

This will return a sequence of RGBA values, actually an Nx4 ndarray. I don't think the inclusion of the 4th column hurts anything, but obviously you can use indexing to remove it if you want to. The last line in the function would change to

     return cmap(np.arange(N))[:,:-1]

cmap_name is a string chosen from the values in cm.datad.keys().

You will want N to be len(lev) - 1.

Eric