matplotlib-users

[Matplotlib-users] Circular colormaps

[Ariel R. <ar...@be...>]

Hi everyone,

I am interested in using a circular colormap, in order to represent a phase
variable, but I don't like 'hsv' (which is circular). In particular, I find
that it induces perceptual distortion, where values in the green/yellow part
of the colormap all look the same. Are there any circular colormaps except
for 'hsv'? If not - how would you go about constructing a new circular
colormap?

Thanks,

Ariel
-- 
Ariel Rokem
Helen Wills Neuroscience Institute
University of California, Berkeley
http://argentum.ucbso.berkeley.edu/ariel

Re: [Matplotlib-users] Circular colormaps

[Gary R. <gr...@bi...>]

Hi Ariel,

You might find the attached function helpful here. Try creating a new 
colormap using the example in the docstring (you could also try setting 
high=0.8) - basically this will let you turn down the saturation which 
will hopefully solve your problem. You may also find the plot option 
useful to see what the individual colour channels are doing if you 
decide to make a new colormap of your own - you just need to ensure that 
the r, g, and b values match at both ends.

Gary


Ariel Rokem wrote:
> Hi everyone,
> 
> I am interested in using a circular colormap, in order to represent a 
> phase variable, but I don't like 'hsv' (which is circular). In 
> particular, I find that it induces perceptual distortion, where values 
> in the green/yellow part of the colormap all look the same. Are there 
> any circular colormaps except for 'hsv'? If not - how would you go about 
> constructing a new circular colormap? 
> 
> Thanks,
> 
> Ariel
> -- 
> Ariel Rokem
> Helen Wills Neuroscience Institute
> University of California, Berkeley
> http://argentum.ucbso.berkeley.edu/ariel

Re: [Matplotlib-users] Circular colormaps

[Chloe L. <ch...@be...>]

... and for dessert, is there a circular colormap that would work for  
the colorblind?

My department is practicing presenting-science-for-the-general-public,  
and the problems 'heat maps' have for the colorblind keep coming up.

handy: http://konigi.com/tools/submissions/color-deficit-simulators

&C


On Nov 8, 2009, at 3:34 AM, Gary Ruben wrote:

> Hi Ariel,
>
> You might find the attached function helpful here. Try creating a  
> new colormap using the example in the docstring (you could also try  
> setting high=0.8) - basically this will let you turn down the  
> saturation which will hopefully solve your problem. You may also  
> find the plot option useful to see what the individual colour  
> channels are doing if you decide to make a new colormap of your own  
> - you just need to ensure that the r, g, and b values match at both  
> ends.
>
> Gary
>
>
> Ariel Rokem wrote:
>> Hi everyone,
>> I am interested in using a circular colormap, in order to represent  
>> a phase variable, but I don't like 'hsv' (which is circular). In  
>> particular, I find that it induces perceptual distortion, where  
>> values in the green/yellow part of the colormap all look the same.  
>> Are there any circular colormaps except for 'hsv'? If not - how  
>> would you go about constructing a new circular colormap? Thanks,
>> Ariel
>> -- 
>> Ariel Rokem
>> Helen Wills Neuroscience Institute
>> University of California, Berkeley
>> http://argentum.ucbso.berkeley.edu/ariel
> import numpy as np
> import matplotlib.pyplot as plt
> import matplotlib.colors as colors
> import matplotlib._cm as _cm
>
>
> def rescale_cmap(cmap_name, low=0.0, high=1.0, plot=False):
>    '''
>    Example 1:
>    my_hsv = rescale_cmap('hsv', low = 0.3)     # equivalent scaling  
> to cplot_like(blah, l_bias=0.33, int_exponent=0.0)
>    Example 2:
>    my_hsv = rescale_cmap(cm.hsv, low = 0.3)
>    '''
>    if type(cmap_name) is str:
>        cmap = eval('_cm._%s_data' % cmap_name)
>    else:
>        cmap = eval('_cm._%s_data' % cmap_name.name)
>    LUTSIZE = plt.rcParams['image.lut']
>    r = np.array(cmap['red'])
>    g = np.array(cmap['green'])
>    b = np.array(cmap['blue'])
>    range = high - low
>    r[:,1:] = r[:,1:]*range+low
>    g[:,1:] = g[:,1:]*range+low
>    b[:,1:] = b[:,1:]*range+low
>    _my_data = {'red':   tuple(map(tuple,r)),
>                'green': tuple(map(tuple,g)),
>                'blue':  tuple(map(tuple,b))
>               }
>    my_cmap = colors.LinearSegmentedColormap('my_hsv', _my_data,  
> LUTSIZE)
>
>    if plot:
>        plt.figure()
>        plt.plot(r[:,0], r[:,1], 'r', g[:,0], g[:,1], 'g', b[:,0],  
> b[:,1], 'b', lw=3)
>        plt.axis(ymin=-0.2, ymax=1.2)
>
>    return my_cmap
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Re: [Matplotlib-users] Circular colormaps

[Robert K. <rob...@gm...>]

On 2009-11-09 11:46 AM, Chloe Lewis wrote:
> ... and for dessert, is there a circular colormap that would work for
> the colorblind?

Almost certainly not, at least not without compromising other desirable features 
for circular colormaps. You could do a circle roughly perpendicular to the lines 
of confusion, but this would mean going up and down in lightness, which 
perceptually overemphasizes the light half.

On the other hand, this may not be a bad thing if 0 degrees and/or 180 degrees 
are special as might be the case with phase measurements and other complex 
number-related things.

> My department is practicing presenting-science-for-the-general-public,
> and the problems 'heat maps' have for the colorblind keep coming up.

As a deuteronopic, I heartily thank you for paying attention to these issues.

I've written an application to visualize colormaps in 3D perceptual space as 
well as simulating colorblindness. It uses Mayavi and Chaco, so you will need a 
full Enthought Tool Suite installation:

http://www.enthought.com/~rkern/cgi-bin/hgwebdir.cgi/colormap_explorer/

Of interest for this thread might be the function find_chroma() in hcl_opt.py 
which will, given a lightness value in HCL space, find the largest chroma value 
(roughly similar to saturation) such that a circle at the given lightness value 
will just fit inside of the RGB gamut. A simple maximization on that function 
will find the lightness that gives the largest chroma and hence the largest 
dynamic range of such a colormap. However, it should be noted that I have found 
such colormaps to appear a little washed out and drab. But then, I'm colorblind.

-- 
Robert Kern

"I have come to believe that the whole world is an enigma, a harmless enigma
  that is made terrible by our own mad attempt to interpret it as though it had
  an underlying truth."
   -- Umberto Eco