Re: [Matplotlib-users] get colorlist and values from existing matplotlib colormaps?

[wiswit <cha...@gm...>]

Thanks Eric. This is quite an informative answer about colormap! 
The first part of the answer is exactly what I need. 

cheers,

Chao


efiring wrote:
> 
> On 06/02/2012 03:37 AM, Chao YUE wrote:
>> Dear all,
>>
>> I find I would like to make some change from the existing colormaps. for
>> example, I would like to change the color at the beginning of the
>> colormap (let's say mat.cm.jet) but I still
>> want to use the remaining other colors. So is there way I can easily use
>> some functions already in matplotlib to extract the colorlist and levels
>> from a mat.cm.jet?
>> Then I can just change the first color of the colorlist, and use
>> mat.colors.LinearSegmentedColormap.from_list to easily construct the
>> colormap I want.
> 
> Try playing with something like this (in ipython --pylab):
> 
> jetcmap = cm.get_cmap("jet", 10) #generate a jet map with 10 values
> jet_vals = jetcmap(np.arange(10)) #extract those values as an array
> jet_vals[0] = [0.1, 0, 0.1, 1] #change the first value
> newcmap = mpl.colors.LinearSegmentedColormap.from_list("newjet", jet_vals)
> imshow(rand(18,20), cmap=newcmap, vmin=0, vmax=1, interpolation="nearest")
> colorbar()
> 
> Alternatively, you can copy the cm.datad['jet'] dictionary (datad is a 
> dictionary of dictionaries), modify it, and use it to initialize a 
> custom LinearSegmentedColormap instance.  See 
> http://matplotlib.sourceforge.net/examples/pylab_examples/custom_cmap.html.
> 
> 
>>
>> I can use mat.cm.jet._segmentdata to retrieve the dictionary. I also
>> have a look at the source code
> 
> In general it is not a good idea to work with attributes with leading 
> underscores, which flag them as especially low-level 
> implementation-dependent details. cm.jet._segmentdata can be accessed as 
> cm.datad['jet'].
> 
> Note also that the _segmentdata is not what is used directly to look up 
> the colors; instead it is used to generate the lookup table (_lut 
> attribute).  See below.
> 
>> /usr/local/lib/python2.7/dist-packages/matplotlib/colors.py but I didn't
>> manage to find a solution.
>>
>> both mat.colors.LinearSegmentedColormap and mat.colors.ListedColormap
>> finally calls mat.colors.Colormap.__init__ and then I don't understand
>> how these colorlist are really used for plotting.
>>
> 
> Typically it is a two-stage process.  First, a data array is passed to a 
> Normalize instance which scales it to the range from zero to one. 
> Second, that scaled array is passed to the Colormap instance, which uses 
> its lookup table to map any point in the 0-1 range to a color.
> 
> Less commonly, instead of passing an array of floats to the Colormap 
> instance, one may pass in an integer array, in which case these integers 
> are used directly as indices into the lookup table (which is the _lut 
> attribute of the Colormap instance.)
> 
>> Another question, where can I find the source code where mat.cm.jet is
>> defined?
> 
> Good question; the answer is obscured by somewhat convoluted coding in 
> cm.py.  The relevant part is this:
> 
> for cmapname in datad.iterkeys():
>      cmap_d[cmapname] = _generate_cmap(cmapname, LUTSIZE)
> 
> locals().update(cmap_d)
> 
> 
> The first block is filling a dictionary with LinearSegmentedColormap 
> instances corresponding to the named sets of segment data from _cm.py.
> The "locals" line is the tricky part: it is adding each entry in that 
> dictionary to the local namespace, so that cm.cmap_d["jet"] can be 
> accessed as cm.jet, etc.
> 
> There is a bit more to it, because Colormap instances can handle three 
> special values: over range, under range, and "bad" (masked).  See 
> http://matplotlib.sourceforge.net/examples/pylab_examples/contourf_demo.html
> and
> http://matplotlib.sourceforge.net/examples/pylab_examples/image_masked.html
> 
> Eric
> 
> 
>>
>> thanks et cheers,
>>
>> Chao
> 
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