Re: [Matplotlib-users] how to define a colormap dynamically

[James B. <bo...@ll...>]

Greg,
Thanks for the help. Sometimes it just takes a bit of hand holding to  
get things right.
I was trying the sequence of steps you indicated, but I was making a  
stupid mistake in the
data specification dictionary and mis-interpreted the subsequent error.
Your comment made me look harder at the details and all worked fine.

FWIW I have enclosed code to make a matplotlib color map data  
dictionary from a GMT
.cpt file. As I indicated previously these colormaps are found at:   
http://craik.shef.ac.uk/cpt-city/
there are well over a hundred from which to choose.
The code is only lightly used and not very elegant.

So if you download, for example, the 'spectrum-light.cpt' file from  
http://craik.shef.ac.uk/cpt-city/views/hueshift.html
then run;
cdict = gmtcolormapPylab.gmtcolormapPylab('spectrum-light')
colormap =  
pylab.cm.colors.LinearSegmentedColormap('spectrum- 
light'',cdict,pylab.rcParams['image.lut'])
pylab.contourf(XX,YY,ZZ,cmap=colormap)

you will get filled contours with the indicated colorTable.

Thanks again.

--Jim

def gmtcolormapPylab(fileName,GMTPath = None):
     import colorsys
     import Numeric
     N = Numeric
     if type(GMTPath) == type(None):
         filePath = '/Volumes/IBM1/gmt4/GMT4.0b/share/cpt/'+ fileName  
+'.cpt'
     else:
         filePath = GMTPath+'/'+ fileName +'.cpt'
     try:
         f = open(filePath)
     except:
         print 'file ',filePath, 'not found'
         return None

     lines = f.readlines()
     f.close()

     x = []
     r = []
     g = []
     b = []
     colorModel = 'RGB'
     for l in lines:
         ls = l.split()
         if l[0] == '#':
            if ls[-1] == 'HSV':
                colorModel = 'HSV'
                continue
            else:
                continue
         if ls[0] == 'B' or ls[0] == 'F' or ls[0] == 'N':
            pass
         else:
             x.append(float(ls[0]))
             r.append(float(ls[1]))
             g.append(float(ls[2]))
             b.append(float(ls[3]))
             xtemp = float(ls[4])
             rtemp = float(ls[5])
             gtemp = float(ls[6])
             btemp = float(ls[7])

     x.append(xtemp)
     r.append(rtemp)
     g.append(gtemp)
     b.append(btemp)

     nTable = len(r)
     x = N.array( x , N.Float)
     r = N.array( r , N.Float)
     g = N.array( g , N.Float)
     b = N.array( b , N.Float)
     if colorModel == 'HSV':
        for i in range(r.shape[0]):
            rr,gg,bb = colorsys.hsv_to_rgb(r[i]/360.,g[i],b[i])
            r[i] = rr ; g[i] = gg ; b[i] = bb
     if colorModel == 'HSV':
        for i in range(r.shape[0]):
            rr,gg,bb = colorsys.hsv_to_rgb(r[i]/360.,g[i],b[i])
            r[i] = rr ; g[i] = gg ; b[i] = bb
     if colorModel == 'RGB':
         r = r/255.
         g = g/255.
         b = b/255.
     xNorm = (x - x[0])/(x[-1] - x[0])

     red = []
     blue = []
     green = []
     for i in range(len(x)):
         red.append([xNorm[i],r[i],r[i]])
         green.append([xNorm[i],g[i],g[i]])
         blue.append([xNorm[i],b[i],b[i]])
     colorDict = {'red':red, 'green':green, 'blue':blue}
     return (colorDict)



On Mar 23, 2005, at 9:47 AM, Greg Novak wrote:

> * James Boyle <bo...@ll...> wrote:
>> I would like to be able to define a new colormap dynamically, that
>> is without altering code in cm.py.  It would not appear to be too
>> difficult to some one who knew what they were doing. I have made
>> some attempts following the code in cm.py and other places, but I do
>> not seem to be able to get the normalization correct.
>
> Do you mean that you're having trouble with the
> LinearSegmentedColormap function?  I've had to do the same thing, and
> I've included below my code to generate two colormaps, one is the same
> as "hot" but reversed, and the other fades from read to blue.  The
> form seems to be:
>
> data = {'red': ((0,r0,r0), (i1, r1, r1), (i2, r2, r2), ... (1, rn,  
> rn)),
>         'green': ... similar, 'blue': ... similar}
>
> colormap = pylab.cm.colors.LinearSegmentedColormap('colormap',
>                                                data,
>                                                 
> pylab.rcParams['image.lut'])
>
> where i1, i2... are independent variable values from zero to 1, and
> r1, r2... are red values from zero to one associated with that
> variable value.  Each red value appears twice to allow for
> discontinuous functions, but I haen't had to use that.
>
>> I know that I could just put the data into cm.py but I would rather
>> not have to remember to alter this code for every release of
>> matplotlib and I am sure most people do not want my colormaps.
>
> One of the things I like about python is that you can stuff things
> into other namespaces without altering the original code.  This is
> probably not recommended and will break things, etc, but I use it to
> add functionality that I consider to be missing from the software I
> use.  This allows me to forget whether the function is my own
> extension to the package or a "native" function in the package.
>
> Ie, I'm sure that you don't want to be typing things like:
>
> subplot(211); pcolor(data_1,cmap=cm.hot)
> subplot(212); pcolor(data_2,cmap=mycoolcolormaps.map)
>
> because then you'll have to remember which colormaps are in which
> place.  So you could do something similar to this:
>
> # Add some stuff to the scipy namespace
> def lrange(l, h, dex):
>     """Log-spaced array"""
>     return 10**scipy.arange(scipy.log10(l), scipy.log10(h), dex)
> scipy.lrange = lrange
>
>> I have used GMT (Generic Mapping Tools) extensively in the past and
>> have code to translate the GMT color map into a form usable by
>> matplotlib.
>
> I don't have anything to offer, but I'd like more colormaps, too, so
> I'm interested in whatever comes out of this discussion...
>
> Greg
>