Thanks Eric, That is much better. I am going to try and implement it now and see how I go. I will let you know.
Regards, Marjolaine. >>> Eric Firing <[EMAIL PROTECTED]> 07/09/08 11:12 PM >>> Marjolaine Rouault wrote: > Hi, > > I don't understand how one creates his own colormap. i am using > matplotlib and I have checked the example in > http://www.scipy.org/Cookbook/Matplotlib/Show_colormaps but I can't > really understand how it works. Are there any other examples out > there? I want to create a colormap a bit like the one at this url: > > http://www.pyngl.ucar.edu/Graphics/Images/ViBlGrWhYeOrRe.gif > > best regards, Marjolaine. > > > Marjolaine, Depending on your starting point--what you know about your desired colormap--you can use either a LinearSegmentedColormap or a ListedColormap. If you already have a colormap in the form of a list of evenly-spaced colors, then the simplest way to get it into mpl is by using that list to initialize a ListedColormap. If, instead, you have a general idea of how you want R, G, and B to vary over the range of the map, then you probably need the LinearSegmentedColormap. Every description of the LinearSegmentedColormap class that I have seen is confusing, even though the way it works is fairly simple. Let's see if I can make it a little less confusing. Example: suppose you want red to increase from 0 to 1 over the bottom half, green to do the same over the middle half, and blue over the top half. Then you would use: cdict = { 'red': ((0, 0, 0), (0.5, 1, 1), (1, 1, 1)), 'green': ((0, 0, 0), (0.25, 0, 0), (0.75, 1, 1), (1, 1, 1)), 'blue': ((0, 0, 0), (0.5, 0, 0), (1, 1, 1))} If, as in this example, there are no discontinuities in the r, g, and b components, then it is quite simple: the second and third element of each tuple, above, is the same--call it "y". The first element ("x") defines interpolation intervals over the full range of 0 to 1, and it must span that whole range. In other words, the values of x divide the 0-to-1 range into a set of segments, and y gives the end-point color values for each segment. Now consider the green. cdict['green'] is saying that for 0 <= x <= 0.25, y is zero; no green. 0.25 < x <= 0.75, y varies linearly from 0 to 1. x > 0.75, y remains at 1, full green. If there are discontinuities, then it is a little more complicated. Label the 3 elements in each row in the cdict entry for a given color as (x, y0, y1). Then for values of x between x[i] and x[i+1] the color value is interpolated between y1[i] and y0[i+1]. Going back to the cookbook example, look at cdict['red']; because y0 != y1, it is saying that for x from 0 to 0.5, red increases from 0 to 1, but then it jumps down, so that for x from 0.5 to 1, red increases from 0.7 to 1. Green ramps from 0 to 1 as x goes from 0 to 0.5, then jumps back to 0, and ramps back to 1 as x goes from 0.5 to 1. row i: x y0 y1 / / row i+1: x y0 y1 Above is an attempt to show that for x in the range x[i] to x[i+1], the interpolation is between y1[i] and y0[i+1]. So, y0[0] and y1[-1] are never used. I hope I got all that right--I would welcome close checking. I want to get an adequate and correct explanation into the standard mpl documentation. Eric -- This message is subject to the CSIR's copyright terms and conditions, e-mail legal notice, and implemented Open Document Format (ODF) standard. The full disclaimer details can be found at http://www.csir.co.za/disclaimer.html. This message has been scanned for viruses and dangerous content by MailScanner, and is believed to be clean. MailScanner thanks Transtec Computers for their support. ------------------------------------------------------------------------- Sponsored by: SourceForge.net Community Choice Awards: VOTE NOW! Studies have shown that voting for your favorite open source project, along with a healthy diet, reduces your potential for chronic lameness and boredom. Vote Now at http://www.sourceforge.net/community/cca08 _______________________________________________ Matplotlib-users mailing list Matplotlib-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/matplotlib-users
