How can I create a colormap with a different colorscale for positive and negative numbers

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Richard Walker 2018-6-19

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回答： Richard Walker 2018-6-20

How can I create a colormap with a different colorscale for positive and negative numbers: positive numbers on a white to red scale, and negative numbers on a white to blue scale (zero being white).

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Sandro Lecci 2018-6-19

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编辑：Sandro Lecci 2018-6-19

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Dear Richard,

a colormap is essentially a nx3 matrix with the specification of RGB (3 columns) and the resolution in n lines. You can either create you own "french flag" (I actually did it by myself in some of my scripts),...

% A french-flag with a resolution of 3 would be:
FF = [0 0 1;...
      1 1 1;...
      1 0 0];
% which becomes:
FF = [0    0    1;...
      0.5  0.5  1;...
      1    1    1;...
      1    0.5  0.5;...
      1    0    0];
% by increasing the resolution.

..or download some very nice colormaps like these ones . The one you are looking for is RdBu.

Follow the instruction on how to use the downloaded file and then retreive the colormap as following:

MyMap = brewermap(100, 'RdBu'); %with 100 being the number of steps in your colormap
myMap = flipud(MyMap); %Since you want blue for negative and red for positive values

Hope this helps. Best

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Stephen23 2018-6-19

@Richard Walker: you could:

create your own colormap which has the white offcenter, or
specify the colors explicitly when the data is plotted.

There are infinite possible colormaps so it is clearly not feasible for MATLAB to support them all, but it is easy to define your own to suit your data distribution.

Richard Walker 2018-6-19

Thanks Stephen, "specify the colors explicitly when the data is plotted." sounds like a good solution, but even after searching I can't see how contourf or any other command will do it. I would be grateful for a tip on how to do it.

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Walter Roberson 2018-6-20

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Richard Walker 2018-6-20

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Thank you Walter. I am sure it must be possible using cptcmap, it looks very powerful, but therefore also rather complex. I got the example in the link you sent to work, but I can't immediately see how to change the scales to what I want .. I am sure there must be a simpler way using basic MATLAB commands, but as a non-expert it's not that simple to penetrate the documentation.