Colormap with both positive and negative values

Hi all,

I have a very simple issue, but the solution has been escaping me nonetheless.

I have a big matrix of correlations: some are negative and some are positive. I would like to plot them with a red-green colormap (like the "colormap(redgreencmap)" for "imagesc"). In this map, a correlation of -1 should be bright red, a correlation of +1 bright green and a correlation of close to zero black.

However, it seems like MATLAB can only handle matrices of values between [0,1] for colormap plotting.

This should be pretty simple, but I cannot figure out the solution. Does anyone have any suggestions?

Thanks a lot,

Anders

Answer 1

[Image Analyst]

Try this:

% Create sample data:
correlations = peaks(300);
minValue = min(correlations(:));
maxValue = max(correlations(:));
% Scale the data to between -1 and +1.
correlations = (correlations-minValue) * 2 / (maxValue - minValue) - 1;
% Display - will use some weird color map to start with.
imagesc(correlations);
colorbar
% Create colormap that is green for negative, red for positive,
% and a chunk inthe middle that is black.
greenColorMap = [zeros(1, 132), linspace(0, 1, 124)];
redColorMap = [linspace(1, 0, 124), zeros(1, 132)];
colorMap = [redColorMap; greenColorMap; zeros(1, 256)]';
% Apply the colormap.
colormap(colorMap);

[Anders]

That is perfect. Just in case anyone else is having the same problem, I would add the line:

caxis([-1 1]) % change colorbar limits

To your code in order to control the color axis. Thanks so much for your help! I don't know why it escaped me.

Anders

[Alexandria]

How would you go about changing the color from red/black/green, to say blue/black/yellow?

[Nicolás Casaballe]

If you are still after this, you can modify the line that defines the colorMap matrix. Its columns represent the RGB ratios of the colors that are later used to represent the image values.

Without changing too much of the code above (but the names are going to be quite misleading), the line

 colorMap = [redColorMap; redColorMap; greenColorMap]'; % Actually not red and green

would build a blue/black/yellow colormap of sorts. Choosing better names is advisable (for instance redCol, greenCol and blueCol to make the columns).

Answer 2

[Walter Roberson]

The colormap should always be the RGB values to use, not the data values that are to be mapped into RGB values.

Create your colormap with bright red in the lower-numbered rows, black in the middle rows, and bright green in the last rows, and then imagesc() your data that is in the range -1 to +1

[Jason McCune-Sanders]

How would one easily change the shading of the map, i.e. add more colors or reduce the black zone in the middle? Thanks!

[Walter Roberson]

You might want to use https://www.mathworks.com/help/matlab/ref/colormapeditor.html