Matrix values represented as colormap

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NeGM 2021-5-25

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https://ww2.mathworks.cn/matlabcentral/answers/838735-matrix-values-represented-as-colormap

评论： DGM 2021-5-26

采纳的回答： DGM

E_superimposed.mat

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Hi everyone,

I am trying to represent the values of a matrix as a colormpap. The problem I am currently having is that this matrix does not have any negative values and I'd like my "colorbar" to represent this as well without altering the way the colormap currelty looks. I'd like my color bar to go from 0 to max(E_superimposed) but when I chnage the "caxis" the colormap gets all distorted. This is the image I am getting from my code: Thank you so much in advance

figure(3)
colormap(flipud(colormap));
set(gcf, 'PaperPosition', [0 0 8 5]);
set(gcf, 'PaperSize', [8 5]);
imagesc(E_superimposed);
caxis([-max(E_superimposed(:)) max(E_superimposed(:))])
colorbar
colormap(lbmap(201,'RedWhiteBlue'))
set(gca,'XTick',[])
set(gca,'YTick',[])
title('Superimposed Block Matrix')

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Answer 1

DGM 2021-5-25

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Why do

caxis([-max(E_superimposed(:)) max(E_superimposed(:))])

instead of

caxis([min(E_superimposed(:)) max(E_superimposed(:))])

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DGM 2021-5-25

编辑：DGM 2021-5-25

在 MATLAB Online 中打开

By default, imagesc() scales the output such that the first entry in the color table corresponds to min(data) and the last entry in the color table corresponds to max(data).

If you change the colormap to one that extends from white to blue, you'll have to get rid of this line

% this forces the image to only use half the colormap
% if the colormap is the right size, you don't want this
caxis([-max(E_superimposed(:)) max(E_superimposed(:))])

Just let imagesc do the scaling and it will span from white to blue as it does now.

For demonstration, this is how you started:

% just a random test image (spans [3 44])
x = 1:20;
A = x+x.'+5*rand(20);
imagesc(A)
colorbar
% ignore the difference btw 'RedBlue' and 'RedWhiteBlue'
colormap(lbmap(201,'RedBlue'))
caxis([-max(A(:)) max(A(:))])

If you use a colormap that just spans from white to blue (either by truncating the RedBlue map as I mentioned or using a different Blue map), you lose all the white parts of the image because the caxis() line is still forcing it to only use half the colormap.

% just a random test image (spans [3 44])
x = 1:20;
A = x+x.'+5*rand(20);
imagesc(A)
colorbar
colormap(lbmap(201,'Blue'))
caxis([-max(A(:)) max(A(:))])

So simply don't use caxis() and let imagesc() do the scaling.

% just a random test image (spans [3 44])
x = 1:20;
A = x+x.'+5*rand(20);
imagesc(A)
colorbar
colormap(lbmap(201,'Blue'))

Or if you want to explicitly specify the limits (e.g. if the data doesn't quite go to zero like this case)

% just a random test image (spans [3 44])
x = 1:20;
A = x+x.'+5*rand(20);
imagesc(A)
colorbar
colormap(lbmap(201,'Blue'))
caxis([0 max(A(:))])

NeGM 2021-5-26

hi DGM, yes thats exactly what I want but I cant get "Blue" to work, how did you modify it?

DGM 2021-5-26

In the above example, I simply used this function

https://www.mathworks.com/matlabcentral/fileexchange/17555-light-bartlein-color-maps

I don't know how that differs from your version of lbmap().

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Answer 2

Image Analyst 2021-5-25

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Try

imagesc(E_superimposed);
caxis([0, max(E_superimposed(:))]) % Ignore negative values.

or maybe

cmap = lbmap(201,'RedWhiteBlue')
imshow(E_superimposed, [], 'Colormap', cmap); % Don't use caxis after this.

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Matrix values represented as colormap

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Matrix values represented as colormap

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