How to correctly use contourf with logarithmic color scale?

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Sebastian
Sebastian 2023-9-5
评论： Voss 2024-9-30
Ran in:
Dear Matlab Community,
I have a 1372 x 4118 (double) matrix I want to plot using contourf() function. The data entries of the matrix vary from 1 to 1e-9.In order to see changes throughout the whole scale I want to use a log scale fo caxis. The following code sets the Colorbar to log scale but the color representation in the plot is not in agreement with the colorbar: ( Due to memory constraints I reduce the matrix to 600 x 600 )
matrix = load('matrix.mat');
matrix = matrix.a;
x= linspace(1,600,600);
y=linspace(-1,-600,600);
contourf(x,y,matrix);
set(gca,'ColorScale','log');
c = colorbar;
c.Label.String = 'energy [keV]';
xlim([0 100]);
ylim([-200,0]);
clim([1e-7,1e0]);
Please find attached an image of the wrong plot with the full matrix, a subset of the matrix , and the matrix itself.
Can someone please help me with this challange?
Best Regards
sethi
  2 个评论
Walter Roberson
Walter Roberson 2023-9-5
I do not see any problem. With your sample data, the left edge (near x == 0) has a sharp peak to 1, but most of the rest of the data is around roughly 5e-4
Sebastian
Sebastian 2023-9-5
Dear Mr. Roberson,
thank you for the fast reply. I would expect a more pronounced color gradient than this shown in the attached image (contourf_logscale_2.jpg). In this image, values of 1e-5 have the same greenish color as values of 1e-2. But according to the scale bar there should be a bluish or orange color, respectively
.

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采纳的回答

Voss
Voss 2023-9-5
Ran in:
The problem is that contourf picks the levels in a linear fashion, so the levels in your contour plot are [0 0.1 0.2 0.3 ... 0.9 1], which obviously don't work very well when the color scale is logarithmic.
One way around this is to use use the log of your data in contourf and then adjust the colorbar tick labels appropriately:
matrix = load('matrix.mat');
matrix = matrix.a;
x = linspace(1,600,600);
y = linspace(-1,-600,600);
figure
contourf(x,y,log10(matrix));
c = colorbar;
c.Label.String = 'energy [keV]';
c.Ticks = -7:0;
c.TickLabels = compose('10^{%d}',c.Ticks);
xlim([0,100]);
ylim([-200,0]);
clim([-7,0]);
Another way is to specify the contour levels yourself:
figure
n_levels = 8;
contourf(x,y,matrix,logspace(-7,0,n_levels));
set(gca,'ColorScale','log');
c = colorbar;
c.Label.String = 'energy [keV]';
xlim([0 100]);
ylim([-200,0]);
clim([1e-7,1e0]);
Surface plot (no contours), for reference:
figure
p = pcolor(x,y,matrix);
p.EdgeColor = 'none';
set(gca,'ColorScale','log');
c = colorbar;
c.Label.String = 'energy [keV]';
xlim([0,100]);
ylim([-200,0]);
clim([1e-7,1e0]);
  3 个评论
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Samuel
Samuel 2024-9-30
Thank you very much! This worked perfectly for a similar issue to OP.

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更多回答（1 个）

Nathan Hardenberg
Ran in:
There are two ways I know of:
  1. Define your own (manual) values on which contours should be drawn, or
  2. just plot the logarithmic values instead of the linear ones
matrix = load('matrix.mat');
matrix = matrix.a;
x = linspace( 1, 600, 600);
y = linspace(-1,-600, 600);
figure(1);
% using contourf with manual lines
contourf(x, y, matrix, [1 0.1 0.01 0.001 0.0001 0.00001 0]);
set(gca,'ColorScale','log');
c = colorbar;
c.Label.String = 'energy [keV]';
xlim([0 100]); ylim([-200,0]);
figure(2);
contourf(x, y, log10(matrix)); % draw logarithmic values
c = colorbar;
c.Label.String = 'log10(energy) [keV]';
xlim([0 100]); ylim([-200,0]);

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