How to define color on matrix figure

Hi everyone,

I have a 19x19 matrix and I want all the 0 values to mark with black color on the figure.

this is the code that I am using the generate my figure:

[R2, P2] = corrcoef(x_avg_cor_time2);
figure;
imagesc(R2);
colorbar;
title('x_avg_cor_time1')

Any help would be appriciated! Thanks!

[Jan]

Please mention, what the problem with the current code is.

Answer 1

[David Goodmanson]

Hi Jan,

the following method comes up with black not just for values that are exactly zero (I don't know how likely that might be for a correlation coefficient calculation), but also for values that are near zero, with a precision that is related to the number of rows of the colormap.

n = size(colormap,1)     % assuming you already have a plot up and running
b = max(R2,[],'all');
a = min(R2,[],'all');
ind = round(1+(-a/(b-a))*(n-1));
map = colormap;
map(ind,:) = [0 0 0];
figure(2)
imagesc(R2)
colormap(map)
colorbar 

Answer 2

[Walter Roberson]

Create a black underlayer and make the image transparent everywhere it is 0.

[R2, P2] = corrcoef(x_avg_cor_time2);
figure;
Z = zeros(size(R2,1), size(R2,2), 3);  %black background
image(Z);
hold on
alphadata = double(R2 ~= 0);
h = imagesc(R2);
h.AlphaData = alphadata;
hold off
colorbar;
title('x_avg_cor_time1')

The disadvantage is that the black will not show up in the color bar.

As @David Goodmanson discussed, when you have a color present in the colorbar, it always describes a range of data, never an exact value. You could, for example, make a colormap that had 1024 entries and set one of them to black so that the color would cover (say) +/- 0.003 -- but you cannot get a colorbar entry to match an exact value when you have continuous data.