How do I associate specific colors with specific numbers of a matrix?

Title basically says it all; I want to be able to control which color get associated with which number. I have tried the code stated below to give a color to every number from 0 to 5. The way I thought this code would work was that the first row of cmap would be the color for 0, the second for 1, the third for 2, etc. But it still seems to associate colors from cmap randomly to the numbers in the matrix A.

A=[1:3;2:4;3:5;0 0 0];
cmap = [0 0.5 0.5;0.75 0.75 0;1 0.75 0.25;
        0.25 0 1;1 1 0.75;0.75 0 0.75; 
        0.75 0.75 0;0.25 0.25 0.25;1 0.5 0];
colormap(cmap);
caxis([0 5]); 
imagesc(A); 

Thanks for reading!

Answer 1

[Adam Danz]

You set 9 colors in your colormap but you only have 6 unique intergers in "A". That means you're asking matlab to distribute 9 colors evenly across 6 values. To make see the mapping, add a colorbar to your plot and set the y axis ticks of the colorbar to intergers (see below).

cb = colorbar(); 
set(cb,'YTick', 1:9)

If you want a 1:1 mapping you've got 2 opitions.

Option 1: number of colors = range of values

You've got 6 unique integer values (see below) then you must have only 6 colors in your colormap.

unique(A(:))
ans =
     0
     1
     2
     3
     4
     5

Here's your code with only the first 6 colors and with a colorbar.

 A=[1:3;2:4;3:5;0 0 0];
cmap = [0 0.5 0.5;
    0.75 0.75 0;
    1 0.75 0.25;
    0.25 0 1;
    1 1 0.75;
    0.75 0 0.75];
colormap(cmap);
imagesc(A);


cb = colorbar(); 
set(cb,'YTick', 0:5)

Option 2: control color range with caxis()

The range of integer values in your data is 6 but you've assigned 9 colors. If you want to scale the colors such that only the first 6 colors are used and the last 3 ignored, set the caxis() range from 0:9 instead of 0:5.

A=[1:3;2:4;3:5;0 0 0];
cmap = [0 0.5 0.5;0.75 0.75 0;1 0.75 0.25;
    0.25 0 1;1 1 0.75;0.75 0 0.75;
    0.75 0.75 0;0.25 0.25 0.25;1 0.5 0];
imagesc(A);
colorbar()
colormap(cmap);
caxis([0 9]);  % <----

[Adam Danz]

If you want to center those y-ticks on the colorbar,

segWidth = range(cb.Limits) / (range(A(:))+1); 
cbTick = linspace(cb.Limits(1)+segWidth/2, cb.Limits(2)-segWidth/2, range(A(:))+1); 
set(cb,'YTick', cbTick, 'YTickLabel', min(A(:)):max(A(:)) )

[Ivan Verstraeten]

Thanks for your answer! This definitely made things more clear!

So it's not possible to have more colors in the colormap than there are unique integers in the matrix and have it associate the first n colors in the colormap to the n unique integers?

In order of the integers; 0 to highest, so the first color with 0, the second color with 1, etc.

I will always have 0:n unique integers in my matrix, so not: ex. 0, 2, 6, 7, 9 as the only unique integers.

(I want it this way because the unique integers in my matrix are 0:n, its a variable. So I just want to have more colors in store for if I pick a higher n)

[Adam Danz]

Ivan, I've updated my answer and added "Option 2".

All you have to do is scale the color range from 0 to 9 instead of 0 to 5.

Let me know if you have any other questions.