How do I specify a specific color for a specific value when using the colormap?

I have a matrix that I would like to plot using imagesc. I have done this and then used one of the standard colormaps (jet, summer, etc.). My matrix is a 75x75 matrix and the matrix consist of only five different values that I would like to plot in five specific colors.

For example -1 = blue, 3 = green, 4 = grey, 5 = Brown and 10 = red.

I have only managed to create a range of colors by using the standard colormaps (ranging from blue to red for example). I tried to create my own colormap, however I could only involve three different colors. When I tried to involve four colors I received an error saying that there was something wrong with my dimensions.

In Excel this is very simple and I guess it's the same in Matlab and that I'm missing something very basic here?

[Geoff Hayes]

Robin - how did you construct your custom colormap? Something like

 customColormap = [0 0 1; 0 1 0; 0.5 0.5 0.5; 0.5 0 0; 1 0 0];

[Robin]

Thank you!

Answer 1

[Image Analyst]

Try this:

% Create indexed image.
indexedImage = randi(10, 75, 75);
% Set values other than 1, 3, 4, 5, 10 to 1.
indexedImage(indexedImage == 2 | (indexedImage >= 6 & indexedImage <= 9)) = 1;
% Now we have a sample image like the poster has.
subplot(1, 2, 2);
histogram(indexedImage);
grid on;
% Create colormap with special colors for 1, 3, 4, 5, and 10
% and black for any other values (of which there shouldn't be any).
cmap = zeros(10, 3);
cmap = [0, 0, 1; ...   % Blue for 1
  0, 0, 0; ...       % Black for 2
  0, 1, 1; ...       % Green for 3
  0.5, 0.5, 0.5; ... % Gray for 4
  0.5, 0.5, 0; ...   % Brown for 5
  0, 0, 0; ...       % Black for 6
  0, 0, 0; ...       % Black for 7
  0, 0, 0; ...       % Black for 8
  0, 0, 0; ...       % Black for 9
  1, 0, 0]           % Red for 10
% Display the image with the colormap applied.
subplot(1, 2, 1);
imshow(indexedImage, cmap);
axis on;
colorbar(gca);

[Robin]

Perfect!