change Black and White - Image to Black & Red Image

Hi all,

I have the following problem. I am displaying binary map information, stored in a 100x100 matrix with imshow. This works well. I am also plotting the error between two maps by substracting map A from map B. This leaves us with an new map C. I can plot that with imshow but I want to change the colors to Black & red instead of BW.

Does anyone know how to do that? Any help is appreciated.

Answer 1

[Image Analyst]

See my demo, which handles negative values which will occur. -1 = red, 0 = black, and +1 = blue.

workspace;  % Make sure the workspace panel is showing.
% Create two sample images.
fontSize = 22;
binaryImageA = false(200, 300);
binaryImageB = false(200, 300);
% Make two regions that overlap.
binaryImageA(60:140, 100:200) = true;
binaryImageB(100:150, 150:250) = true;
% Display the sample images.
subplot(2, 2, 1);
imshow(binaryImageA);
title('Binary Image A', 'FontSize', fontSize);
grid on;
% Enlarge figure to full screen.
set(gcf, 'units','normalized','outerposition',[0 0 1 1]);
subplot(2, 2, 2);
imshow(binaryImageB);
title('Binary Image B', 'FontSize', fontSize);
grid on;

% Subtract the two sample images
differenceImage = int32(binaryImageA) - int32(binaryImageB);
% Display the difference image.
figure;
imshow(differenceImage, []);
title('Difference Image: A - B', 'FontSize', fontSize);
grid on;
% Make colormap: red for negative, 0 for 0, blue for positive.
cmap = [1 0 0; 0 0 0; 0 0 1];
% Apply colormap
colormap(cmap);
colorbar;

[Lukas Non]

This works perfect. Thank you so much. I was going trough a lot of frustration. You are my hero!!

Answer 2

[Ryan]

You need to define the colormap:

imshow(BW,[0 0 0; 1 0 0]) % 0's are black and 1's are red

[Lukas Non]

Hi, thanks for you reply. Unfortunately that did not work.

[Ryan]

A general rule of thumb is always go with Image Analyst's answer on image related questions.

The most likely cause is that your matrix C has a -1 as IA pointed out at the beginning of his posts. This works for a logical, 0 & 1, BW image (at least for me). You'd need to specify a third color to handle the other number. I'm fairly sure color maps apply the colors in increasing numerical order so choose wisely.