using imagesc for segmentation

Question

I have observed that whenever I use the command imagesc(image); the resultant image has some regions that are yellow in colour and the background is red.

Is there any way to segment these regions of the images ? If there is not do they have any similar pattern that can be used for thresholding or is the colour representation meaningless?

I am using Matlab R2012a on Windows.

Answer 1

When you are using imagesc (short for image scale), you are visualizing a matrix by mapping the lowest value of that matrix to one end of a colormap, and the highest value of the matrix to the other end.

By default, MATLAB uses the jet() colormap which is the normal RGB-colorrange. If some parts of your image turns out to be yellow, it means that the elements of the matrix has been some spesific place between the highest and lowest value.

The example below hopefully illustrates this more clearly and shows how you can segment out the "yellow" regions of a matrix (which doesn't really have any color per se)

colorRes = 256;

%# In a jet colormap with size 256, yellow is at placement 159
yellow = 159;
yellowScale = ((yellow/256));

image = repmat(1:colorRes,40,1);

figure(1);clf;
colormap(jet(colorRes))

subplot(2,1,1)
imagesc(image)
title('Original image')

%# Segment out yellow
colorDist = 1/colorRes*5; %# Make scalar higher to include colors close to yellow
imageSegmented = zeros(size(image));
imageSegmented(abs(image/colorRes-yellowScale)< colorDist) = 1;

subplot(2,1,2)
imagesc(imageSegmented)
title('Yellow segmented out')