Isolating Pixels by Color

Question

I am new to matlab and trying to teach myself.

My first task is that I am trying to take an image and compare color values in one image to another image. In order to do that I need to gather all R, G, B values between certain thresholds and place them in a vector accordingly. My issue at the moment is how do you go through an image and isolate, for instance, pixels with an R value between [0,31], G value between [0,31], and B value between [0,31]?

Given image X I know how to find intensities within the image but when it comes to looking for the color I am at a loss. Let me know if I need to explain further.

Thanks

edit: The images are 2D .jpegs (don't know if that helps)

Answer 1

First, read your jpeg image into a Matlab 3D array (first two dimensions indicate position, third dimension indicates R,G,B):

X = imread('image.jpg');

Then:

index = find(X(:,:,1)<=31 & X(:,:,2)<=31 & X(:,:,3)<=31);
R = X(index);
G = X(index + size(X,1)*size(X,2));
B = X(index + 2*size(X,1)*size(X,2));

does what you want. It uses the concept of linear indexing.