How to understand segmentation colour in image?

Hi all Right now I am in process to understand and learn how to segmented colour in image. I found example coding of programming but I am not very clear with that. Here I attached the example of code in matlab:

    cform = makecform('srgb2lab');
    lab_he = applycform(he,cform);
    ab = double(lab_he(:,:,2:3)); 
    nrows = size(ab,1);
    ncols = size(ab,2);
    ab = reshape(ab,nrows*ncols,2);

Here is my question; 1. why need to double only 2:3?Why luminosity value no need to be double? 2. What is function to reshape value of ab?

Answer 1

[Image Analyst]

The image is converted to LAB color space - see picture below.

Then the a and b channels are extracted into a 3D image. Then a list of all [a,b] pairs are formed into a 2D list where each row is the a and b value for that pixel.

I have several color segmentation demos in my File Exchange. http://www.mathworks.com/matlabcentral/fileexchange/?term=authorid%3A31862