Creating a matrix array for training images. Whats going on?

Can someone please explain what is going on in the second loop and how its happening.

N = 90

for i=1:N
    % importing files
    FName=sprintf('%d.bmp',i);
    image1=imread([FFolder1, FName]);   
    image2=imread([FFolder2, FName]);
    image3=imread([FFolder3, FName]);

    % taking the 2D image (image1-3) and sticks it into the i'th 
    % slice (plane) of a 3D image
    im1(:,:,i)=image1;      % "0"
    im2(:,:,i)=image2;      % "1"
    im3(:,:,i)=image3;      % "2"
end

% What is going on in this loop?
for i=1:N   
    im(:,:,i)=im1(:,:,i);
    im(:,:,i+N)=im2(:,:,i);
    im(:,:,i+2*N)=im3(:,:,i);
end

Answer 1

[Image Analyst]

Basically you have 3 volumetric images: im1, im2, and im3. That code stacks them on top of each other to form a new volumetric image that is equal to the height of all 3 summed together (because they're stacked in the Z direction).

[Recap]

I kind of get it but can you explain it a little bit more for me, please?

[Image Analyst]

Imagine you get have 3 boxes of cereal. Put the Corn flakes on the ground and write im1 on it with a Sharpie marker. Then take the box of Raisin Bran, write im2 on it, and set it on top of the Corn Flakes box. Now take the box of Cheerios, write im3 on it with the marker, and place it on top of the Raisin Bran box. So now all three boxes are stacked on top of each other.

[Recap]

I get that now but what about when it loops? where does it stack then?

[Image Analyst]

If im is already preallocated, it's basically inserting the three layers in at the proper level. If it's not, then the last line basically "grows" the stack taller and then those planes are there and the next iteration the two lower planes will be inserted and another layer added on the top. Put this line into the for loop and you'll see what it's doing

fprintf('Now inserting planes %d, %d, and %d\n', i, i+N, i+2*n);

You should be able to simply do

im = cat(3, im1, im2, im3);

instead of that for loop. This should be faster.