CAN ANY ONE TELL ME WHAT IS THIS PART OF CODE DOING??

Answer 1

[Guillaume]

This badly written code recolour a rectangle of 4 rows by 8 column above the centroid of the largest objects. The red and green channels of that rectangle are set to 0, the blue is set to 255.

I say badly written since none of the for loops are required (the second one is particularly absurd). The first loop could just be replaced with:

[armax, armaxloc] = max(ar);
%no need to calculate mx (which by the way is just mx = numel(ar))
centroid = stats(armaxloc).Centroid;

The second loop is just:

i_aft(y-4:y, x-4:x+4, [1 2]) =  0; %set red and green channel to 0
i_aft(y-4:y, x-4:x+4, 3) = 255; %set blue channel to 255

Note that the above, just like the original code will give an error if the centroid is less than 4 pixels away from the left, top or right edge of the image.

Possibly, the original author intended to recolour a square around the centroid, but that's not what the code is doing.

[Image Analyst]

Looks like he's trying to burn a blue box into the RGB "after" image. I'm not sure why. I'd probably just use plot(x,y,'bs', 'LineWidth', 4, 'MarkerSize', 10), or rectangle() or line(), to place a blue rectangle in the overlay above the image.