matlab code for face detection using RGB space color

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MatLab

image-processing

I have this matlab code for face detection using RGB space color and it is works 100%

clc
I=imread('my.jpg');
figure,imshow(I);

%%%%%%%%%%%%%%%%%%%%%%%%%%%skin region in rgb image%%%%%%%%%
if(size(I, 3) > 1)
final_image = zeros(size(I,1), size(I,2));
for i = 1:size(I,1)
for j = 1:size(I,2)
R = I(i,j,1);
G = I(i,j,2);
B = I(i,j,3);

if(R > 95 && G > 40 && B > 20)
v = [R,G,B];
if((max(v) - min(v)) > 15)
if(abs(R-G) > 15 && R > G && R > B)

%it is a skin
final_image(i,j) = 1;
end
end
end
end
end
end %%% added
figure, imshow(final_image);
disp('before');
BW=final_image;
%else
% BW=im2bw(I);
% figure, imshow(BW);
%disp('convert');
%end

%%%%%%%%%%%Connected Components-Detection of face%%%%%%%%%%%%%
L = bwlabel(BW,4);%BWLABEL Label==>connected components in 2-D binary image
BB = regionprops(L, 'BoundingBox');%Measure properties of image regions
BB1 =struct2cell(BB);%struct2cell==>Convert structure array to cell array
BB2 = cell2mat(BB1);%cell2mat==>Convert a cell array into a single matrix.
[s1 s2]=size(BB2);%sizing BB2 in matrix format
mx=0;
for k=3:4:s2-1
p=BB2(1,k)*BB2(1,k+1);
if p>mx && (BB2(1,k)/BB2(1,k+1))<1.8
mx=p;
j=k;
end
end

the questions
1. why for- loop "for k=3:4:s2-1" ??
2. what does this loop ??
3. what dose "if p>mx && (BB2(1,k)/BB2(1,k+1))<1.8" ?
4. what step of loop is 3 ??