Problem converting formula of Welford algorthm

Hai, please someone help me because using code below, for different image, i still get the same value for alpha, theta and image normalization.

 a = imread('3.JPG');
  imshow(a);
  b = roipoly(a);

%to set point
[X,Y] = size(b);

%to calculate mean image
m = mean2(b);

%to calculate standard deviation
sd = std2(b);

% initialise
sMinusMean=0;

% mean-variance moving window operation
for i=1:X
    for j=1:Y

         iMean = [X,Y]-m;
         sMinusMean=sMinusMean+iMean; 
      end
  end

      %image normalised
       iNeu=sMinusMean/sd;
       [X,Y]= size(iNeu);

% gradient orientation

g1= [X,Y+1]-[X,Y-1];
g2= [X+1,Y]-[X-1,Y];

theta=atan2(g1,g2);

% magnitude
a1= [X*X,(Y+1)*X]-[X*X,(Y-1)*X];
a2= [(X+1)*Y,Y*Y]-[(X-1),Y*Y];
a3= a1+a2;
alpha=sqrt(a3);