Trying to compute mahalanobis Distance without using the in built function

Hello I am trying to wrtite a function where i am trying to compute mahalanobis Distance. I am stuck as it throws an error that dimensions of the Matrices do not match now i know the error is clear but i am trying to follow the equation defination Is x in this equation represents pixel value at each row and column? If yes then where am i missing the trick if any one can guide me Thanks anyways guys

if true
  % code
function imDistance = mahalanobisDistance(im, modelMean, modelSigmaInv)
  %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
  for x=size(im,1)
      for y=size(im,2)
   a=im(x,y)-modelMean;
      end
  end
  b=a.*modelSigmaInv;
  c=b.*a;
   imDistance=sqrt(c);
end

Answer 1

[Youssef Khmou]

Lora, The build in function is in square unite , but here is an example on how to write a function :

 x=randn(4,100);
 R=cov(x');
 for n=1:100

    d(n)=sqrt(x(:,n)'*inv(R)*x(:,n));
 end
  plot(d.^2)
hold on
plot(mahal(x',x'),'r')

[Youssef Khmou]

the last task is to generalize that script for any x,y .

[Lora]

Well thanks a lot for the help but i wrote it in description as wekk that i cannot use Mat lab in built function Mahal.

[Youssef Khmou]

try to study the following function :

 function d=Mahald(x,y)
% this a simple version : x and y must have the sime size, rows represent
% the dimensions, columns the observations.
[K,N]=size(x);
N=length(x); % length(y)
R=(x*x')/N;
d=zeros(1,N);
for n=1:N
    d(n)=sqrt(x(:,n)'*inv(R)*y(:,n));
end

 % More enhancements can be made for arbitrary sizes of x,y .