I'm new to image processing and cannot trace this code?

hi I'm new to image processing and I can not trace this code,can any body say a brief explanation about what is that function?? thanks

function H = circconvmatx2(h, M,N)

[mh, nh] = size(h);
if mh~=nh,
    error('blur kernel must be square');
end

blockheight = N;
blockwidth = M;

for i=((nh+1)/2):nh,

      %Take the kernel
      h0 = h(:,i);
      h0 = h0(:)';
      h0 = fliplr(h0);

      h0 = circshift( padarray( h0, [0 N-nh], 'post' ), [0 -(nh+1)/2+1] );

      if sum(find(abs(h0)>0)) == 0,
          H0 = sparse(M,M);
      else
          H0 = sparse(circulant(h0'));
      end
      if i==(nh+1)/2,
          H = H0;
      else
          H = [H H0];
      end
  end

for i=(nh+1)/2-1:-1:1,

      %Take the kernel
      h0 = h(:,i);
      h0 = h0(:)';
      h0 = fliplr(h0);

      h0 = circshift( padarray( h0, [0 N-nh], 'post' ), [0 -(nh+1)/2+1] );

      if sum(find(abs(h0)>0)) == 0,
          H0 = sparse(M,M);
      else
          H0 = sparse(circulant(h0'));
      end
      H = [H0 H];
  end

H = [H, sparse(zeros(N,N*(N-nh)))];

H = circshift( H, [0 -N*((nh+1)/2-1)] );
H0 = H;
for i=1:N-1,
    H = [H; circshift( H0, [0 N*i] )];
end

[Walter Roberson]

Some kind of 2D circular convolution. circulant() is not a Mathworks-provided function but might be http://www.mathworks.com/matlabcentral/fileexchange/22858-circulant-matrix