cconv2.m

% RES = CCONV2(MTX1, MTX2, CTR)
%
% Circular convolution of two matrices.  Result will be of size of
% LARGER vector.
% 
% The origin of the smaller matrix is assumed to be its center.
% For even dimensions, the origin is determined by the CTR (optional) 
% argument:
%      CTR   origin
%       0     DIM/2      (default)
%       1     (DIM/2)+1  

% Eero Simoncelli, 6/96.  Modified 2/97.

function c = cconv2(a,b,ctr)

if (exist('ctr') ~= 1)
  ctr = 0;
end

if (( size(a,1) >= size(b,1) ) & ( size(a,2) >= size(b,2) ))
    large = a; small = b;
elseif  (( size(a,1) <= size(b,1) ) & ( size(a,2) <= size(b,2) ))
    large = b; small = a;
else
  error('one arg must be larger than the other in both dimensions!');
end

ly = size(large,1);
lx = size(large,2);
sy = size(small,1);
sx = size(small,2);

%% These values are the index of the small mtx that falls on the
%% border pixel of the large matrix when computing the first
%% convolution response sample:
sy2 = floor((sy+ctr+1)/2);
sx2 = floor((sx+ctr+1)/2);

% pad:
clarge = [ ...
    large(ly-sy+sy2+1:ly,lx-sx+sx2+1:lx), large(ly-sy+sy2+1:ly,:), ...
	large(ly-sy+sy2+1:ly,1:sx2-1); ...
    large(:,lx-sx+sx2+1:lx), large, large(:,1:sx2-1); ...
    large(1:sy2-1,lx-sx+sx2+1:lx), ...
	large(1:sy2-1,:), ...
	large(1:sy2-1,1:sx2-1) ];

c = conv2(clarge,small,'valid');