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+## COLFILT Apply filter to matrix blocks
+## colfilt(A,[r c],[m n],'sliding',f,...)
+##   For each r x c overlapping subblock of A, add a column in matrix C
+##   f(C,...) should return a row vector which is then reshaped into a
+##   a matrix of size A and returned.  A is processed in chunks of size m x n.
+## colfilt(A,[r c],[m n],'distinct',f,...)
+##   For each r x c non-overlapping subblock of A, add a column in matrix C
+##   f(C,...) should return a matrix of size C each column of which is
+##   placed back into the subblock from whence it came.  A is processed
+##   in chunks of size m x n.
+##
+## The present version requires [m n], but for compatibility it should
+## be optional.  Use colfilt(A,[r c],size(A),...)
+##
+## The present version requires that [m n] divide size(A), but for
+## compatibility it should work even if [m n] does not divide A. Use
+## the following instead:
+##    [r c] = size(A);
+##    padA = zeros (m*ceil(r/m),n*ceil(c/n));
+##    padA(1:r,1:c) = A;
+##    B = colfilt(padA,...);
+##    B = B(1:r,1:c);
+##
+## The present version does not handle 'distinct'
+
+## This software is granted to the public domain
+function B = colfilt(A,filtsize,blksize,blktype,f,...)
+
+   [m,n]=size(A);
+   r = filtsize(1);
+   c = filtsize(2);
+   mblock = blksize(1);
+   nblock = blksize(2);
+
+   switch blktype
+   case 'sliding'
+     # pad with zeros
+     padm = (m+r-1);
+     padn = (n+c-1);
+     padA = zeros(padm, padn);
+     padA([1:m]+floor((r-1)/2),[1:n]+floor((c-1)/2)) = A;
+     padA = padA(:);
+
+     # throw away old A to save memory.
+     B=A; clear A;  
+
+     # build the index vector
+     colidx = [0:r-1]'*ones(1,c) + padm*ones(r,1)*[0:c-1];
+     offset = [1:mblock]'*ones(1,nblock) + padm*ones(mblock,1)*[0:nblock-1];
+     idx = colidx(:)*ones(1,mblock*nblock) + ones(r*c,1)*offset(:)';
+     clear colidx offset;
+
+     # process the matrix, one block at a time
+     idxA = zeros(r*c,mblock*nblock);
+     tmp = zeros(mblock,nblock);
+     for i = 0:m/mblock-1
+       for j = 0:n/nblock-1
+         idxA(:) = padA(idx + (i*mblock + padm*j*nblock));
+         tmp(:) = feval(f,idxA,all_va_args);
+         B(1+i*mblock:(i+1)*mblock, 1+j*nblock:(j+1)*nblock) = tmp;
+       end
+     end
+
+   case 'old-sliding'  # processes the whole matrix at a time
+     padA = zeros(m+r-1,n+c-1);
+     padA([1:m]+floor(r/2),[1:n]+floor(c/2)) = A;
+     [padm,padn] = size(padA);
+     colidx = [0:r-1]'*ones(1,c) + padm*ones(r,1)*[0:c-1];
+     offset = [1:m]'*ones(1,n) + padm*ones(m,1)*[0:n-1];
+     idx = colidx(:)*ones(1,m*n) + ones(r*c,1)*offset(:)';
+     idxA = zeros(r*c,m*n);
+     idxA(:) = padA(:)(idx);
+     B = zeros(size(A));
+     B(:) = feval(f,idxA,all_va_args);
+   case 'old-distinct' # processes the whole matrix at a time
+     if (r*floor(m/r) != m || c*floor(n/c) != n)
+        error("colfilt expected blocks to exactly fill A");
+     endif
+     colidx = [0:r-1]'*ones(1,c) + m*ones(r,1)*[0:c-1];
+     offset = [1:r:m]'*ones(1,n/c) + m*ones(m/r,1)*[0:c:n-1];
+     idx =colidx(:)*ones(1,m*n/r/c) + ones(r*c,1)*offset(:)';
+     idxA = zeros(r*c,m*n/r/c);
+     idxA(:) = A(:)(idx);
+     B = zeros(prod(size(A)),1);
+     B(idx) = feval(f,idxA,all_va_args);
+     B = reshape(B,size(A));
+   endswitch
+endfunction
+
+%!test
+%! A = reshape(1:36,6,6);
+%! assert(colfilt(A,[2,2],[3,3],'sliding','sum'), conv2(A,ones(2),'same');