comparison Problems/private/sampgrid.m @ 61:42fcbcfca132

(none)

+function y = sampgrid(x,blocksize,varargin)
+%SAMPGRID Sample a multi-dimensional matrix on a regular grid.
+%  Y = SAMPGRID(X,BLOCKSIZE,I1,I2,...,Ip) extracts block samples of size
+%  BLOCKSIZE from the p-dimensional matrix X, arranging the samples as the
+%  column vectors of the matrix Y. The locations of the (1,1,..,1)-th
+%  elements of each block are given in the index vectors I1,I2,..Ip. The
+%  total number of samples taken is length(I1)xlength(I2)x...xlength(Ip).
+%  BLOCKSIZE should either be a p-element vector of the form [N1,N2,...Np],
+%  or a scalar N which is shorthand for the square block size [N N ... N].
+%
+%  Example: Sample a set of blocks uniformly from a 2D image.
+%
+%    n = 512; blocknum = 20000; blocksize = [8 8];
+%    im = rand(n,n);
+%    [i1,i2] = reggrid(size(im)-blocksize+1, blocknum);
+%    blocks = sampgrid(im, blocksize, i1, i2);
+%
+%  See also REGGRID.
+
+%  Ron Rubinstein
+%  Computer Science Department
+%  Technion, Haifa 32000 Israel
+%  ronrubin@cs
+%
+%  November 2007
+
+
+p = ndims(x);
+if (p==2 && any(size(x)==1) && length(blocksize)==1)
+  p = 1;
+end
+
+if (numel(blocksize)==1)
+  blocksize = ones(1,p)*blocksize;
+end
+
+n = zeros(1,p);
+for i = 1:p
+  n(i) = length(varargin{i});
+end
+
+nsamps = prod(n);
+
+% create y of the same class as x
+y = zeros(prod(blocksize),nsamps,class(x));
+
+% ids() contains the index of the current block in I1..Ip
+ids = ones(p,1);
+
+% block_ids contains the indices of the current block in X
+block_ids = cell(p,1);
+for j = 1:p
+  block_ids{j} = varargin{j}(1) : varargin{j}(1)+blocksize(j)-1;
+end
+
+for k = 1:nsamps
+  block = x(block_ids{:});
+  y(:,k) = block(:);
+  
+  % increment ids() and block_ids{}
+  if (k<nsamps)
+    j = 1;
+    while (ids(j) == n(j))
+      ids(j) = 1;
+      block_ids{j} = varargin{j}(1) : varargin{j}(1)+blocksize(j)-1;
+      j = j+1;
+    end
+    ids(j) = ids(j)+1;
+    block_ids{j} = varargin{j}(ids(j)) : varargin{j}(ids(j))+blocksize(j)-1;
+  end
+end
+