qtgetblk.m

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function [val,r,c] = qtgetblk(A,S,dim)
%QTGETBLK Get block values in quadtree decomposition.
%   [VALS,R,C] = QTGETBLK(I,S,DIM) returns in VALS an array
%   containing the DIM-by-DIM blocks in the quadtree
%   decomposition of I. S is the sparse matrix returned by
%   QTDECOMP; it contains the quadtree structure. VALS is a
%   DIM-by-DIM-by-K array, where K is the number of DIM-by-DIM
%   blocks in the quadtree decomposition; if there are no blocks
%   of the specified size, all outputs are returned as empty
%   matrices. R and C are vectors containing the row and column
%   coordinates of the upper-left corners of the blocks.
%
%   [VALS,IDX] = QTGETBLK(I,S,DIM) returns in IDX a vector
%   containing the linear indices of the upper-left corners of
%   the blocks.
%
%   Class Support
%   -------------
%   I can be of class uint8 or double. S is of class sparse.
%
%   Remarks
%   -------
%   The ordering of the blocks in VALS matches the columnwise
%   order of the blocks in I. For example, if VALS is
%   4-by-4-by-2, VALS(:,:,1) contains the values from the first
%   4-by-4 block in I, and VALS(:,:,2) contains the values from
%   the second 4-by-4 block.
%
%   Example
%   -------
%       I = [1    1    1    1    2    3    6    6
%            1    1    2    1    4    5    6    8
%            1    1    1    1   10   15    7    7
%            1    1    1    1   20   25    7    7
%           20   22   20   22    1    2    3    4
%           20   22   22   20    5    6    7    8
%           20   22   20   20    9   10   11   12
%           22   22   20   20   13   14   15   16];
%
%       S = qtdecomp(I,5);
%       [vals,r,c] = qtgetblk(I,S,4);
%
%   See also QTDECOMP, QTSETBLK.

%   Steven L. Eddins, March 1995
%   Copyright 1993-1998 The MathWorks, Inc.  All Rights Reserved.
%   $Revision: 1.9 $  $Date: 1997/11/24 15:36:09 $

error(nargchk(3,3,nargin));

[M,N] = size(A);

Sind = find(S == dim);
numBlocks = length(Sind);
if (numBlocks == 0)
    % Didn't find any blocks.
    val = zeros(dim,dim,0);
    r = zeros(0,1);
    c = zeros(0,1);
    return;
end

% Compute block indices for a dim-by-dim block.
rows = (0:dim-1)';
cols = 0:M:(dim-1)*M;
rows = rows(:,ones(1,dim));
cols = cols(ones(dim,1),:);
ind = rows + cols;
ind = ind(:);

% Compute index matrix for block computations.
tmp = repmat(Sind', length(ind), 1);
ind = ind(:, ones(1,numBlocks));
ind = ind + tmp;

val = A(ind);

val = reshape(val, [dim dim numBlocks]);

if (nargout == 2)
    r = Sind;
    
elseif (nargout == 3)
    [r,c] = find(S == dim);
end