problem with dividing matrix into subblocks using blockproc and global varaible!

Dear All, I want to divide a 3 by 15 matrix into 3 by 3 blocks (no overlap) blockproc. I use global variables to extract those blocks. Please have a look at this code shown below:

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
  function test()

    global all_blocks item

     item=0; all_blocks=[];

     x=rand(3,15)

    fun = @(block_struct) block1(block_struct.data);
    y = blockproc(x,[3 3],fun,'BorderSize',[0 0],...
    'TrimBorder',true,'PadPartialBlocks',true);

    z=all_blocks

    end

    function output= block1(in_array)
    global all_blocks item
    output=in_array;
    item=item+1;
    all_blocks{item}=in_array;
    end
  %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

the output of this code, z, is a 1 by 8 cells whereas the true output is 1 by 5 cells !. The cell no. 8, 7, and 6 of z are repeated (extra blocks). I do not want to use other functions such as mat2cell. Any suggestion why the output is like that?.. many thanks, wissam

[Geoff Hayes]

Hi wissam,

I can't comment on why the code isn't working as intended (I don't have the blockproc command) but have you tried stepping through the code to see what is happening? Placing breakpoints at the update to all_blocks{item} may give you a clue as to why it is being updated with the additional three blocks.

Or an alternative to blockproc is to write the code yourself to extract the blocks (especially since you are not doing any sort of processing on the data other than to extract it from the matrix):

m = 3;  % the number of rows in the block to extract
n = 3;  % the number of columns in the block to extract

[r,c] = size(x);  % get the number of rows and cols in x

rBlks = floor((r-1)/m) + 1;  % determine the number of row blocks
cBlks = floor((c-1)/n) + 1;  % determine the number of col blocks

z = cell(rBlks,cBlks);  % size your output cell matrix

% extract all blocks from the matrix x
for i=1:rBlks

      % determine the start row (xra) and the end row (xrb)
      % for row block i
      xra = (i-1)*m+1; xrb = min(xra+m-1,r);

      for j=1:cBlks

          % determine the start col (xca) and the end col (xcb)
          % for column block j
          xca = (j-1)*n+1; xcb = min(xca+n-1,c);

          % reset the empty block to all zeros (zeros will be the
          % padding in the event the block we extract from x is
          % not complete)
          blk = zeros(m,n);

          % extract the data (which may now be padded)
          blk(1:(xrb-xra+1),1:(xcb-xca+1)) = x(xra:xrb,xca:xcb);

          % save to cell array
          z{i,j} = blk;
      end
  end

Geoff

[wissam]

Dear Geoff, many thanks for the valuable feedback. Yes, I traced the code using breakpoints and I found that, blockproc produces some repeated blocks (cell no. 8, 7, and 6). I don't know what the problem is.

regarding your code, It works very fine and thank you for that. How about if there is an overlap of 50% among blocks? is it possible to modify your code to extracted overlapped blocks?

best regards

[Geoff Hayes]

I suspect that it will be relatively easy to add a 50% overlap…I will leave that for you though! :)