Question

I have a piece of code that divides a image matrix img into small piece and work on them parallel. But Matlab says parfor loop cannot be used, because the way outC{i,j} is indexed. How do I fix this?

The sub matrices are of different size. If img=[4x7], then

C=[3x3 3x3 3x1;
   1x3 1x3 1x1]

On a side note, I’m not sure if using cell array is a good idea here. If not, feel free to give suggestion on how to divide up the img as well.

C=mat2cell(img, rowSplit, colSplit);
[rowc,colc]=size(C);
outC=cell(rowc,colc);
parfor i=1:rowc
    for j=1:colc
       outC{i,j}=doWork(C{i,j}); 
    end
end

Answer 1

You can use linear indexing for both the input and output.

First I make a pretend input of your shape, and a simple doWork function:

>> C= {rand(3) rand(3) rand(3,1); rand(1,3) rand(1,3) rand(1)};
>> C
C = 
    [3x3 double]    [3x3 double]    [3x1 double]
    [1x3 double]    [1x3 double]    [    0.3922]
>> doWork = @(x)2*x;

Then use linear indexing:

>> outC=cell(size(C));
>> parfor ci=1:numel(C)
     outC{ci} = doWork(C{ci});
   end

A quick check that it’s worked:

>> outC{2,1}./C{2,1}
ans =
     2     2     2