MATLAB: Parallel computing for images processing

Question

Good morning everyone, I've recently embarked on using the parallel computing toolbox and one problem I'd like to solve concerns a very basic scenario related to images processing. Suppose you have a directory where N different images, which can be distinguished by their name that is in the form of "imgX.jpg" where X is an increasing index, are stored. I want to create a pool of M workers that ought to perform some predefined operations over these images. In particular, the essential tasks that these workers are requested to accomplish are: 1. importing the i-th image 2. creating a filter by using the "fspecial" function 3. filtering the i-th image by employing the filter created at step 2 4. saving the processed image Of course, the list of the tasks may be further extended in the future, depending on what kind of operations are required. I wrote some code from scratch trying to employ a FSM-like structure and fit in with the SPMD model:

workers     = Open_Pool(profile, numWorkers);
state     = 1;
directory   = 'Immagini_Esercizio2/';
list    = dir([directory '*.jpg']);
images     = cell(1, length(list));
indexImage   = 1;
for i = 1 : length(images), images{i} = [directory, list(i).name];
end
spmd
  pre  = mod(labindex - 2 + numlabs, numlabs) + 1;
  post = mod(labindex, numlabs) + 1;
  
  while(indexImage <= length(images))
    % fsm update
    switch state
      case 1
        I     = imread(images{indexImage});
        labSend(I, post, 1);
        state = 2;
      case 2
        H     = fspecial('laplacian');
        I     = labReceive(pre, 1);
        labSend(post, I, 1);
        labSend(post, H, 2);
        state = 3;
      case 3
        I   = labReceive(pre, 1);
        H  = labReceive(pre, 2);
        Out     = imfilter(I, H);
        results  = Out
        labSend(post, Out, 3);
        state   = 4;
      case 4
        Out      = labReceive(pre, 3);
        filename   = sprintf('risultato%d.jpg', indexImage);
        imwrite(Out, filename);
        state      = 1;
        indexImage = indexImage + 1;
    end
  end
end

In the previous code, "Open_Pool" is a function I created to manage pools of workers (essentially, it allows the creation of M workers by using a specified profile). However, not surprisingly the execution of this code leads to deadlocks because I can't figure out how to enable a proper communication among all the workers. Instead, all the aforementioned steps must be executed orderly in a chain-like fashion.

Answer 1

Rather than using labSend and labReceive inside an spmd block, I would suggest simply re-writing this as a parfor loop where each iteration of the loop loads one file, processes it, and then writes out the result.