MATLAB: Trouble with accessing variable in parfor loop

Question

Hello all,

I am trying to do batch processing and I am having trouble implementing the parfor loop. I have a variable (a movie frame), that I want to subtract from every frame in a movie after I load in the frames. The basics of my loop look like this.

load('gray_mean_frame.mat');
set(handles.testMovieButton, 'String', 'Please Wait');
poolobj = parpool('local',4);
addAttachedFiles(poolobj,{'gray_mean_frame.mat'});
parfor frameNumber = 1:numFrames
      movieFrame = read(readerobj, frameNumber);
      ghostFrame = gray_mean_frame - movieFrame;
end

I always get the error

"An UndefinedFunction error was thrown on the workers for 'gray_mean_frame'.
This might be because the file containing 'gray_mean_frame' is not accessible
on the workers.  Use addAttachedFiles(pool, files) to specify the required
files to be attached.  See the documentation for
'parallel.Pool/addAttachedFiles' for more details."

But I think I am correctly passing the file gray_mean_frame to the workers. gray_mean_frame.mat is a file which contains the variable gray_mean_frame, among a couple others. I've also tried making gray_mean_frame a global variable but it did not help. It is not clear to me what I am doing wrong. Thank you for any help.

Answer 1

The problem here is that unfortunately the PARFOR analysis is based on the text of your program, and in this case the analysis cannot deduce that gray_mean_frame is a variable. You should be able to fix this by making a minor change:

    data = load('gray_mean_frame.mat');
    gray_mean_frame = data.gray_mean_frame;