Error in classifying variable in parallel for loop.

Hi! In the following code M1 is a 4d matrix. The error message I receive upon running this code is : The variable this_volume in a parfor cannot be classified. From what I understand I am just using the variable as a sliced input and output variable at the same time since I am assigning values to some of it's elements. I am not sure if the two extra for loops after the parfor are the cause of the problem. I will be grateful for any suggestions.

tic

fill_thresh = 12;

for a_fill = 1:size_M1(4)

    this_volume = M1(:,:,:,a_fill);
    this_volume_copy = this_volume;
    parfor b_fill = 1:size_M1(3)

        fill_factor = zeros(size_M1(1),size_M1(2));
        for c_fill = 1+2:size_M1(1)-2
            for d_fill = 1+2:size_M1(2)-2

                fill_factor(c_fill,d_fill) = sum(sum(this_volume_copy(c_fill-2:c_fill+2,d_fill-2:d_fill+2,b_fill )));
                if fill_factor(c_fill,d_fill ) > fill_thresh
                    this_volume(c_fill,d_fill,b_fill ) = 1;
                end
            end
        end

        toc
    end
    M1(:,:,:,a_fill) = this_volume;
    toc
end

Answer 1

[Tejas M U]

The reason for the error is documented as - “A parfor-loop generates an error if it contains any variables that cannot be uniquely categorized or if any variables violate their category restrictions.” in the link

Here is a link that might help you figure out how to sort this out.