MATLAB: How to make the pool to parallel? “Error: The variable hj in a parfor cannot be classified.” We always get that error

parfor

clear clc
tic ticBytes(gcp);
I = imread('test1.tif'); imshow(I); I1 = imadjust(I,stretchlim(I),[0,1]); figure; imshow(I1); [N,M] = size(I)
h = [0.125,0.125,0.125,0.125]; g = [0.5,-0.5]; delta = [1,0,0];
tic;
    J = 3;
    a(1:N,1:M,1,1:J+1) = 0;
    dx(1:N,1:M,1,1:J+1) = 0;
    dy(1:N,1:M,1,1:J+1) = 0;
    d(1:N,1:M,1,1:J+1) = 0;
    a(:,:,1,1) = conv2(h,h,I,'same');
    dx(:,:,1,1) = conv2(delta,g,I,'same');
    dy(:,:,1,1) = conv2(g,delta,I,'same');
    x = dx(:,:,1,1);
    y = dy(:,:,1,1);
    d(:,:,1,1) = sqrt(x.^2+y.^2);
    I1 = imadjust(d(:,:,1,1),stretchlim(d(:,:,1,1)),[0 1]);
    figure;imshow(I1);
    lh = length(h);
    lg = length(g);
    parfor j = 1:J+1
        lhj = 2^j*(lh-1)+1;
        lgj = 2^j*(lg-1)+1;
        hj(1:lhj)=0;
        gj(1:lgj)=0;
        for n = 1:lh
             hj(2^j*(n-1)+1)=h(n);
        end
        for n = 1:lg
            gj(2^j*(n-1)+1)=g(n);
        end
        a(:,:,1,j+1) = conv2(hj,hj,a(:,:,1,j),'same');
        dx(:,:,1,j+1) = conv2(delta,gj,a(:,:,1,j),'same');
        dy(:,:,1,j+1) = conv2(gj,delta,a(:,:,1,j),'same');
        x = dx(:,:,1,j+1);
        y = dy(:,:,1,j+1);
        dj(:,:,1,j+1) = sqrt(x.^2+y.^2);
        I1 = imadjust(dj(:,:,1,j+1),stretchlim(dj(:,:,1,j+1)),[0 1]);
        figure;imshow(I1);
    end
tocBytes(gcp) toc

Best Answer

The problem here is that you are not completely overwriting hj on each iteration of the parfor loop, and so the iterations are order dependent. parfor can only run if the iterations can be proved (by the MATLAB infrastructure that supports parfor) to be independent. You can convince parfor that your iterations are independent (at least with respect to hj) by completely overwriting it at the start of each loop iteration. I.e. replace
hj(1:lhj) = 0;
with
hj = zeros(1, lhj);
however this is not completely equivalent to the code you had previously, so it might not work as you expect.
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