draw principal directions on a plot

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Jay 2014-5-8

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链接

此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/128929-draw-principal-directions-on-a-plot

I am trying to draw principal directions from strain tensors on a plot. I attached some part of my code.

for j=1:15 ..

    for k=1:(sizetemp(1,2)-1)
..

% e2_j_k is 2 X 2 strain tensors.

        [V,D] = eig(e2_j_k);
        if (D(1,1) > D(2,2))
           eigen_j_k = D(1,1);
           p= [V(1,1);V(2,1)];
           if (V(2,1) < 0)
               theta_j_k = -acosd(dot([1 0],p));
           else
               theta_j_k = acosd(dot([1 0],p));
           end
        else
           eigen_j_k = D(2,2);
           p= [V(1,2);V(2,2)];
           if (V(2,2) < 0)
               theta_j_k = -acosd(dot([1 0],p));
           else
               theta_j_k = acosd(dot([1 0],p));
           end
        end
        eval(sprintf('e_%d_%d =e_j_k;',j,k));
        eval(sprintf('eigen_%d_%d = eigen_j_k',j,k));
        eval(sprintf('theta_%d_%d = theta_j_k',j,k));

% I already know xc and zc.

        xc(j,k) = xx(1,k);
        zc(j,k) = xz(1,k);
        eigen(j,k) = eigen_j_k;
        theta(j,k) = theta_j_k;

% xc2 and zc2 are new points to draw lines representing principal directions.

        xc2(j,k) = xc(j,k) + 0.3*cosd(theta(j,k));
        zc2(j,k) = zc(j,k) + 0.3*sind(theta(j,k));
        figure(6);
        plot([xc(j,k),xc2(j,k)],[zc(j,k),zc2(j,k)]);
        hold on;
    end

end

======================================

I know what results are supposed to be, but I don't get the reasonable results with this code. Can you see anything that I am doing wrong? Or do you have any suggestions for different method? I would appreciate any help. Thanks,

Jay