%--------------------------
 % @Author: Jingqiao Hu
 % @Date: 2021-06-03 13:51:44
 % @LastEditTime: 2021-06-03 14:13:29
%--------------------------
function green_strain(F, globalx, globaly)
    m = size(F{1}, 2);
    I = repmat([1, 0, 0, 1]', 1, m);
    E = zeros(4, 4, m);
    for gp = 1:4
        Fg = F{gp}; % 4,m
        FT = Fg([1,3,2,4], :);
        E(gp, :, :)  = (matrix_3d_multiply(FT, Fg) - I) * 0.5;
    end
    Em = squeeze(mean(E, 1));
    s1 = reshape(Em(1, :), globaly, globalx);
    s2 = reshape(Em(2, :), globaly, globalx);
    s3 = reshape(Em(4, :), globaly, globalx);
    figure; imagesc(s1); colormap(jet); axis equal; axis off; caxis([-0.1, 0.1]); 
    figure; imagesc(s2); colormap(jet); axis equal; axis off; caxis([-0.15, 0.03]); 
    figure; imagesc(s3); colormap(jet); axis equal; axis off; caxis([-0.1, 0.1]); 
end