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h = fspecial('sobel');I4 = imfilter((I3),h')+imfilter((I3),h);

Mat I4_x, I4_y;Sobel(I3, I4_x, CV_8U, 1, 0);Sobel(I3, I4_y, CV_8U, 0, 1);I4 = I4_x + I4_y;