I think some of the edges aren t getting detected becuase of the sobel filters not catching the edges, but I m not sure. I think the thick lines are from not doing the supression and hysteresis properly. Here is my canny edge code and convolution

detect edes by canny edge detection based method

code of edge detection canny in m file for lena pic

利用传统canny算法进行边缘检测，详细(Edge detection using traditional Canny algorithm in detail)