f gradient operators provided:
<B>grad3</B> which takes difference between horizontally and vertically 
neighboring pixels,
<B>prewitt</B> and <B>sobel</B>.
<EM>method</EM>=0,1 selects <B>grad3</B>, 2 selects <B>prewitt</B> and 3 selects
<B>sobel</B>.
Pixels that have edge intensity greater than <EM>th1</EM> are identified as
strong edge pixels.
After thinning edges referring to edge intensities and
directions of gradient, isolated edge pixels are marked.
Starting from end points of these strong edges, weak