 edge pixels
that are consistent with the strong edge's direction are searched for and linked
to compose elongated lines.
Weak edge pixels that have greater edge intensity than <EM>th2</EM> are
unconditionally linked.
Even very weak edge pixels that have less edge intensity than <EM>th2</EM>
can be linked as long as they connect to another weak or strong edge
within <EM>run</EM> length.
<B>edge1</B> returns a pixel-image object
each pixel of which represents either a strong edge pixel (=1),
a weak and elong