The algorithm searches for the next neighbour pixel from
the starting point (only border pixels) according to the
connectedness definition used. Except for the transition
point, i.e. “9”, these pixels which change the orientation of
boundary, such as downward turn to upward or leftward
turn to rightward, are encoded “8”. If the code of the next
neighbour pixel is repeated then only one code is recorded.
Contour tracing ends when the algorithm reaches its starting
point