~F82. CONTOURING.

	Contouring,   converts  the  bit arrays  HSEG  and VSEG  into
vectors  and polygons.  The  contouring itself,  is  done by a single
subroutine named MKPGON,  make polygon.   When MKPGON is called,   it
looks for the upper most left non-zero  bit in the VSEG array. If the
VSEG  array is empty, MKPGON returns a  NIL. However, when the bit is
found, MKPGON traces and  erases the polygonal outline to  which that
bit belongs and returns a polygon node with a ring of vectors.

	To belabor the details  (for the sake of later complexities);
the MKGON trace  can go  in four  directions: north  and south  along
vertical columns of  bits in the VSEG  array, or east and  west along
horizontal rows of  the HSEG array. The trace starts by heading south
until it hits a turn; when heading south MKPGON must check  for first
a turn  to the east (indicated  by a bit  in HSEG); next for  no turn
(continue  south); and last  for a turn  to the west. When  a turn is
encountered MKPGON  creates a  vector  node representing  the run  of
bits  between the  previous turn  and the  present turn.    The trace
always ends heading west bound.  The outline so traced can be  either
the edge  of a blob  or a  hole, the two  cases are distinguished  by
looking  at the VIC-polygon's  uppermost  left  pixel in  the PAC bit
array.

	There  are   two  complexities:  contrast   accumulation  and
dekinking.    The  contrast  of  a  vector  is defined  as  (QUOTIENT
(DIFFERENCE (Sum of pixel  values on one  side of the vector)(Sum  of
pixel values on the other side  of the vector)) (length of the vector
in pixels)).   Since vectors are always either horizontal or vertical
and  are  construed as  being  on  the  cracks  between  pixels;  the
specified summations  refer to the pixels immediately  to either side
of the vector. Notice that  this definition of constrast will  always
give  a  positive  constrast for  vectors  of  a  blob  and  negative
contrast for the vectors of a hole.

	The terms "jaggies",   "kinks" and "sawtooth" all are used to
express what seems to  be wrong about  the lowermost left polygon  on
page  25.   The problem  involves  doing something  to a  rectilinear
quantized  set of segments,  to make  its linear nature more evident.
The CRE jaggies solution (in the subroutine  MKPGON) merely positions
the  turning  locus diagonally  off  its grid  point  alittle in  the
direction (northeast,    northwest,   southwest  or  southeast)  that
bisects the  turn's  right angle.   The  distance  of dekink  vernier
positioning  is  always  less than  half  a  pixel;  but greater  for
brighter cuts and less for the darker cuts; in order to  preserve the
nesting of contours.  The saw  toothed and the dekinked versions of a
polygon  have the  same number of  vectors.  I  am very  fond of this
dekinking algorithm because of its incredible  efficiency; given that
you have  a north,  south,  east,  west polygon  trace routine (which
handles image  coordinates packed row, column  into  one  accumulator
word);  then  dekinking  requires  only   one  more  ADD  instruction
execution per vector ! ~I1973,800;F8- 26 -