~F81. THRESHOLDING.

	Thresholding, the  first and easiest  step,  consists  of two
subroutines,    called THRESH  and  PACXOR. THRESH  converts  a 6-bit
image into a 1-bit image with respect to a given threshold  cut level
between zero  for black and  sixty-three for light. All  pixels equal
to or  greater than the cut, map into a one; all the pixels less than
the cut, map into zero. The resulting 1-bit image is  stored in a bit
array  of  216  rows by  288  columns  (1728  words) called  the  PAC
(picture accumulator)  which  was  named  in  memory  of  McCormick's
ILLIAC-III.  After THRESH,   the PAC contains blobs of bits.   A blob
is defined as  "rook's move" simply connected; that is every bit of a
blob can be reached  by horizontal or  vertical moves from any  other
bit without having to  cross a zero bit or having  to make a diagonal
(bishop's)  move.  Blobs may of course  have holes.  Or equalvalently
a blob always  has one outer  perimeter polygon,   and may have  one,
several or  no inner perimeter polygons. This  blob and hole topology
is recoverible  from the  CRE  data structure  and  is built  by  the
nesting step.

	Next,   PACXOR copies the  PAC into  two slightly larger  bit
arrays named HSEG and  VSEG. Then the PAC is shifted down one row and
exclusive OR'ed into  the HSEG array;  and the  PAC is shifted  right
one column  and exclusive OR'ed  into the  VSEG array to  compute the
horizontal and  vertical border bits of the PAC blobs.  Notice,  that
this is the very heart  of the "edge finder" of CRE. Namely,   PACXOR
is the mechanism that converts regions into edges.
~I1973,800;F8- 24 -