decision : we'll study the sign of a integer parameter whose value is proportional
to the difference between the separations of the two pixel positions from the
actual line path.
recursive process
step 0
from k to k+1 : choice (xk + 1, yk)
or (xk + 1, yk
+ 1)
y = m (xk + 1) + b
d1 = y - yk = m (xk + 1) + b - yk
d2 = (yk + 1) - y = yk + 1 - m (xk
+ 1) -b
what we want to know : which of d1 and d2 is smaller, what we'll study
: the sign of d1 - d2