493
SECTION 6.4 Halftones
NAME APPEARANCE DEFINITION
Square −max ( | x | , | y | )
{ abs exch abs 2 copy lt
{ exch }
if
pop neg }
Cross −min ( | x | , | y | )
{ abs exch abs 2 copy gt
{ exch }
if
pop neg }
Rhomboid 0.9 × x + y
-
--------------------------------
2
{ abs exch abs 0.9 mul add 2 div }
Diamond if | x | + | y | ≤ 0.75 then 1 − (x 2 + y 2)
else if | x | + | y | ≤ 1.23 then 1 − (0.85 × | x | + | y | )
else ( | x | – 1) 2 + ( | y | – 1) 2 – 1
{ abs exch abs 2 copy add 0.75 le
{ dup mul exch dup mul add 1 exch sub }
{ 2 copy add 1.23 le
{ 0.85 mul add 1 exch sub }
{ 1 sub dup mul exch 1 sub dup mul add 1 sub }
ifelse }
ifelse }
Figure 6.1 illustrates the effects of some of the predefined spot functions.