SECTION 6.4
489
Halftones
definition of each function along with the corresponding PostScript language
code as it would be defined in a PostScript calculator function (see Section 3.9.4,
function shows how the relative values of the function are distributed over the
halftone cell, indicating the approximate order in which pixels are whitened. Pix-
els corresponding to darker points in the image are whitened later than those cor-
responding to lighter points. (See implementation note 70 in Appendix H.)
TABLE 6.1 Predefined spot functions
NAME
APPEARANCE
DEFINITION
SimpleDot
1
−
(x
2
+
y
2
)
{ dup mul exch dup mul add 1 exch sub }
InvertedSimpleDot
x
2
+
y
2
−
1
{ dup mul exch dup mul add 1 sub }
DoubleDot
sin
(
360
×
x
)
sin
(
360
×
y
)
-
----------------------------- + -----------------------------
-
2
2
{ 360 mul sin 2 div exch 360 mul sin 2 div add }
InvertedDoubleDot
sin
(
360
×
x
)
sin
(
360
×
y
)
–
⎛
----------------------------- + -----------------------------
⎞
-
-
⎠
⎝
2
2
{ 360 mul sin 2 div exch 360 mul sin 2 div add neg }