CHAPTER 6
502
Rendering
Any halftone cell can be divided in this way. The side of the upper square
(X)
is
equal to the horizontal displacement from a point in one halftone cell to the cor-
responding point in the adjacent cell, such as those marked by asterisks in Figure
(Y)
is the vertical displacement between the
same two points. The frequency of a halftone screen constructed from squares
with sides
X
and
Y
is thus given by
resolution
-
frequency
= ----------------------
2
2
X
+
Y
and the angle by
Y
angle
=
atan
⎛
---
⎞
⎝
X
⎠
Like a type 6 halftone, a type 10 halftone is represented as a stream containing
the threshold values, with the parameters defining the halftone specified by en-
tries in the stream dictionary. This dictionary can contain the entries shown in
Xsquare
and
Ysquare
entries replace the type 6 halftone’s
Width
and
Height
entries.
TABLE 6.5 Additional entries specific to a type 10 halftone dictionary
KEY
TYPE
VALUE
Type
name
integer
byte string
integer
integer
function or
name
(Optional)
The type of PDF object that this dictionary describes; if
present, must be
Halftone
for a halftone dictionary.
(Required)
A code identifying the halftone type that this dictionary
describes; must be 10 for this type of halftone.
(Optional)
The name of the halftone dictionary.
(Required)
The side of square
X,
in device pixels; see below.
(Required)
The side of square
Y,
in device pixels; see below.
(Optional)
A transfer function, which overrides the current transfer func-
tion in the graphics state for the same component. This entry is required
if the dictionary is a component of a type 5 halftone (see “Type 5 Half-
primary color component (see Section 6.3, “Transfer Functions”). The
name
Identity
may be used to specify the identity function.
HalftoneType
HalftoneName
Xsquare
Ysquare
TransferFunction