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
6.4. The side of the lower square
(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
Table 6.5 in addition to the usual entries common to all streams (see Table 3.4 on
page 62). The
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-
tones” on page 505) and represents either a nonprimary or nonstandard
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
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