CHAPTER 6
496
Rendering
dictionary are interpreted according to this type. PDF supports the halftone types
listed in Table 6.2.
TABLE 6.2 PDF halftone types
TYPE
MEANING
1
5
Defines a single halftone screen by a
frequency, angle,
and
spot function.
Defines an arbitrary number of halftone screens, one for each colorant or color
component (including both primary and spot colorants). The keys in this dic-
tionary are names of colorants; the values are halftone dictionaries of other
types, each defining the halftone screen for a single colorant.
Defines a single halftone screen by a threshold array containing 8-bit sample
values.
Defines a single halftone screen by a threshold array containing 8-bit sample
values, representing a halftone cell that may have a nonzero screen angle.
(PDF 1.3)
Defines a single halftone screen by a threshold array containing 16-
bit sample values, representing a halftone cell that may have a nonzero screen
angle.
6
10
16
The dictionaries representing these halftone types contain the same entries as
the corresponding PostScript language halftone dictionaries (as described in Sec-
tion 7.4 of the
PostScript Language Reference,
Third Edition), with the following
exceptions:
•
The PDF dictionaries may contain a
Type
entry with the value
Halftone
, identi-
fying the type of PDF object that the dictionary describes.
•
Spot functions and transfer functions are represented by function objects in-
stead of PostScript procedures.
•
Threshold arrays are specified as streams instead of files.
•
In type 5 halftone dictionaries, the keys for colorants must be name objects;
they may not be strings as they may in PostScript.
Halftone dictionaries have an optional entry,
HalftoneName
, that identifies the
halftone by name. In PDF 1.3, if this entry is present, all other entries, including
HalftoneType
, are optional. At rendering time, if the output device has a halftone
with the specified name, that halftone is used, overriding any other halftone pa-
rameters specified in the dictionary. This provides a way for PDF documents to