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CHAPTER 6                                                                         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           Defines a single halftone screen by a frequency, angle, and spot function.

5           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.

6           Defines a single halftone screen by a threshold array containing 8-bit sample
            values.

10          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.

16          (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.

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

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