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                                                     497
            SECTION 6.4                                                                     Halftones



            select the proprietary halftones supplied by some device manufacturers, which
            would not otherwise be accessible because they are not explicitly defined in PDF.
            If there is no HalftoneName entry, or if the requested halftone name does not ex-
            ist on the device, the halftone’s parameters are defined by the other entries in the
            dictionary, if any. If no other entries are present, the default halftone is used.

            See Section 7.6.4, “Rendering Parameters and Transparency,” and in particular,
            “Halftone and Transfer Function” on page 573, for further discussion of the role
            of halftones in the transparent imaging model.


            Type 1 Halftones

            Table 6.3 describes the contents of a halftone dictionary of type 1, which defines a
            halftone screen in terms of its frequency, angle, and spot function.

                              TABLE 6.3 Entries in a type 1 halftone dictionary
KEY                 TYPE                VALUE

Type                name                (Optional) The type of PDF object that this dictionary describes; if
                                        present, must be Halftone for a halftone dictionary.

HalftoneType        integer             (Required) A code identifying the halftone type that this dictionary
                                        describes; must be 1 for this type of halftone.

HalftoneName        byte string         (Optional) The name of the halftone dictionary.

Frequency           number              (Required) The screen frequency, measured in halftone cells per inch in
                                        device space.

Angle               number              (Required) The screen angle, in degrees of rotation counterclockwise
                                        with respect to the device coordinate system. (Most output devices
                                        have left-handed device spaces. On such devices, a counterclockwise
                                        angle in device space corresponds to a clockwise angle in default user
                                        space and on the physical medium.)

SpotFunction        function or name    (Required) A function object defining the order in which device pixels
                                        within a screen cell are adjusted for different gray levels, or the name of
                                        one of the predefined spot functions (see Table 6.1 on page 489).

AccurateScreens     boolean             (Optional) A flag specifying whether to invoke a special halftone al-
                                        gorithm that is extremely precise but computationally expensive; see
                                        below for further discussion. Default value: false.

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