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          SECTION 6.4                                                                      Halftones



          Type 6 Halftones

          A type 6 halftone defines a halftone screen with a threshold array. The halftone is
          represented as a stream containing the threshold values; the parameters defining
          the halftone are specified by entries in the stream dictionary. This dictionary can
          contain the entries shown in Table 6.4 in addition to the usual entries common to
          all streams (see Table 3.4 on page 62). The Width and Height entries specify the
          dimensions of the threshold array in device pixels; the stream must contain
          Width × Height bytes, each representing a single threshold value. Threshold val-
          ues are defined in device space in the same order as image samples in image space
          (see Figure 4.26 on page 338), with the first value at device coordinates (0, 0) and
          horizontal coordinates changing faster than vertical coordinates.


          Type 10 Halftones

          Although type 6 halftones can be used to specify a threshold array with a zero
          screen angle, they make no provision for other angles. The type 10 halftone re-
          moves this restriction and allows the use of threshold arrays for halftones with
          nonzero screen angles as well.

                   TABLE 6.4 Additional entries specific to a type 6 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 6 for this type of halftone.

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

Width              integer             (Required) The width of the threshold array, in device pixels.

Height             integer             (Required) The height of the threshold array, in device pixels.

TransferFunction   function or name    (Optional) A transfer function, which overrides the current transfer
                                       function in the graphics state for the same component. This entry is re-
                                       quired if the dictionary is a component of a type 5 halftone (see “Type
                                       5 Halftones” on page 505) and represents either a nonprimary or non-
                                       standard primary color component (see Section 6.3, “Transfer Func-
                                       tions”). The name Identity may be used to specify the identity
                                       function.

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