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                                                               502
          CHAPTER 6                                                                                 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

          angle = atan ⎛ --- ⎞
                         Y
                       ⎝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                     (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 10 for this type of halftone.

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

Xsquare               integer                  (Required) The side of square X, in device pixels; see below.

Ysquare               integer                  (Required) The side of square Y, in device pixels; see below.

TransferFunction      function or              (Optional) A transfer function, which overrides the current transfer func-
                      name                     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.

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