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504
CHAPTER 6 Rendering
sented as 2 bytes, with the high-order byte first. The stream must therefore
contain 2 × Width × Height bytes if there is only one rectangle or
2 × (Width × Height + Width2 × Height2) bytes if there are two rectangles. The
contents of the first rectangle are specified first, followed by those of the second
rectangle. Threshold values within each rectangle 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 chang-
ing faster than vertical coordinates.
TABLE 6.6 Additional entries specific to a type 16 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 16 for this type of halftone.
HalftoneName byte string (Optional) The name of the halftone dictionary.
Width integer (Required) The width of the first (or only) rectangle in the threshold
array, in device pixels.
Height integer (Required) The height of the first (or only) rectangle in the threshold
array, in device pixels.
Width2 integer (Optional) The width of the optional second rectangle in the threshold
array, in device pixels. If this entry is present, the Height2 entry must
be present as well. If this entry is absent, the Height2 entry must also be
absent, and the threshold array has only one rectangle.
Height2 integer (Optional) The height of the optional second rectangle in 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,” below) and represents either a nonprimary or nonstand-
ard primary color component (see Section 6.3, “Transfer Functions”).
The name Identity may be used to specify the identity function.
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