CHAPTER 6
504
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
integer
byte string
integer
integer
integer
(Optional)
The type of PDF object that this dictionary describes; if
present, must be
Halftone
for a halftone dictionary.
(Required)
A code identifying the halftone type that this dictionary
describes; must be 16 for this type of halftone.
(Optional)
The name of the halftone dictionary.
(Required)
The width of the first (or only) rectangle in the threshold
array, in device pixels.
(Required)
The height of the first (or only) rectangle in the threshold
array, in device pixels.
(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.
(Optional)
The height of the optional second rectangle in the threshold
array, in device pixels.
(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
ard primary color component (see Section 6.3, “Transfer Functions”).
The name
Identity
may be used to specify the identity function.
HalftoneType
HalftoneName
Width
Height
Width2
Height2
integer
function or name
TransferFunction