SECTION 6.4
497
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,
of halftones in the transparent imaging model.
Type 1 Halftones
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
integer
byte string
number
number
(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 1 for this type of halftone.
(Optional)
The name of the halftone dictionary.
(Required)
The screen frequency, measured in halftone cells per inch in
device space.
(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.)
(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).
(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
.
HalftoneType
HalftoneName
Frequency
Angle
SpotFunction
function or name
AccurateScreens
boolean