SECTION 6.4
499
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
integer
byte string
integer
integer
function or name
(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 6 for this type of halftone.
(Optional)
The name of the halftone dictionary.
(Required)
The width of the threshold array, in device pixels.
(Required)
The height of 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
standard primary color component (see Section 6.3, “Transfer Func-
Identity
may be used to specify the identity
function.
HalftoneType
HalftoneName
Width
Height
TransferFunction