Table 130 – Entries in a type 1 halftone dictionary

Key

Type

Value

Type

name

(Optional) The type of PDF object that this dictionary describes; if present, shall be Halftone for a halftone dictionary.

HalftoneType

integer

(Required) A code identifying the halftone type that this dictionary describes; shall be 1 for this type of halftone.

HalftoneName

byte string

(Optional) The name of the halftone dictionary.

Frequency

number

(Required) The screen frequency, measured in halftone cells per inch in device space.

Angle

number

(Required) The screen angle, in degrees of rotation counterclockwise with respect to the device coordinate system.

NOTE 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.

SpotFunction

function or name

(Required) A function object defining the order in which device pixels within a screen cell shall be adjusted for different gray levels, or the name of one of the predefined spot functions (see Table 128 ).

AccurateScreens

boolean

(Optional) A flag specifying whether to invoke a special halftone algorithm that is extremely precise but computationally expensive; see Note 1 for further discussion. Default value: false.

TransferFunction

function or name

(Optional) A transfer function, which overrides the current transfer function in the graphics state for the same component. This entry shall be present if the dictionary is a component of a type 5 halftone (see “Type 5 Halftones” in 10.5.5.6 ) and represents either a nonprimary or nonstandard primary colour component (see 10.4, "Transfer Functions" ). The name Identity may be used to specify the identity function.