SECTION 4.5
275
Color Spaces
KEY
TYPE
VALUE
DotGain
dictionary
(Optional)
A dictionary specifying the
dot gain
of inks to be used in blending
calculations when used as an alternative to the tint transformation function.
Dot gain (or loss) represents the amount by which a printer’s halftone dots
change as the ink spreads and is absorbed by paper.
For each entry, the key is a colorant name, and the value is a function that maps
values in the range 0 to 1 to values in the range 0 to 1. The dictionary may list
colorants unused by this specific
DeviceN
instance and need not list all colo-
rants. An entry with a key of
Default
specifies a function to be used by all colo-
rants for which a dot gain function is not explicitly specified.
PDF consumer applications may ignore values in this dictionary when other
sources of dot gain information are available, such as ICC profiles associated
with the process color space or tint transformation functions associated with
individual colorants.
Each entry in the mixing hints dictionary refers to colorant names, which include
spot colorants referenced by the
Colorants
dictionary. Under some circumstanc-
es, they may also refer to one or more individual process components called
Cyan
,
Magenta
,
Yellow
, or
Black
when
DeviceCMYK
is specified as the process col-
or space in the process dictionary. However, applications should ignore these pro-
cess component entries if they can obtain the information from an ICC profile.
Note:
The mixing hints subdictionaries (as well as the colorants dictionary) may
specify colorants that are not used in any given instance of a
DeviceN
color space.
This allows them to be referenced from multiple
DeviceN
color spaces, which can
produce smaller file sizes as well as consistent color definitions across instances.
For consistency of color, PDF consumers should follow these guidelines:
•
The consumer should apply either the specified tint transformation function or
invoke the same alternative blending algorithm for all
DeviceN
instances in the
document.
Note:
When the tint transformation function is used, the burden is on the produc-
er to guarantee that the individual function definitions chosen for all
DeviceN
in-
stances produce similar color appearances throughout the document.
•
Blending algorithms should produce a similar appearance for colors when they
are used as separation colors or as a component of a
DeviceN
color space.