SECTION 4.5
269
Color Spaces
the PANTONE Hexachrome system, which uses six colorants: the usual cyan,
magenta, yellow, and black, plus orange and green.
•
Multitone
color systems use a single-component image to specify multiple color
components. In a
duotone,
for example, a single-component image can be used
to specify both the black component and a spot color component. The tone
reproduction is generally different for the different components. For example,
the black component might be painted with the exact sample data from the sin-
gle-component image; the spot color component might be generated as a
nonlinear function of the image data in a manner that emphasizes the shadows.
Plate 6 shows an example that uses black and magenta color components. In
Plate 7, a single-component grayscale image is used to generate a
quadtone
re-
sult that uses four colorants: black and three PANTONE spot colors. See Exam-
DeviceN
was designed to represent color spaces containing multiple components
that correspond to colorants of some target device. As with
Separation
color
spaces, PDF consumer applications must be able to approximate the colorants if
they are not available on the current output device, such as a display. To accom-
plish this, the color space definition provides a tint transformation function that
can be used to convert all the components to an alternate color space.
PDF 1.6 extends the meaning of
DeviceN
to include color spaces that are referred
to as
NChannel
color spaces.
Such color spaces may contain an arbitrary number
of spot and process components, which may or may not correspond to specific
device colorants (the process components must be from a single process color
space). They provide information about each component that allows applications
more flexibility in converting colors. For example, they may use their own blend-
ing algorithms for on-screen viewing and composite printing, rather than being
required to use a specified tint transformation function. These color spaces are
identified by a value of
NChannel
for the
Subtype
entry of the attributes dictio-
nary (see Table 4.21). A value of
DeviceN
for the
Subtype
entry, or no value,
means that only the previous features are supported. PDF consumer applications
that do not support PDF 1.6 treat these color spaces as normal
DeviceN
color
spaces and use the tint transformation function as appropriate. Producer applica-
tions using the
NChannel
features should follow certain guidelines, as noted
throughout this section, to achieve good backward compatibility.
DeviceN
color spaces are defined in a similar way to
Separation
color spaces—in
fact, a
Separation
color space can be defined as a
DeviceN
color space with only
one component.