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                                       269
SECTION 4.5                                                            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-
  ple 4.21 on page 282 for the code used to generate this image.

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.

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