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SECTION 4.5                                                              Color Spaces



the component names in the color space do not correspond to a colorant avail-
able on the device, the PDF consumer application can perform subsequent paint-
ing operations in the alternate color space specified by this parameter.

Note: For NChannel color spaces, the components are evaluated individually; that
is, only the ones not present on the output device use the alternate color space.

The tintTransform parameter specifies a function (see Section 3.9, “Functions”)
that is used to transform the tint values into the alternate color space. It is called
with n tint values and returns m color component values, where n is the number
of components needed to specify a color in the DeviceN color space and m is the
number required by the alternate color space.

Note: Painting in the alternate color space may produce a good approximation of
the intended color when only opaque objects are painted. However, it does not cor-
rectly represent the interactions between an object and its backdrop when the object
is painted with transparency or when overprinting (see Section 4.5.6, “Overprint
Control”) is enabled.

The color component name None, which may be present only for DeviceN color
spaces that do not have the NChannel subtype, indicates that the corresponding
color component is never painted on the page, as in a Separation color space for
the None colorant. (However, see implementation note 48 in Appendix H.) When
a DeviceN color space is painting the named device colorants directly, color com-
ponents corresponding to None colorants are discarded. However, when the
DeviceN color space reverts to its alternate color space, those components are
passed to the tint transformation function, which can use them as desired.

Note: A DeviceN color space whose component colorant names are all None always
discards its output, just the same as a Separation color space for None; it never
reverts to the alternate color space. Reversion occurs only if at least one color com-
ponent (other than None) is specified and is not available on the device.

The optional attributes parameter is a dictionary (see Table 4.21) containing addi-
tional information about the components of color space that PDF consumer ap-
plications may use. PDF consumers are not required to use the alternateSpace and
tintTransform parameters, and may instead use custom blending algorithms, along
with other information provided in the attributes dictionary if present. (If the val-
ue of the Subtype entry in the attributes dictionary is NChannel, such informa-
tion must be present.) However, alternateSpace and tintTransform must always be
provided for applications that want to use them or do not support PDF 1.6.

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