SECTION 4.5
271
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
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.