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SECTION 4.5                                                             Color Spaces



Implicit Conversion of CIE-Based Color Spaces

In workflows in which PDF documents are intended for rendering on a specific
target output device (such as a printing press with particular inks and media), it is
often useful to specify the source colors for some or all of a document’s objects in
a CIE-based color space that matches the calibration of the intended device. The
resulting document, although tailored to the specific characteristics of the target
device, remains device-independent and will produce reasonable results if re-
targeted to a different output device. However, the expectation is that if the docu-
ment is printed on the intended target device, source colors that have been
specified in a color space matching the calibration of the device will pass through
unchanged, without conversion to and from the intermediate CIE 1931 XYZ
space as depicted in Figure 4.14 on page 245.

In particular, when colors intended for a CMYK output device are specified in an
ICCBased color space using a matching CMYK printing profile, converting such
colors from four components to three and back is unnecessary and results in a
loss of fidelity in the black component. In such cases, PDF consumer applications
may provide the ability for the user to specify a particular calibration to use for
printing, proofing, or previewing. This calibration is then considered to be that of
the native color space of the intended output device (typically DeviceCMYK), and
colors expressed in a CIE-based source color space matching it can be treated as if
they were specified directly in the device’s native color space. Note that the condi-
tions under which such implicit conversion is done cannot be specified in PDF,
since nothing in PDF describes the calibration of the output device (although an
output intent dictionary, if present, may suggest such a calibration; see Section
10.10.4, “Output Intents”). The conversion is completely hidden by the applica-
tion and plays no part in the interpretation of PDF color spaces.

When this type of implicit conversion is done, all of the semantics of the device
color space should also apply, even though they do not apply to CIE-based spaces
in general. In particular:

• The nonzero overprint mode (see Section 4.5.6, “Overprint Control”) deter-
  mines the interpretation of color component values in the space.
• If the space is used as the blending color space for a transparency group in the
  transparent imaging model (see Sections 7.2.3, “Blending Color Space”; 7.3,
  “Transparency Groups”; and 7.5.5, “Transparency Group XObjects”), compo-
  nents of the space, such as Cyan, can be selected in a Separation or DeviceN col-

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