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      SECTION 7.6                                         Color Space and Rendering Issues



      The opaque imaging model also allows process color components to be addressed
      individually, as if they were spot colors. For instance, it is possible to specify a
      Separation color space named Cyan, which paints just the cyan component on a
      CMYK output device. However, this capability is very difficult to extend to trans-
      parency groups. In general, the color components in a group are not the process
      colorants themselves, but are converted to process colorants only after the com-
      pletion of all color compositing computations for the group (and perhaps some of
      its parent groups as well). For instance, if the group’s color space is ICCBased, the
      group has no Cyan component to be painted. Consequently, treating a process
      color component as if it were a spot color is permitted only within a group that
      inherits the native color space of the output device (or is implicitly converted to
      DeviceCMYK, as discussed in “Implicit Conversion of CIE-Based Color Spaces”
      on page 259). Attempting to do so in a group that specifies its own color space re-
      sults in conversion of the requested spot color to its alternate color space.

7.6.3 Overprinting and Transparency

      In the opaque imaging model, overprinting is controlled by two parameters of the
      graphics state: the overprint parameter and the overprint mode (see Section 4.5.6,
      “Overprint Control”). Painting an object causes some specific set of device colo-
      rants to be marked, as determined by the current color space and current color in
      the graphics state. The remaining colorants are either erased or left unchanged,
      depending on whether the overprint parameter is false or true. When the current
      color space is DeviceCMYK, the overprint mode parameter additionally enables
      this selective marking of colorants to be applied to individual color components
      according to whether the component value is zero or nonzero.

      Because this model of overprinting deals directly with the painting of device
      colorants, independently of the color space in which source colors have been
      specified, it is highly device-dependent and primarily addresses production
      needs rather than design intent. Overprinting is usually reserved for opaque colo-
      rants or for very dark colors, such as black. It is also invoked during late-stage
      production operations such as trapping (see Section 10.10.5, “Trapping Sup-
      port”), when the actual set of device colorants has already been determined.

      Consequently, it is best to think of transparency as taking place in appearance
      space, but overprinting of device colorants in device space. This means that colo-
      rant overprint decisions should be made at output time, based on the actual re-
      sultant colorants of any transparency compositing operation. On the other hand,
      effects similar to overprinting can be achieved in a device-independent manner
      by taking advantage of blend modes, as described in the next section.

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