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                                               563
      SECTION 7.6                                          Color Space and Rendering Issues



      If a group’s color space—whether specified explicitly or inherited from the parent
      group—is CIE-based, any use of device color spaces for painting objects is subject
      to special treatment. Device colors cannot be painted directly into such a group,
      since there is no generally defined method for converting them to the CIE-based
      color space. This problem arises in the following cases:

      • DeviceGray, DeviceRGB, and DeviceCMYK color spaces, unless remapped to de-
        fault CIE-based color spaces (see “Default Color Spaces” on page 257)
      • Operators (such as rg) that specify a device color space implicitly, unless that
        space is remapped
      • Special color spaces whose base or underlying space is a device color space, un-
        less that space is remapped

      It is recommended that the default color space remapping mechanism always be
      employed when defining a transparency group whose color space is CIE-based. If a
      device color is specified and is not remapped, it is converted to the CIE-based color
      space in an implementation-dependent fashion, producing unpredictable results.

      Note: The foregoing restrictions do not apply if the group’s color space is implicitly
      converted to DeviceCMYK, as discussed in “Implicit Conversion of CIE-Based Color
      Spaces” on page 259.


7.6.2 Spot Colors and Transparency

      The foregoing discussion of color spaces has been concerned with process colors—
      those produced by combinations of an output device’s process colorants. Process
      colors may be specified directly in the device’s native color space (such as Device-
      CMYK), or they may be produced by conversion from some other color space,
      such as a CIE-based (CalRGB or ICCBased) space. Whatever means is used to
      specify them, process colors are subject to conversion to and from the group’s col-
      or space.

      A spot color is an additional color component, independent of those used to pro-
      duce process colors. It may represent either an additional separation to be
      produced or an additional colorant to be applied to the composite page (see “Sep-
      aration Color Spaces” on page 264 and “DeviceN Color Spaces” on page 268).
      The color component value, or tint, for a spot color specifies the concentration of
      the corresponding spot colorant. Tints are conventionally represented as subtrac-
      tive, rather than additive, values.

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