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                                                262
      CHAPTER 4                                                                       Graphics



      NAME                     DESCRIPTION

      Perceptual               Colors are represented in a manner that provides a pleasing per-
                               ceptual appearance. To preserve color relationships, both in-
                               gamut and out-of-gamut colors are generally modified from
                               their precise colorimetric values. A typical use might be for
                               scanned images.


4.5.5 Special Color Spaces

      Special color spaces add features or properties to an underlying color space.
      There are four special color space families: Pattern, Indexed, Separation, and
      DeviceN.


      Pattern Color Spaces

      A Pattern color space (PDF 1.2) enables a PDF content stream to paint an area
      with a pattern rather than a single color. The pattern may be either a tiling pattern
      (type 1) or a shading pattern (type 2). Section 4.6, “Patterns,” discusses patterns in
      detail.


      Indexed Color Spaces

      An Indexed color space allows a PDF content stream to use small integers as indi-
      ces into a color map or color table of arbitrary colors in some other space. A PDF
      consumer application treats each sample value as an index into the color table
      and uses the color value it finds there. This technique can considerably reduce the
      amount of data required to represent a sampled image—for example, by using
      8-bit index values as samples instead of 24-bit RGB color values.

      An Indexed color space is defined by a four-element array:

         [ /Indexed base hival lookup ]

      The first element is the color space family name Indexed. The remaining ele-
      ments are parameters that an Indexed color space requires; their meanings are
      discussed below. Setting the current stroking or nonstroking color space to an
      Indexed color space initializes the corresponding current color to 0.

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