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                                               244
      CHAPTER 4                                                                     Graphics



4.5.4 CIE-Based Color Spaces

      Calibrated color in PDF is defined in terms of an international standard used in
      the graphic arts, television, and printing industries. CIE-based color spaces enable
      a page description to specify color values in a way that is related to human visual
      perception. The goal is for the same color specification to produce consistent re-
      sults on different output devices, within the limitations of each device; Plate 2 il-
      lustrates the kind of variation in color reproduction that can result from the use
      of uncalibrated color on different devices. PDF 1.1 supports three CIE-based col-
      or space families, named CalGray, CalRGB, and Lab; PDF 1.3 adds a fourth, named
      ICCBased.

      Note: In PDF 1.1, a color space family named CalCMYK was partially defined, with
      the expectation that its definition would be completed in a future version. However,
      this is no longer being considered. PDF 1.3 and later versions support calibrated
      four-component color spaces by means of ICC profiles (see “ICCBased Color Spaces”
      on page 252). PDF consumer applications should ignore CalCMYK color space at-
      tributes and render colors specified in this family as if they had been specified using
      DeviceCMYK.

      The details of the CIE colorimetric system and the theory on which it is based are
      beyond the scope of this book; see the Bibliography for sources of further in-
      formation. The semantics of CIE-based color spaces are defined in terms of the
      relationship between the space’s components and the tristimulus values X, Y, and
      Z of the CIE 1931 XYZ space. The CalRGB and Lab color spaces (PDF 1.1) are
      special cases of three-component CIE-based color spaces, known as CIE-based
      ABC color spaces. These spaces are defined in terms of a two-stage, nonlinear
      transformation of the CIE 1931 XYZ space. The formulation of such color spaces
      models a simple zone theory of color vision, consisting of a nonlinear trichro-
      matic first stage combined with a nonlinear opponent-color second stage. This
      formulation allows colors to be digitized with minimum loss of fidelity, an impor-
      tant consideration in sampled images.

      Color values in a CIE-based ABC color space have three components, arbitrarily
      named A, B, and C. The first stage transforms these components by first forcing
      their values to a specified range, then applying decoding functions, and then mul-
      tiplying the results by a 3-by-3 matrix, producing three intermediate components
      arbitrarily named L, M, and N. The second stage transforms these intermediate
      components in a similar fashion, producing the final X, Y, and Z components of
      the CIE 1931 XYZ space (see Figure 4.14).

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