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      SECTION 4.5                                                            Color Spaces



4.5.3 Device Color Spaces

      The device color spaces enable a page description to specify color values that are
      directly related to their representation on an output device. Color values in these
      spaces map directly (or by simple conversions) to the application of device colo-
      rants, such as quantities of ink or intensities of display phosphors. This enables a
      PDF document to control colors precisely for a particular device, but the results
      may not be consistent from one device to another.

      Output devices form colors either by adding light sources together or by subtract-
      ing light from an illuminating source. Computer displays and film recorders typi-
      cally add colors; printing inks typically subtract them. These two ways of forming
      colors give rise to two complementary methods of color specification, called ad-
      ditive and subtractive color (see Plate 1). The most widely used forms of these two
      types of color specification are known as RGB and CMYK, respectively, for the
      names of the primary colors on which they are based. They correspond to the fol-
      lowing device color spaces:

      • DeviceGray controls the intensity of achromatic light, on a scale from black to
        white.
      • DeviceRGB controls the intensities of red, green, and blue light, the three addi-
        tive primary colors used in displays.
      • DeviceCMYK controls the concentrations of cyan, magenta, yellow, and black
        inks, the four subtractive process colors used in printing.

      Although the notion of explicit color spaces is a PDF 1.1 feature, the operators for
      specifying colors in the device color spaces—G, g, RG, rg, K, and k—are available
      in all versions of PDF. Beginning with PDF 1.2, colors specified in device color
      spaces can optionally be remapped systematically into other color spaces; see
      “Default Color Spaces” on page 257.

      Note: In the transparent imaging model (PDF 1.4), the use of device color spaces is
      subject to special treatment within a transparency group whose group color space is
      CIE-based (see Sections 7.3, “Transparency Groups,” and 7.5.5, “Transparency
      Group XObjects”). In particular, the device color space operators should be used
      only if device color spaces have been remapped to CIE-based spaces by means of the
      default color space mechanism. Otherwise, the results are implementation-
      dependent and unpredictable.

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