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                                             486
    CHAPTER 6                                                                   Rendering



    duce device-dependent effects, a page description that is intended to be device-
    independent should not alter them.

    Note: When the current color space is DeviceGray and the output device’s native
    color space is DeviceCMYK, the interpreter uses only the gray transfer function. The
    normal conversion from DeviceGray to DeviceCMYK produces 0.0 for the cyan,
    magenta, and yellow components. These components are not passed through their
    respective transfer functions but are rendered directly, producing output containing
    no colored inks. This special case exists for compatibility with existing applications
    that use a transfer function to obtain special effects on monochrome devices, and
    applies only to colors specified in the DeviceGray color space.

    See Section 7.6.4, “Rendering Parameters and Transparency,” and in particular,
    “Halftone and Transfer Function” on page 573, for further discussion of the role
    of transfer functions in the transparent imaging model.


6.4 Halftones

    Halftoning is a process by which continuous-tone colors are approximated on an
    output device that can achieve only a limited number of discrete colors. Colors
    that the device cannot produce directly are simulated by using patterns of pixels
    in the colors available. Perhaps the most familiar example is the rendering of gray
    tones with black and white pixels, as in a newspaper photograph.

    Some output devices can reproduce continuous-tone colors directly. Halftoning is
    not required for such devices; after gamma correction by the transfer functions,
    the color components are transmitted directly to the device. On devices that do
    require halftoning, it occurs after all color components have been transformed by
    the applicable transfer functions. The input to the halftone function consists of
    continuous-tone, gamma-corrected color components in the device’s native color
    space. Its output consists of pixels in colors the device can reproduce.

    PDF provides a high degree of control over details of the halftoning process. For
    example, in color printing, independent halftone screens can be specified for each
    of several colorants. When rendering on low-resolution displays, fine control
    over halftone patterns is needed to achieve the best approximations of gray levels
    or colors and to minimize visual artifacts.

    Note: Remember that everything pertaining to halftones is, by definition, device-
    dependent. In general, when a PDF document provides its own halftone specifica-

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