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     CHAPTER 6                                                                Rendering



6.2.4 Conversion from DeviceCMYK to DeviceRGB

     Conversion of a color value from CMYK to RGB is a simple operation that does
     not involve black generation or undercolor removal:
        red = 1.0 – min (1.0, cyan + black)
      green = 1.0 – min (1.0, magenta + black)
       blue = 1.0 – min (1.0, yellow + black)

     In other words, the black component is simply added to each of the other compo-
     nents, which are then converted to their complementary colors by subtracting
     them each from 1.0.


 6.3 Transfer Functions

     In PDF 1.2, a transfer function adjusts the values of color components to compen-
     sate for nonlinear response in an output device and in the human eye. Each com-
     ponent of a device color space—for example, the red component of the
     DeviceRGB space—is intended to represent the perceived lightness or intensity of
     that color component in proportion to the component’s numeric value. Many de-
     vices do not actually behave this way, however; the purpose of a transfer function
     is to compensate for the device’s actual behavior. This operation is sometimes
     called gamma correction (not to be confused with the CIE-based gamut mapping
     function performed as part of CIE-based color rendering).

     In the sequence of steps for processing colors, the consumer application applies
     the transfer function after performing any needed conversions between color
     spaces, but before applying a halftone function, if necessary. Each color compo-
     nent has its own separate transfer function; there is no interaction between com-
     ponents.

     Transfer functions always operate in the native color space of the output device,
     regardless of the color space in which colors were originally specified. (For exam-
     ple, for a CMYK device, the transfer functions apply to the device’s cyan, magen-
     ta, yellow, and black color components, even if the colors were originally
     specified in, for example, a DeviceRGB or CalRGB color space.) The transfer func-
     tion is called with a numeric operand in the range 0.0 to 1.0 and must return a
     number in the same range. The input is the value of a color component in the de-
     vice’s native color space, either specified directly or produced by conversion from

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