Previous Next

                                               284
      CHAPTER 4                                                                    Graphics



      As in the preceding examples, an Indexed color space based on a DeviceN space is
      used to paint the grayscale image shown on the left in the plate with four colo-
      rants: black and three PANTONE spot colors. The alternate color space is a sim-
      ple calibrated RGB. Thus, the DeviceN color space has seven components: the
      four desired colorants plus the three components of the alternate space. The ex-
      ample shows the image XObject (see Section 4.8.4, “Image Dictionaries”) repre-
      senting the quadtone image, followed by the color space used to interpret the
      image data. (See implementation note 49 in Appendix H.)


4.5.6 Overprint Control

      The graphics state contains an overprint parameter, controlled by the OP and op
      entries in a graphics state parameter dictionary. Overprint control is useful main-
      ly on devices that produce true physical separations, but it is available on some
      composite devices as well. Although the operation of this parameter is device-
      dependent, it is described here rather than in the chapter on color rendering,
      because it pertains to an aspect of painting in device color spaces that is impor-
      tant to many applications.

      Any painting operation marks some specific set of device colorants, depending
      on the color space in which the painting takes place. In a Separation or DeviceN
      color space, the colorants to be marked are specified explicitly; in a device or
      CIE-based color space, they are implied by the process color model of the output
      device (see Chapter 6). The overprint parameter is a boolean flag that determines
      how painting operations affect colorants other than those explicitly or implicitly
      specified by the current color space.

      If the overprint parameter is false (the default value), painting a color in any color
      space causes the corresponding areas of unspecified colorants to be erased (paint-
      ed with a tint value of 0.0). The effect is that the color at any position on the page
      is whatever was painted there last, which is consistent with the normal painting
      behavior of the opaque imaging model.

      If the overprint parameter is true and the output device supports overprinting, no
      such erasing actions are performed; anything previously painted in other colo-
      rants is left undisturbed. Consequently, the color at a given position on the page
      may be a combined result of several painting operations in different colorants.
      The effect produced by such overprinting is device-dependent and is not defined
      by the PDF language.

Previous Next