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                                              557
      SECTION 7.5                                            Specifying Transparency in PDF



KEY   TYPE            VALUE

CS    name or array   (Sometimes required, as discussed below) The group color space, which is used
                      for the following purposes:
                      • As the color space into which colors are converted when painted into the
                        group
                      • As the blending color space in which objects are composited within the group
                        (see Section 7.2.3, “Blending Color Space”)
                      • As the color space of the group as a whole when it in turn is painted as an ob-
                        ject onto its backdrop
                      The group color space may be any device or CIE-based color space that treats its
                      components as independent additive or subtractive values in the range 0.0 to 1.0,
                      subject to the restrictions described in Section 7.2.3, “Blending Color Space.”
                      These restrictions exclude Lab and lightness-chromaticity ICCBased color spac-
                      es, as well as the special color spaces Pattern, Indexed, Separation, and DeviceN.
                      Device color spaces are subject to remapping according to the DefaultGray,
                      DefaultRGB, and DefaultCMYK entries in the ColorSpace subdictionary of the
                      current resource dictionary (see “Default Color Spaces” on page 257).
                      Ordinarily, the CS entry is allowed only for isolated transparency groups (those
                      for which I, below, is true), and even then it is optional. However, this entry is re-
                      quired in the group attributes dictionary for any transparency group XObject
                      that has no parent group or page from which to inherit—in particular, one that
                      is the value of the G entry in a soft-mask dictionary of subtype Luminosity (see
                      “Soft-Mask Dictionaries” on page 552).
                      In addition, it is always permissible to specify CS in the group attributes dictio-
                      nary associated with a page object, even if I is false or absent. In the normal case
                      in which the page is imposed directly on the output medium, the page group is
                      effectively isolated regardless of the I value, and the specified CS value is there-
                      fore honored. But if the page is in turn used as an element of some other page
                      and if the group is non-isolated, CS is ignored and the color space is inherited
                      from the actual backdrop with which the page is composited (see Section 7.3.6,
                      “Page Group”).

                      Default value: the color space of the parent group or page into which this trans-
                      parency group is painted. (The parent’s color space in turn can be either explicit-
                      ly specified or inherited.)
                      Note: For a transparency group XObject used as an annotation appearance (see
                      Section 8.4.4, “Appearance Streams”), the default color space is inherited from the
                      page on which the annotation appears.

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