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                                             372
CHAPTER 4                                                                             Graphics



  <<                                               % In the resources dictionary
        /Properties << /oc1 5 0 R >>               % This dictionary maps the name oc1 to an
  ...                                              % optional content group (object 5)
  >>

  5 0 obj                                          % The OCG controlling the visibility
  <<                                               % of the text.
      /Type /OCG
      /Name (Show Greeting)
  >>
  endobj

The example above shows one piece of content associated with one optional con-
tent group. There are other possibilities:
• More than one section of content can refer to the same group or membership
  dictionary, in which case the visibility of both sections is always the same.
• Equivalently, although less space-efficient, different sections can have separate
  membership dictionaries with the same OCGs and P entries. The sections will
  have identical visibility behavior.
• Two sections of content can belong to membership dictionaries that refer to the
  same group(s) but with different P settings. For example, if one section has no P
  entry, and the other has a P entry of AllOff, the visibility of the two sections of
  content are opposite. That is, the first section is visible when the second is hid-
  den, and vice versa.

The following example demonstrates both the direct use of optional content
groups and the indirect use of groups through a membership dictionary. The
content (a black rectangle frame) is drawn if either of the images controlled by
the groups named Image A or Image B is shown. If both groups are hidden, the
rectangle frame is hidden.

Example 4.36

  % Within a content stream
  ...
  /OC /OC2 BDC                    % Draws a black rectangle frame
      0g
      4w
      100 100 412 592 re s
  EMC

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