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                                                   614
         CHAPTER 8                                                           Interactive Features




                             TABLE 8.19 Entries in an appearance dictionary
KEY   TYPE                      VALUE

N     stream or dictionary      (Required) The annotation’s normal appearance.

R     stream or dictionary      (Optional) The annotation’s rollover appearance. Default value: the value of
                                the N entry.

D     stream or dictionary      (Optional) The annotation’s down appearance. Default value: the value of the
                                N entry.


         Each entry in the appearance dictionary may contain either a single appearance
         stream or an appearance subdictionary. In the latter case, the subdictionary de-
         fines multiple appearance streams corresponding to different appearance states of
         the annotation.

         For example, an annotation representing an interactive check box might have two
         appearance states named On and Off. Its appearance dictionary might be defined
         as

             /AP << /N << /On    formXObject1
                          /Off    formXObject2
                       >>
                    /D << /On    formXObject3
                          /Off    formXObject4
                       >>
                 >>

         where formXObject1 and formXObject2 define the check box’s normal appearance
         in its checked and unchecked states, and formXObject3 and formXObject4 provide
         visual feedback, such as emboldening its outline, when the user clicks it. (No R
         entry is defined because no special appearance is needed when the user moves
         the cursor over the check box without pressing the mouse button.) The choice be-
         tween the checked and unchecked appearance states is determined by the AS en-
         try in the annotation dictionary (see Table 8.15 on page 606).

         Note: Some of the standard PDF annotation types, such as movie annotations—as
         well as all custom annotation types defined by third parties—are implemented
         through plug-in extensions. If the plug-in for a particular annotation type is not
         available, PDF viewer applications should display the annotation with its normal
         (N) appearance. Viewer applications should also attempt to provide reasonable be-

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