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                                                    808
          CHAPTER 9                                                           Multimedia Features



          to be corrected in cases where the original content cannot be re-authored. See
          “3D Lighting Scheme Dictionaries” on page 817.”

          The SA entry provides cross section information for clipping 3D artwork while its
          associated view is active. This allows view authors to be more clear in calling out
          the intended areas of interest for a particular view, some of which might other-
          wise be completely obscured. See “3D Cross Section Dictionaries” on page 819.”

          The NR and NA entries are meant to give a more accurate representation of the 3D
          artwork at a given state. These keys give view authors finer granularity in manip-
          ulating the artwork to be presented in a particular way. They also provide a
          means for returning node parameters to a known state after potential changes by
          interactive features such as keyframe animations and JavaScript. See “3D Node
          Dictionaries” on page 828.”


          Projection Dictionaries

          A projection dictionary (see Table 9.40) defines the mapping of 3D camera coordi-
          nates onto the target coordinate system of the annotation. Each 3D view can
          specify a projection dictionary by means of its P entry.

          Note: Although view nodes can specify projection information, PDF consumers ig-
          nore it in favor of information in the projection dictionary.

          PDF 1.6 introduces near/far clipping. This type of clipping defines a near plane
          and a far plane (as shown in Figure 9.5 on page 810). Objects, or parts of objects,
          that are beyond the far plane or closer to the camera than the near plane are not
          drawn. 3D objects are projected onto the near plane and then scaled and posi-
          tioned within the annotation’s target coordinate system, as described below.

                              TABLE 9.40 Entries in a projection dictionary
KEY       TYPE        VALUE

Subtype   name        (Required) The type of projection. Valid values are O (orthographic) or P (perspective).

CS        name        (Optional) The clipping style. Valid values are XNF (explicit near/far) or ANF (automatic
                      near/far). Default value: ANF.

F         number      (Optional; meaningful only if the value of CS is XNF) The far clipping distance, expressed
                      in the camera coordinate system. No parts of objects whose z coordinates are greater
                      than the value of this entry are drawn. If this entry is absent, no far clipping occurs.

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