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                                                    835
          SECTION 9.5                                                                   3D Artwork



            The first two steps can be expressed as a single equation, as follows:
                                                      –1
             x c y c z c 1 = x a y a z a 1 × ( aw × cw )

          3. Finally, the camera coordinates are projected into two dimensions, eliminating
             the z coordinate, then scaled and positioned within the annotation’s target co-
             ordinate system.


  9.5.5 3D Markup

          Beginning with PDF 1.7, users can comment on specific views of 3D artwork by
          using markup annotations (see “Markup Annotations” on page 616). Markup an-
          notations (as other annotations) are normally associated with a location on a
          page. To associate the markup with a specific view of a 3D annotation, the anno-
          tation dictionary for the markup annotation contains an ExData entry (see Table
          8.21 on page 618) that specifies the 3D annotation and view. Table 9.48 describes
          the entries in an external data dictionary used to markup 3D annotations.

            TABLE 9.48 Entries in an external data dictionary used to markup 3D annotations
KEY          TYPE          VALUE

Type         name          (Required) The type of PDF object that this dictionary describes; if present, must be
                           ExData for an external data dictionary.

Subtype      name          (Required) The type of external data that this dictionary describes; must be
                           Markup3D for a 3D comment. In PDF 1.7, the only defined value is Markup3D.

3DA          dictionary    (Required) The 3D annotation to which this markup annotation applies. The 3D
             or text       annotation may be specified as a child dictionary or as the name of a 3D annota-
             string        tion, as specified by its NM entry. In the latter case, the 3D annotation and the
                           markup annotation must be on the same page of the document.

3DV          dictionary    (Required) The 3D view that this markup annotation is associated with. The anno-
                           tation will be hidden unless this view is currently being used for the 3D annotation
                           specified by 3DA.

MD5          byte string   (Optional) A 16-byte string that contains the checksum of the bytes of the 3D
                           stream data that this 3D comment is associated with. The checksum is calculated
                           by applying the standard MD5 message-digest algorithm (described in Internet
                           RFC 1321, The MD5 Message-Digest Algorithm; see the Bibliography) to the bytes
                           of the stream data. This value is used to determine if artwork data has changed
                           since this 3D comment was created.

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