Viewing Transformation

Each time an application draws the model space, it specifies a different viewing transformation to transfer a view of the picture-to-page coordinate space. GpiSetViewingTransformMatrix is used to apply a viewing transformation. There is one viewing transformation for each part or whole model space to be incorporated in page-coordinate space.

GpiSetViewingTransformMatrix cannot be called while there is an open segment, and it has no effect on primitives outside segments. When it has been specified, the viewing transformation applies to all subsequently created segments until it is next changed. The viewing transformation that is current when a segment is created is a fixed part of the segment. Once specified, the viewing transformation cannot be queried, nor can it be altered, unless you re-create the segment. This is inconvenient when several versions of the same picture are being drawn. The problem can be solved by:

• Defining the picture in one or more unchained segments.

• Creating a segment chain and calling the unchained segments from each root segment.

• Setting the viewing transformation before each root segment.

Each time the segment chain is drawn, multiple views of the same picture are produced. This sequence is illustrated in the following example, where the segment chain comprises three root segments, each of which calls a single unchained segment.

GpiSetInitialSegmentAttrs      /* Switches off the chained attribute   */
GpiOpenSegment                 /* Creates an unchained segment         */
                               /* containing the picture definition    */

GpiCloseSegment
GpiSetInitialSegmentAttrs      /* Switches on the chained attribute    */

GpiSetViewingTransformMatrix   /* Sets the viewing transformation      */
                               /* for segment 1                        */

GpiOpenSegment (segment 1)
GpiCallSegmentMatrix           /* Calls the unchained segment          */

GpiCloseSegment
GpiSetViewingTransformMatrix   /* Sets the viewing transformation      */
                               /* for segment 2                        */

GpiOpenSegment (segment 2)
GpiSetViewingLimits            /* Specifies the area of interest       */
                               /* in the model space.                  */

GpiCallSegmentMatrix           /* Calls the unchained segment          */

GpiCloseSegment
GpiSetViewingTransformMatrix   /* Sets the viewing transformation      */
                               /* for segment 3                        */

GpiOpenSegment (segment 3)
GpiCallSegmentMatrix           /* Calls the unchained segment          */
GpiCloseSegment

When a segment chain has been created using this method, an application cannot change the viewing transformation unless it re-creates the segment chain. The viewing transformation of a segment is permanently recorded and cannot be edited. The application would not, however, have to re-create the picture definition in the unchained segment.

If the picture definition comprises a number of unchained segments, an application must create an intermediate segment to contain the GpiCallSegmentMatrix calls for those segments. Each root segment would then call the intermediate segment.

The viewing transformation applies to the entire root segment and cannot be overridden from within the segment. It is particularly useful for positioning and scaling one or more segments of a subpicture within the presentation page when a segment transformation cannot be used. A segment transformation can be overridden by any model or instance transformations within the segment. A typical example of its use is when importing a subpicture using GpiPutData or GpiPlayMetaFile.

Note: The viewing transformation must be set to its default value before an application defines an unchained segment to be called from another segment.