       Various  resources  are created during the definition of a
       scene, these resources are referenced in the Tcl code with
       handles.  A handle is similar to a file id number returned
       by the open system call.  It is an  abstract  entity  that
       provides a reference to a resource that is internally man-
       aged.   The following handles for the following  resources
       are provided:


              o  camera  -  A  camera  which specifies a point of
              view.  The handle "STDCAMERA" is predefined  camera
              located  at (0.0, 0.0 10.0), looking at the origin,
              with the world y-axis as the  up  direction  and  a
              focal factor of 0.25.

              o light - A light source or spot light.

              o  surface  -  Surfaces  that  are constructed from
              polygons that  have  been  pushed  on  the  polygon
              stack.

              o shader - A shader defines a shading algorithm and
              surface attributes required by  the  shader.   This
              handle  can be used to define the way surfaces will
              be shaded, including their color and attributes.

              o object - Objects defined  in  the  SIPP  internal
              database. A special handle `WORLD' is used to spec-
              ify the world object.

              o pbmfile - A PBMPlus Portable Pixel Map  (PPM)  or
              Portable Bit Map (PBM) file.

              o rlefile - Utah Raster Toolkit RLE files.

              o pixmap - Memory resident pixel or bit map.

       When  a  unreference operation is performed on a handle, a
       reference count is decremented, the resource is not  actu-
       ally deleted unless the reference count goes to zero.  The
       handle itself is deleted.  Thus an object is not  released
       until  its is handle is unreferenced and it is no longer a
       subobject  of  any  object.   This  behavior  applies   to
       objects, surfaces, shaders and pixmaps.
