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       vpCreateMinMaxOctree,  vpMinMaxOctreeThreshold, vpDestroyMinMaxOctree -
       create/destroy a min-max octree for optimized rendering


       #include <volpack.h>

       vpCreateMinMaxOctree(vpc, root_node_size, base_node_size)
           vpContext *vpc;
           int root_node_size, base_node_size;

       vpMinMaxOctreeThreshold(vpc, param_num, range)
           vpContext *vpc;
           int param_num, range;

           vpContext *vpc;


       vpc    VolPack context from vpCreateContext.

              Number of voxels per side in the top level  (largest  nodes)  of
              the octree.  Not currently used.

              Number  of  voxels per side in the bottom level (smallest nodes)
              of the octree.  Must be a power of 2.

              Identifying number of one  parameter  of  the  opacity  transfer

       range  Range  threshold  for  one  parameter  of  the  opacity transfer
              function.  If an octree node spans a larger  range  then  it  is


       These  routines  are  used  to  create and destroy a min-max octree, an
       internal data structure that is  used  to  accelerate  classifying  and
       rendering   volume  data  in  a  3D  voxel  array.   The  octree  is  a
       hierarchical data structure that contains minimum  and  maximum  values
       for  each  voxel  field.   It  does not duplicate the data in the voxel
       array.   The  octree  is  not  used  with  preclassified  volumes  (see

       Before   creating   an   octree,   declare   a  3D  voxel  array  using
       vpSetVolumeSize, vpSetVoxelSize,  vpSetVoxelField  and  vpSetRawVoxels,
       and  initialize parameters for the opacity transfer function by calling
       vpSetClassifierTable.  Then initialize the voxel  data  in  the  array.
       The  lookup  tables for the opacity transfer function do not need to be

       Now call vpMinMaxOctreeThreshold once for each parameter to the opacity
       transfer  function.   The  parameter  number  is the same number as the
       parameter number  you  would  pass  to  vpClassifierTable.   The  range
       argument  specifies  a  "small"  range  of values for the corresponding
       parameter.  The opacity of a voxel should  not  vary  much  for  voxels
       within  this  range  of parameter space.  Choosing a value which is too
       small or too large may result in a reduced performance  benefit  during
       rendering.   The  function  vpOctreeMask  can  be used to visualize the
       effectiveness of various parameter ranges.  Note that you  must  choose
       the   voxel  field  associated  with  each  opacity  transfer  function
       parameter   in   advance   of   creating   the   octree   (by   calling
       vpSetClassiferTable), but the opacity transfer function itself does not
       need to be known since the lookup tables do not need to be initialized.
       In the current implementation all of the parameters must be one or two-
       byte voxel fields; four-byte fields are not allowed.

       Finally, call vpCreateMinMaxOctree to actually compute the octree.  The
       arguments specify the largest and smallest node size.  These values can
       be used  to  trade  off  storage  size  of  the  octree  for  increased
       resolution.   The octree is stored in the rendering context and is used
       automatically   in   subsequent   calls   to    vpClassifyVolume    and
       vpRenderRawVolume.   Only  one  octree  may be stored in a context at a
       time.  If an octree already exists when vpCreateMinMaxOctree is  called
       then the old octree is destroyed.

       If  the data in the voxel array changes then the min-max octree must be
       explicitly recomputed by calling vpCreateMinMaxOctree again before  the
       next call to vpClassifyVolume or vpRenderRawVolume.

       vpDestroyMinMaxOctree destroys the current min-max octree and frees the
       associated storage.


       The current octree parameters can be retrieved with the following state
       variable     codes     (see     vpGeti(3)):    VP_CLASSIFY_FIELD_COUNT,


       The  normal  return  value  for   all   three   functions   is   VP_OK.
       vpMinMaxOctreeThreshold can return the following error value:

              The parameter number is invalid.

       vpCreateMinMaxOctree can return the following error values:

              The node size arguments are not a power of two.

              The  voxel  size  or the voxel fields have not been specified or
              have been incorrectly specified, or the number of parameters  to
              the opacity transfer function has been incorrectly specified.

              The  volume  array or volume dimensions have not been specified,
              or the size of the  volume  array  does  not  match  the  volume

              The  fields  specified  as  parameters  to  the opacity transfer
              function are invalid or do not have valid sizes.

              The volume is too large, so the maximum size of the  octree  has
              been  exceeded.   Change  the  constant  VP_MAX_OCTREE_LEVELS in
              volpack.h and recompile the VolPack library.


       VolPack(3), vpCreateContext(3),  vpClassifyVolume(3),  vpOctreeMask(3),