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       vpOctreeMask  -  compute  a  mask  representing  one level of a min-max


       #include <volpack.h>

       vpOctreeMask(vpc, array, array_size, max_level)
           vpContext *vpc;
           unsigned char *array;
           int array_size;
           int max_level;


       vpc    VolPack context from vpCreateContext.

       array  A 3D array for storing the mask.

              Size of array in bytes.

              Maximum octree level to descend to.


       vpOctreeMask is used for performance debugging when  rendering  volumes
       with  a min-max octree.  During rendering the min-max octree is used to
       help determine which voxels are transparent.  The time required to make
       this  determination  can  be minimized by an appropriate choice for the
       range  parameters  for  vpMinMaxOctreeThreshold  and  the   node   size
       parameters for vpCreateMinMaxOctree.  vpOctreeMask provides information
       that  can  help  to  determine  whether  a  set  of  parameters   works
       effectively or not.

       The output of the routine is stored in a 3D array of bytes that has the
       same dimensions as the volume (although each element is only one  byte,
       regardless  of  the size of a voxel).  For each voxel in the volume the
       corresponding byte in the output array is set by vpOctreeMask to one of
       three  values:  0  if  the  voxel is definitely transparent, 255 if the
       voxel may  be  non-transparent,  or  128  if  the  voxel  may  be  non-
       transparent  and  there  is  more  detailed  information  available  at
       unvisited levels of the octree.  The max_level argument  specifies  the
       deepest  level of the octree to be visited (the root level is level 0).

       A suggested way to use this routine is to compute  an  octree  using  a
       particular  set  of  parameters and then call vpOctreeMask with several
       different values for max_level.  For each resulting mask array, look at
       slices  (or create a volume rendering!) and see how accurately the mask
       captures the general shape of the data in  the  original  volume.   The
       best  set  of  parameters  results  in a mask that captures the general
       shape without an excessively small value for the  minimum  octree  node
       size  (which  would  result in a very large data structure and a lot of
       traversal overhead).


       The normal return value is VP_OK.  The following  error  return  values
       are possible:

              There is no octree or the output array has the wrong size.

              The  voxel  size  or the voxel fields have not been specified or
              have been incorrectly specified.

              The opacity transfer function tables have invalid sizes  or  are
              associated  with  invalid  voxel fields or have been incorrectly


       VolPack(3), vpCreateContext(3), vpCreateMinMaxOctree(3)