SYNOPSIS
#include <volpack.h>
vpResult
 vpCreateMinMaxOctree(vpc, root_node_size, base_node_size)
 vpContext *vpc;

int root_node_size, base_node_size;
vpResult
 vpMinMaxOctreeThreshold(vpc, param_num, range)
 vpContext *vpc;

int param_num, range;
vpResult
 vpDestroyMinMaxOctree(vpc)
 vpContext *vpc;
ARGUMENTS
 vpc
 VolPack context from vpCreateContext.
 root_node_size
 Number of voxels per side in the top level (largest nodes) of the octree. Not currently used.
 base_node_size
 Number of voxels per side in the bottom level (smallest nodes) of the octree. Must be a power of 2.
 param_num
 Identifying number of one parameter of the opacity transfer function.
 range
 Range threshold for one parameter of the opacity transfer function. If an octree node spans a larger range then it is subdivided.
DESCRIPTION
These routines are used to create and destroy a minmax 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 vpClassifyVolume(3)).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 initialized.
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 twobyte voxel fields; fourbyte 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 minmax octree must be explicitly recomputed by calling vpCreateMinMaxOctree again before the next call to vpClassifyVolume or vpRenderRawVolume.
vpDestroyMinMaxOctree destroys the current minmax octree and frees the associated storage.
STATE VARIABLES
The current octree parameters can be retrieved with the following state variable codes (see vpGeti(3)): VP_CLASSIFY_FIELD_COUNT, VP_CLASSIFY_FIELDS, VP_MMOCTREE_THRESHOLDS, VP_MMOCTREE_SIZE.ERRORS
The normal return value for all three functions is VP_OK. vpMinMaxOctreeThreshold can return the following error value: VPERROR_BAD_VALUE
 The parameter number is invalid.
vpCreateMinMaxOctree can return the following error values:
 VPERROR_BAD_VALUE
 The node size arguments are not a power of two.
 VPERROR_BAD_VOXEL
 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.
 VPERROR_BAD_VOLUME
 The volume array or volume dimensions have not been specified, or the size of the volume array does not match the volume dimensions.
 VPERROR_BAD_CLASSIFIER
 The fields specified as parameters to the opacity transfer function are invalid or do not have valid sizes.
 VPERROR_LIMIT_EXCEEDED
 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.