vpSetClassifierTable  specify an opacity transfer function
#include <volpack.h>
vpResult
vpSetClassifierTable(vpc, param_num, param_field,
table, table_size)

 vpContext *vpc;

 int param_num, param_field;

 float *table;

 int table_size;
 vpc
 VolPack context from vpCreateContext.
 param_num
 Parameter number identifying one parameter of the opacity transfer
function.
 param_field
 Field number identifying a voxel field that contains the parameter.
 table
 Lookup table that defines the mapping from the voxel field value to the
opacity.
 table_size
 Size of table in bytes.
vpSetClassifierTable is used to specify lookup tables that define an
opacity transfer function. The opacity transfer function must be of the form:

 opacity = f0(param0) * f1(param1) * ... * fN(paramN)
where each of the factors is a function of one voxel field (see below for
implementing more general classification models).
vpSetClassifierTable
must be called once for each factor to specify a lookup table that defines the
factor. After the function has been specified, opacities are computed by
calling one of the classification functions:
vpClassifyScalars,
vpClassifyVolume, or
vpClassifyScanline. To classify a voxel, these
functions use the values in the fields of the voxel as indexes into the
opacity transfer function tables, retrieve one value from each table, and
multiply the values together to compute the opacity for the voxel. The opacity
should be a number in the range 0.01.0, so normally each table contains
numbers that are also in this range.
The parameters of the opacity transfer function are numbered from zero, and the
total number of parameters must be specified using
vpSetVoxelSize
before calling
vpSetClassifierTable. The number of parameters is
independent of the number of voxel fields: some fields may not be used as
parameters, and one field could be used multiple times (although this is
inefficient since all of the tables indexed by the same parameter could be
merged into one table).
Each table should be a 1D array of numbers with one entry for each possible
value of the corresponding voxel field. The voxel field must contain an
unsigned integer quantity. Normally each table would be declared:

 float table[MAX_FIELD_VALUE + 1];
where MAX_FIELD_VALUE is the maximum possible value in the voxel field as
specified with
vpSetVoxelField. The table may also be dynamically
allocated. Note that VolPack will never free the table, even if the VolPack
context is destroyed with
vpDestroyContext, so the application is
responsible for freeing the memory when appropriate. The values in the table
may be initialized or modified at any time before or after calling
vpSetClassifierTable, but before calling one of the classification
routines. See the function
vpRamp for a convenient way to initialize
tables with piecewise linear ramps.
Classification functions that are not expressible in the form above can be
handled as follows. First, write your own routine that computes the opacity
for each voxel. Next, quantize the opacities using somewhere between 8 and 16
bits per voxel, and store the quantized values in one field of each voxel.
Finally, define an opacity transfer function table that maps the quantized
opacities into a floating point number in the range 0.01.0 and use the table
to define a singleparameter opacity transfer function by calling
vpSetClassifierTable.
The minimum opacity threshold should be set at the same time as the opacity
transfer function. See the VP_MIN_VOXEL_OPACITY argument to
vpSetd.
The current classification function parameters can be retrieved with the
following state variable codes (see vpGeti(3)): VP_CLASSIFY_FIELD_COUNT,
VP_CLASSIFY_FIELDS, VP_CLASSIFY_TABLES, VP_CLASSIFY_TABLE_SIZES,
VP_MIN_VOXEL_OPACITY
The normal return value is VP_OK. The following error return value is possible:
 VPERROR_BAD_VALUE
 Either param_num or field_num is out of range.
VolPack(3), vpCreateContext(3), vpClassifyScanline(3), vpClassifyScalars(3),
vpClassifyVolume(3), vpRamp(3)