vpsetclassifiertable(3)
NAME
vpSetClassifierTable - specify an opacity transfer function
SYNOPSIS
#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;
ARGUMENTS
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.
DESCRIPTION
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
0.0-1.0, 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
piece-wise 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 0.0-1.0 and use the table to define a single-parameter 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.
STATE VARIABLES
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
ERRORS
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.