oracular (3) VolumeNormals.3.gz
NAME
vpVolumeNormals - compute surface normal vectors and gradient magnitudes for a volume
SYNOPSIS
#include <volpack.h>
vpResult
vpVolumeNormals(vpc, scalar_data, length, scalar_field, grad_field, norm_field)
vpContext *vpc;
unsigned char *scalar_data;
int length;
int scalar_field;
int grad_field;
int norm_field;
ARGUMENTS
vpc VolPack context from vpCreateContext.
scalar_data
3D array of scalar samples.
length Size of scalar_data array in bytes.
scalar_field
Voxel field number indicating the field in which to store the scalar samples.
grad_field
Voxel field number indicating the field in which to store the gradient magnitudes of the scalar
samples.
norm_field
Voxel field number indicating the field in which to store encoded surface normal vectors.
DESCRIPTION
vpVolumeNormals is used to precompute values for voxel fields used in classification and shading
algorithms. The input is a 3D array of scalar values; a separate routine is provided for processing
scanlines of voxel data (see vpScanlineNormals(3)). For each voxel, the routine computes an
approximation for the gradient of the scalar field (using a central-difference operator). From this
vector the routine can compute the magnitude of the gradient, which is useful for some classification
schemes (see vpSetClassifierTable(3)), and a surface normal vector in the direction of the gradient
vector, which is used to implement lighting models (see vpShadeTable(3)). The scalar value, gradient
magnitude and surface normal vector are then stored into the fields of the voxel array, which is separate
from the array of scalar values.
The scalar array must be an array of 8-bit values (other sizes are not currently supported). The
dimensions of the array must match the volume dimensions previously specified with vpSetVolumeSize. The
size argument is used to make sure the array has the correct size. The voxel array must also be
initialized by calling vpSetRawVoxels prior to calling vpVolumeNormals.
The three voxel field number arguments indicate which voxel fields the computed information should be
stored into, and which field the scalar samples should be copied to. The field numbers must correspond
to appropriately-sized voxel fields previously specified with vpSetVoxelField. Any of these arguments
can be the constant VP_SKIP_FIELD, in which case the corresponding quantity is not computed or copied.
The voxel fields for the scalar value and the gradient magnitude must be one-byte fields, and the field
for the surface normal vector must be a two-byte field. The constants VP_SCALAR_MAX, VP_GRAD_MAX and
VP_NORM_MAX give the maximum value that might be stored in each field, respectively. The value stored in
the surface normal vector field is actually an encoded surface normal; see vpNormalIndex(3).
ERRORS
The normal return value is VP_OK. The following error return values are possible:
VPERROR_BAD_SIZE
The size of the scalar array does not match the volume dimensions.
VPERROR_BAD_VALUE
One or more of the voxel fields specified is invalid, or the corresponding voxel field has the
wrong size.
VPERROR_BAD_VOXEL
The voxel size or the voxel fields have not been specified or have 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.
SEE ALSO
VolPack(3), vpCreateContext(3), vpNormalIndex(3), vpScanlineNormals(3), vpSetClassifierTable(3), vpSetLookupShader(3)