NAME
vpSetVoxelField, vpFieldOffset - define the size and location of a
voxel field
SYNOPSIS
#include <volpack.h>
vpResult
vpSetVoxelField(vpc, field_num, field_size, field_offset, field_max)
vpContext *vpc;
int field_num;
int field_size;
int field_offset;
int field_max;
int
vpFieldOffset(voxel_ptr, field_name)
vpContext *vpc;
LITERAL field_name;
ARGUMENTS
vpc VolPack context from vpCreateContext.
field_num
Field number for the field to define (first field is 0).
field_size
Size of the field in bytes.
field_offset
Number of bytes from the beginning of the voxel to the beginning
of the field.
field_max
Maximum possible value of the field.
voxel_ptr
Pointer to a C structure defining the fields in the voxel.
field_name
Literal name of the voxel field in the C structure.
DESCRIPTION
vpSetVoxelField is used to define the size and location of each field
in a voxel. Once you have chosen the sizes and order for the voxel
fields, call vpSetVoxelSize once and then call vpSetVoxelField once for
each field. Each field has a unique field number (the field_num
argument); the first field is 0, the next is 1, and so on, up to one
less than the total number of fields defined with VpSetVoxelSize. The
field_max argument is used to determine the required size for lookup
tables indexed by the value in a particular field; the maximum field
value may be any number from 0 to the maximum unsigned integer
representable by the field size.
If you have declared the layout of a voxel using a C structure then you
can use the sizeof operator and the vpFieldOffset macro to compute the
field sizes and offsets. vpFieldOffset returns the number of bytes
from the beginning of a C structure to the beginning of a particular
field in the structure. For example, the following code shows how to
declare the size and offset for one voxel field:
struct voxel {
char field0;
char field1;
short field2;
} *dummy_voxel;
#define SIZE_0 sizeof(char)
#define OFFSET_0 vpFieldOffset(dummy_voxel, field0)
#define MAX_0 127
vpSetVoxelField(vpc, 0, SIZE_0, OFFSET_0, MAX_0);
The size of each field must be 1, 2 or 4 bytes.
You must also obey any byte-alignment restrictions required by your
hardware. On many machines, two-byte fields must begin on a two-byte
boundary and four-byte fields must begin on a four-byte boundary.
The total size of a voxel may also need to be padded so that voxels
packed one after the other still obey the alignment restrictions.
If you declare your voxel as a C structure and use the sizeof
operator and the vpFieldOffset macro then you should always get
correct results. However, if you do not consider alignment
restrictions when choosing the voxel field ordering the compiler may
have to insert padding for you, resulting in wasted memory.
Some of the VolPack routines require you to declare fields of a
particular size in order to store their results. The functions to
precompute surface normals and gradient magnitudes (see
vpVolumeNormals(3), vpScanlineNormals(3), vpClassifyVolume(3),
vpClassifyScalars(3) and vpClassifyScanline(3)) can optionally store a
one-byte scalar value, a one-byte
gradient magnitude, and a two-byte surface normal value in each voxel.
The following constants defined in volpack.h give the corresponding
maximum field values: VP_SCALAR_MAX, VP_GRAD_MAX, and VP_NORM_MAX.
If you choose not to use these functions then there is no need to
declare these fields.
It is usually not necessary to declare a field for the voxel opacity
if the opacity can be computed from the other voxel fields. See
vpSetClassifierTable(3) for further discussion.
If the call to vpSetVoxelSize succeeds, any existing precomputed
volume data structures in the context are destroyed.
STATE VARIABLES
The current voxel field parameters can be retrieved with the following
state variable codes (see vpGeti(3)): VP_FIELD_SIZES, VP_FIELD_OFFSETS,
VP_FIELD_MAXES.
ERRORS
The normal return value is VP_OK. The following error return value is
possible:
VPERROR_BAD_VALUE
The field number, size or offset is out of range.
SEE ALSO
VolPack(3), vpCreateContext(3), vpSetVoxelSize(3)