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vliso_surface
NAME
vliso_surface - Define the isosurfaces
for a specific variable in a given volume.
SYNOPSIS
#include <parvox.h>
void vliso_surface(vlvolume *vol, vlvariable
*variable,
int time_step, vlsurface *surface,
int num_surfaces)
DESCRIPTION
vliso_surface()
classifies the data volume of a specific variable
at a given time_step with the
isosurface definition defined in an array of vlsurface data structure
pointed by surface. num_surfaces
specifies the number of isosurfaces defined. vlsurface
is defined in vltypes.h as follows:
typedef struct surf
{
float threshold,
/* iso-surface threshold */
thickness; /* thickness in voxels */
vlcolor col;
/* color of this surface */
} vlsurface;
The color (col) of the surface can be specified
by the users, or determined by the threshold value and a predefined colormap
using vlpick_iso_color(). ParVox
renders multiple isosurfaces, each with a different opacity and color.
Levoy's classification algorithm ("Display of Surfaces from Volume Data",
IEEE Computer Graphics & Applications, May 1988) is used to define
the surfaces. When the voxel value is equal to the threshold
value, the opacity is set to the specified opacity (col.o); the
voxel opacities fall off as we move away from the threshold value at a
rate inversely proportional to the magnitude of the local gradient vector.
The thickness is used to define the transition region from the full
opacity to zero. When more than one isosurface is defined,
the classification has to be done all at once, in other words, all the
surfaces have to be defined and stored in surface array, then vliso_surface()
is called once to classify the volume. Prior to this function,
each isosurface may be individually enabled or disabled by vlenable_surface()
or vldisable_surface().
By default, all the surfaces are disabled. Whenever there is any
changes in the surface definition, such as add a new surface, change an
existing surface, or disable/enable an existing surface, vliso_surface()
has to be called again to reclassify the volume.
Levoy's classification algorithm requires the magnitudes
of the local gradient vectors of each voxel. vliso_surface()
assumes the magnitudes are precalculated and stored. vlgradient_shade()
calculates the gradient vectors and stores the magnitudes of the gradient
vectors in vlvariable data structure, then do the shading.
vliso_shade() combines vlgradient_shade()
and vliso_surface() in one functions. Therefore, prior to
calling vliso_surface(), either vliso_shade() or vlgradient_shade()
should be called first.
SEE ALSO
vlgradient_shade,
vliso_shade , vlpick_iso_color,
vlenable_surface, vldisable_surface
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