Provided by: libvolpack1-dev_1.0b3-10_amd64 
NAME
vpRenderRawVolume, vpRenderClassifiedVolume - render a volume
SYNOPSIS
#include <volpack.h>
vpResult
vpRenderRawVolume(vpc)
vpContext *vpc;
vpResult
vpRenderClassifiedVolume(vpc)
vpContext *vpc;
ARGUMENTS
vpc VolPack context from vpCreateContext.
DESCRIPTION
These routines are used to render a volume using the parameters in a rendering context.
vpRenderRawVolume renders the data in the current voxel array and stores the result in the current image
array. If an octree is present in the context (see vpCreateMinMaxOctree(3)) then it is used to
accelerate rendering. vpRenderClassifiedVolume is identical in function except that it renders the
current preclassified volume (see vpClassifyVolume(3)) instead of the voxel array, and the octree is not
used.
vpRenderClassifiedVolume should be used when the same volume is rendered multiple times with the same
opacity transfer function. This rendering mode is the fastest. The opacity transfer function must not
change because the voxel opacities must be fixed to compute a preclassified volume.
vpRenderRawVolume should be used when the volume data or the opacity transfer function changes in between
renderings. If the volume data remains fixed then a min-max octree should be computed to accelerate
rendering. Rendering is significantly faster with an octree than without, although it is still slower
than rendering with vpRenderClassifiedVolume.
Before the rendering routines are called the rendering context must be initialized to set the volume
parameters, classification parameters (vpRenderRawVolume only), viewing parameters, shading and lighting
parameters, and image array. See VolPack(3) for a list of relevant functions.
One important state variable should be adjusted before calling the rendering routines: the maximum
opacity threshold. This number specifies a threshold value for the opacity of an image pixel. If the
opacity of the pixel reaches the threshold then no more voxels are composited into the pixel. The
threshold should be a number slightly less than one (0.95 is a good value); numbers closer to 1.0 result
in longer rendering times, but less error. The value of the threshold is changed by calling vpSetd with
the VP_MAX_RAY_OPACITY option. The default value is 1.0.
There are two additional state variables that can be adjusted to set the size of an internal data
structure. During rendering, a work buffer called the intermediate image is used to hold temporary
results. The size of this data structure depends on the size of the volume and the current view
transformation matrix. Over the course of an animation sequence the intermediate image may have to be
enlarged multiple times, resulting in many calls to the memory allocator. Normally the overhead of
reallocating the data structure is negligible and can be ignored. However, this overhead can be
eliminated by specifying hints for the maximum size of the intermediate image. The data structure will
then be allocated once, and will never be reallocated unless an even larger intermediate image is
required. To set the size hints, call vpSeti with the VP_INT_WIDTH_HINT and VP_INT_HEIGHT_HINT state
variable codes. If the viewing transformation does not include any scaling, then the maximum possible
size of each dimension of the intermediate image is twice the size of the largest dimension of the
volume. To get a more precise bound, the intermediate image size required for rendering a volume with
the current viewing parameters can be found by calling vpGeti with the VP_INTERMEDIATE_WIDTH and
VP_INTERMEDIATE_HEIGHT state variable codes.
ERRORS
The normal return value is VP_OK. The following error return values are possible:
VPERROR_BAD_SHADER
The shading lookup tables are sized incorrectly, or one of the voxel fields used to index the
tables is invalid, or the number of material types is invalid.
VPERROR_SINGULAR
One of the current view transformation matrices is singular.
VPERROR_BAD_VOLUME
The volume size has not been set or is invalid, or there is no volume data.
VPERROR_BAD_VOXEL
The voxel size or the voxel fields have not been specified or have been incorrectly specified.
VPERROR_BAD_CLASSIFIER
The opacity transfer function tables have invalid sizes or are associated with invalid voxel
fields or have been incorrectly specified. (vpRenderRawVolume only)
VPERROR_BAD_SHADOW
The angle between the shadow light vector and the viewing direction is too large (must be less
than 45 degrees). Either change the light direction or disable shadows (see
vpSetShadowLookupShader).
SEE ALSO
VolPack(3), vpCreateContext(3)