vprenderrawvolume(3)
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).