NAME¶
vpSetShadowLookupShader - specify shading lookup tables for rendering shadows
SYNOPSIS¶
#include <volpack.h>
vpResult
vpSetShadowLookupShader(vpc, color_channels,
num_materials, color_field, color_table, color_table_size,
weight_field, weight_table, weight_table_size, shadow_table,
shadow_table_size )
-
- vpContext *vpc;
-
- int color_channels, num_materials;
-
- int color_field;
-
- float *color_table;
-
- int color_table_size;
-
- int weight_field;
-
- float *weight_table;
-
- int weight_table_size;
-
- float *shadow_table;
-
- int shadow_table_size;
ARGUMENTS¶
- vpc
- VolPack context from vpCreateContext.
- color_channels
- The number of color channels per pixel (1 or 3).
- num_materials
- The number of material types.
- color_field
- Field number for voxel field containing color lookup table index.
- color_table
- Color lookup table.
- color_table_size
- Size of color lookup table in bytes.
- weight_field
- Field number for voxel field containing material weight lookup table
index.
- weight_table
- Material weight lookup table.
- weight_table_size
- Size of material weight lookup table in bytes.
- shadow_table
- Shadow color lookup table.
- shadow_table_size
- Size of shadow color lookup table in bytes.
DESCRIPTION¶
vpSetShadowLookupShader is used to specify lookup tables that define a
shading function for rendering a volume with shadows. It should be used
instead of
vpSetLookupShader when shadows are enabled.
VolPack supports a fast, one-pass shadow algorithm. The algorithm computes the
fraction of light from a given light source that reaches each voxel. The
fraction is then used to attenuate the diffuse and specular shading terms
associated with that light source, producing a dark shadow in areas that are
hidden from the light source. The implementation uses lookup tables so that
most of the shading calculation can be precomputed.
In order to compute the shadows in a single pass the algorithm places a
restriction on the direction of the light source: the light casting the
shadows must not be more than 45 degrees from the viewing direction. The
quality of the shadows may degrade if the angle approaches 45 degrees. The
current implementation allows shadows to be cast only from one light source.
Additional lights may be enabled, but they will not cast shadows.
To make a rendering with shadows, the following steps must be performed:
- Call vpSetShadowLookupShader to define the lookup tables (see
discussion below).
- Call vpSeti with the VP_SHADOW_LIGHT option to specify which light
will cast shadows. The current implementation only allows one light to be
specified.
- Call vpSeti with the VP_SHADOW_BIAS option to set the shadow bias
value (see discussion below).
- Call vpEnable with the VP_SHADOW option to enable shadows.
- Call vpShadeTable as usual to initialize the lookup tables.
- Call one of the rendering routines.
vpSetShadowLookupShader defines the lookup tables required for the
shading and shadowing algorithm. The first nine arguments are identical to the
arguments for
vpSetLookupShader (see the corresponding man page). The
remaining two arguments specify an additional color lookup table,
shadow_table, with the same dimensions as
color_table. The
contents of the table will be initialized by
vpShadeTable.
The call to
vpSeti with the VP_SHADOW_BIAS option specifies an offset to
eliminate self-shadowing. Self-shadowing is an intrinsic problem when
implementing shadow algorithms for volume rendering. Consider a single
voxelized object consisting of an opaque core surrounded by a "halo"
of lower-opacity voxels (necessary for the scene to be band-limited). As a
light ray pierces the halo its strength is attenuated. By the time the light
reaches the high-opacity region a significant fraction of the light may be
obscured, resulting in a general darkening of the image even if no shadows
should be present. The problem can be corrected by moving the shadow a small
distance along the direction of the light rays. VP_SHADOW_BIAS specifies the
distance of the bias in units of voxels. The optimal value depends on the data
set. Increase the bias until it has no more effect on overall image
brightness, but do not increase it too far or small features in the data will
no longer produce correct shadows.
vpShadeTable initializes the shading lookup tables. It operates
differently when shadows are enabled. Instead of computing one color for each
surface normal vector and storing the results in
color_table, the
routine computes two colors terms. The first term is the portion of the voxel
color due to the diffuse and specular components of the shadow light. This
value is stored in
shadow_table. The second term contains the
contribution of all other light source and the ambient light term, and is
stored in
color_table. During rendering the color of a voxel is
computed by extracting a surface normal from the voxel, using the surface
normal to index both
color_table and
shadow_table, attenuating
the value from
shadow_table by the local strength of the shadow light,
and then adding the two terms together. The local strength of the shadow light
is found by extracting a value from the shadow buffer, an internal data
structure that is updated during rendering.
The values in the shading lookup tables may be initialized before or after
calling
vpSetShadowLookupShader. Typically
vpSetShadowLookupShader is called once at the beginning of a rendering
session, and then
vpShadeTable is called whenever the user changes the
lighting and shading parameters or the viewing direction.
The shadow buffer is an internal 2D array used to maintain state during
rendering. There are several state variables that can be used to query its
current size in pixels (VP_SHADOW_WIDTH and VP_SHADOW_HEIGHT) and to suggest a
size (VP_SHADOW_WIDTH_HINT and VP_SHADOW_HEIGHT_HINT). The required size
depends on the volume size and the shadow light's direction. Normally the
buffer is automatically resized when necessary.
STATE VARIABLES¶
Information about the current shading table parameters can be retrieved with the
following state variable codes (see vpGeti(3)): VP_COLOR_CHANNELS,
VP_SHADE_COLOR_TABLE, VP_SHADE_COLOR_SIZE, VP_SHADE_WEIGHT_TABLE,
VP_SHADE_WEIGHT_SIZE, VP_SHADE_COLOR_FIELD, VP_SHADE_WEIGHT_FIELD,
VP_MATERIAL_COUNT, VP_SHADOW, VP_SHADOW_LIGHT, VP_SHADOW_WIDTH_HINT,
VP_SHADOW_HEIGHT_HINT, VP_SHADOW_WIDTH, VP_SHADOW_HEIGHT,
VP_SHADOW_COLOR_TABLE, VP_SHADOW_COLOR_SIZE, VP_SHADOW_BIAS
ERRORS¶
The normal return value is VP_OK. The following error return values are
possible:
- VPERROR_BAD_VALUE
- One or more of the arguments has an invalid value or is out of range.
- VPERROR_LIMIT_EXCEEDED
- The num_materials argument has exceeded an internal limit. Change
the value of VP_MAX_MATERIAL in volpack.h and recompile the VolPack
library.
SEE ALSO¶
VolPack(3), vpCreateContext(3), vpShadeTable(3), vpSetLookupShader(3)