Ubuntu Manpage: vpSetLookupShader - specify shading lookup tables

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synopsis
arguments
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state variables
errors
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Provided by: libvolpack1-dev_1.0b3-10_amd64

NAME

       vpSetLookupShader - specify shading lookup tables

SYNOPSIS

       #include <volpack.h>

       vpResult
       vpSetLookupShader(vpc,   color_channels,   num_materials,   color_field,  color_table,  color_table_size,
               weight_field, weight_table, weight_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;

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.

DESCRIPTION

vpSetLookupShader is used to specify lookup tables that define a shading function. The lookup-table
shading algorithm provided by VolPack is designed to support the following shading model. Each voxel is
assumed to contain a mixture of some set of basic material types. Each material type has its own shading
parameters for the Phong shading equation (for example, the diffuse color components and the shinyness).
The color of a voxel is found by computing a color for each material type and then combining the colors
in proportion to the fraction of each material in the voxel. The shading model also supports directional
(infinite-distance) light sources.

This shading model is implemented using lookup tables. The tables can be precomputed so that the shading
equation does not have to be evaluated for every voxel. VolPack provides routines to compute the lookup
tables (see vpShadeTable(3)). The lookup tables can also be used to implement other shading models by
using a user-supplied routine to compute the contents of the tables. See vpSetDepthCueing(3) for
information about depth cueing.

It is also possible to define a callback function that will be called to shade each voxel during
rendering, instead of using a lookup table. This method allows more general shading models but slows
down rendering. To define a callback shader, call vpSetCallback with the VP_GRAY_SHADE_FUNC or
VP_RGB_SHADE_FUNC option code (see vpSetCallback(3)). The lookup tables are used if the last successful
call to vpSetLookupShader occurred after the last successful call to vpSetCallback with one of the
VP_*_SHADE_FUNC options.

The color_channels argument determines whether the shading table will contain grayscale (1 channel) or
RGB (3 channel) pixels. The num_materials argument specifies the number of basic material types out of
which each voxel can be composed (at least 1). The color_table and weight_table arguments specify two
lookup tables, and the color_field and weight_field arguments specify two voxel fields that are used to
index the tables. The size of the color_table array must be: float
color_table[n][num_materials][color_channels]; where n is the number of possible values in the
color_field field of the voxel (the maximum value plus one). The size of the weight_table argument must
be: float weight_table[m][num_materials]; where m is the number of possible values in the weight_field
field of the voxel (the maximum value plus one). If there is only one material then weight_table is not
used and may be a NULL pointer.

During rendering the shading lookup tables are accessed as follows. Suppose a particular voxel to be
shaded has the value color_index in the field specified by color_field and the value weight_index in the
field specified by weight_field. For each material number material_index, a weighted color is computed
from the formula: color = color_table[color_index][material_index] *
168u' weight_table[weight_index][material_index] Then the weighted colors for each material type are
added up to give the voxel color. If color_table contains RGB values then each component is treated
separately using the above formula. If there is only one material type then the constant 1.0 is
substituted for the weight_table value.

The suggested way to use the tables is as follows. The voxel field specified by color_field should
contain an encoded surface normal vector; this should be the same field as the norm_field argument to
vpVolumeNormals or vpClassifyScalars. In the color_table array, for each possible encoded surface normal
(the color index) and material type (the material index) the array should contain the color of the
material given the current viewing direction and lighting arrangement. The color is independent of the
voxel location because the light sources are assumed to be infinitely distant. The function vpShadeTable
initializes the contents of color_table in this way.

The voxel field specified by weight_field should contain some quantity that determines the material
composition of the voxel. For example, a field containing the scalar value could be used. In the
weight_table array, for each possible value of weight_field there should be one row of material occupancy
fractions. Each entry should be a number in the range 0.0-1.0 that specifies the fraction of the voxel
occupied by the corresponding material. For example, if there are two material types and a voxel's
weight_field contains the value 62, then weight_table[62][0] gives the fraction of the voxel occupied by
material 0, and weight_table[62][1] gives the fraction of the voxel occupied by material 1. Normally,
the numbers in a given row of weight_table should add up to 1.0 (although special effects can be achieved
by bending the rules). The function vpRamp is useful for initializing the weight table with piece-wise
linear ramps.

The values in the shading lookup tables may be initialized before or after calling vpSetLookupShader.
Typically vpSetLookupShader is called once at the beginning of a rendering session, and then the values
in the shading tables are modified as the user changes the lighting and shading parameters or the viewing
direction.

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

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.