vpSetShadowLookupShader  -  specify shading lookup tables for rendering


   #include <volpack.h>

   vpSetShadowLookupShader(vpc,       color_channels,       num_materials,
           color_field,   color_table,   color_table_size,   weight_field,
           weight_table,         weight_table_size,          shadow_table,
       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;


   vpc    VolPack context from vpCreateContext.

          The number of color channels per pixel (1 or 3).

          The number of material types.

          Field  number  for  voxel  field  containing  color lookup table

          Color lookup table.

          Size of color lookup table in bytes.

          Field number for voxel field containing material  weight  lookup
          table index.

          Material weight lookup table.

          Size of material weight lookup table in bytes.

          Shadow color lookup table.

          Size of shadow color lookup table in bytes.


   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

          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

   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


   Information about the current shading table parameters can be retrieved
   with   the   following   state   variable   codes   (see    vpGeti(3)):


   The normal return value is VP_OK.  The following  error  return  values
   are possible:

          One  or  more of the arguments has an invalid value or is out of

          The num_materials  argument  has  exceeded  an  internal  limit.
          Change  the  value of VP_MAX_MATERIAL in volpack.h and recompile
          the VolPack library.


   VolPack(3), vpCreateContext(3), vpShadeTable(3), vpSetLookupShader(3)


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