NAME

       genBSDF - generate BSDF description from Radiance or MGF input

SYNOPSIS

       genBSDF  [  -c  Nsamp  ][  -n  Nproc  ][  -r  'rtcontrib  opts...'   ][  -t{3|4} Nlog2 ][ {+|-}forward ][
       {+|-}backward ][ {+|-}mgf ][ {+|-}geom unit ][ -dim Xmin Xmax Ymin Ymax Zmin Zmax ] [ geom ..  ]

DESCRIPTION

       GenBSDF computes  a  bidirectional  scattering  distribution  function  from  a  Radiance  or  MGF  scene
       description  given  on  the  input.   The program assumes the input is in Radiance format unless the +mgf
       option is specified.  The output conforms to the LBNL Window 6 XML  standard  for  BSDF  data,  and  will
       include  an  MGF  representation  of  the input geometry if the +geom option is given, followed by one of
       "meter," "foot," "inch," "centimeter," or "millimeter," depending on the scene units.  The default is  to
       include  the  provided geometry, which is assumed to be in meters.  Geometry output can be supressed with
       the -geom option, which must also be followed by one of the above length units.

       Normally, genBSDF computes components needed by a backwards  ray-tracing  process,  +backward.   If  both
       forward  and  backward  (front  and back) distributions are needed, the +forward option may be given.  To
       turn off backward components, use the -backward option.  Computing both components takes about  twice  as
       long as one component.

       The  geometry  must  fit a rectangular profile, whose width is along the X-axis, height is in the Y-axis,
       and depth is in the Z-axis.  The positive Z-axis points into the room, and the input geometry should  not
       extend  into  the  room.  (I.e., it should not contain any positive Z values, since the putative emitting
       surface is assumed to lie at Z=0.)  The entire window system should be modeled, including sills and  edge
       geometry  anticipated  in  the  final  installation,  otherwise  accuracy  will  be impaired.  Similarly,
       materials in the description should be carefully measured.

       Normally, the input geometry will be positioned according to its actual bounding box,  but  this  may  be
       overridden  with  the  -dim option.  Use this in cases where the fenestration system is designed to fit a
       smaller (or larger) opening or is offset somehow.

       The variance in the results may be reduced by increasing the number of  samples  per  incident  direction
       using the -c option.  This value defaults to 2000 samples distributed over the incoming plane for each of
       the 145 Klems hemisphere directions.

       In  some  cases,  the  processing  time  may  be  reduced by the -n option, which specifies the number of
       simultaneous rtrace(1) processes to run in rtcontrib(1).  The -r option may be used to specify a  set  of
       quoted arguments to be included on the rtcontrib command line.

       The  -t4  mode  computes  a non-uniform BSDF represented as a rank 4 tensor tree, suitable for use in the
       Radiance rendering tools.  The parameter given to this option is the log to the base 2  of  the  sampling
       resolution  in  each  dimension,  and  must  be an integer.  The -c setting should be adjusted so that an
       appropriate number of samples lands in each region.  A -t4 parameter of 5 corresponds to  32x32  or  1024
       output  regions, so a -c setting of 10240 would provide 10 samples per region on average.  Increasing the
       resolution to 6 corresponds to 64x64 or 4096 regions, so the -c setting would need to be increased  by  a
       factor of 4 to provide the same accuracy in each region.

       The  -t3  mode  is  similar to -t4 but computes a rank 3 tensor tree rather than rank 4.  This provides a
       much faster computation, but only works in special circumstances.  Specifically, do NOT use  this  option
       if  the  system  is not in fact isotropic.  I.e., only use -t3 when you are certain that the system has a
       high degree of radial symmetry.  Again, the parameter to this option sets the  maximum  resolution  as  a
       power of 2 in each dimension, but in this case there is one less dimension being sampled.

EXAMPLE

       To create a BSDF description including geometry from a set of venetian blinds:

         genblinds blind_white blind1 .07 3 1.5 30 40 | xform -rz -90 -rx 90 > blind1.rad
         genBSDF -r @rtc.opt blind_white.mat glazing.rad blind1.rad > blind1.xml

       To create a non-uniform, anisotropic BSDF distribution with a maximum resolution of 128x128 from the same
       description:

         genBSDF -r @rtc.opt -t4 7 -c 160000 blind_white.mat glazing.rad blind1.rad > blind12.xml

NOTES

       The  variable  resolution  (tensor  tree)  BSDF  representation  is  not  supported  by  all software and
       applicatons, and should be used with caution.  It provides practical, high-resolution data for use in the
       Radiance rendering programs, but does not work in the matrix  formulation  of  the  daylight  coefficient
       method  for  example.   Also,  third  party  tools  generally  expect or require a fixed number of sample
       directions using the Klems directions or similar.

AUTHOR

       Greg Ward

SEE ALSO

       dctimestep(1), genklemsamp(1), genskyvec(1), mkillum(1), pkgBSDF(1), rtcontrib(1), rtrace(1)

RADIANCE                                            9/3/2010                                          GENBSDF(1)