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)