SYNOPSIS

DESCRIPTION

EXAMPLE

NOTES

AUTHOR

SEE ALSO

genBSDF - generate BSDF description from Radiance or MGF input

**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 ..** ]

*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.

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 |

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.

Greg Ward

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