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.\" RCSid $Id: bsdf2klems.1,v 1.3 2013/09/26 17:05:00 greg Exp $
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.TH BSDF2KLEMS 1 4/24/2013 RADIANCE
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.SH NAME
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bsdf2klems - generate XML Klems matrix description of a BSDF
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.SH SYNOPSIS
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.B bsdf2klems
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[
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.B "\-n spp"
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][
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.B "\-h|\-q"
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][
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.B "\-pC"
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][
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.B "\-l maxlobes"
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]
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[
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.B "bsdf.sir .."
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]
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.br
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or
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.br
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.B bsdf2klems
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[
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.B "\-n spp"
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][
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.B "\-h|\-q"
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][
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.B "\-pC"
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]
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.B bsdf_in.xml
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.br
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or
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.br
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.B bsdf2klems
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[
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.B "\-n spp"
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][
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.B "\-h|\-q"
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][
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.B "\-pC"
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][
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.B "{+|-}forward"
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][
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.B "{+|-}backward"
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][
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.B "\-e expr"
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][
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.B "\-f file"
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]
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.B bsdf_func
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.SH DESCRIPTION
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.I Bsdf2klems
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produces a Klems matrix representation of a
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bidirectional scattering distribution function (BSDF)
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based on an intermediate representation (in the first form),
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an input XML representation (in the second form),
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or a functional description (in the third form).
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A complete XML description is written to the standard output,
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which is normally redirected to a file.
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.PP
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The Klems matrix representation divides the input and output
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hemisphere into a default 145 patches.
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The
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.I \-h
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option may be used to reduce this number to 73 patches per hemisphere.
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The
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.I \-q
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option may be used to reduce this number to 41 patches.
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Neither option is recommended unless the distribution is known to
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be approximately diffuse.
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.PP
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The
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.I \-p
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option by itself turns off the progress bar, whose length may be set
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by an immediately following integer argument.
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(The default progress bar length is 79 characters.)\0
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.PP
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The
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.I \-l
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option may be used to specify the maximum number of lobes in any
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interpolated radial basis function.
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The default value is 15000, which generally keeps the interpolation tractable.
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Setting the value to 0 turns off this limit.
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.PP
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Normally, multiple samples are taken from random points on each input
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and output patch to improve accuracy.
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The number of samples to take for each input-output patch pair may
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be controlled using the
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.I \-n
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option, which defaults to 256.
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.PP
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The first invocation form takes a intermediate scattering representation
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as produced by
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.I pabopto2bsdf(1)
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or similar, and produces a Klems representation with as many
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components as there are independent input distributions.
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Each intermediate scattering file contains one of
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the four components, and if the first component
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is isotropic, all components must be isotropic.
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A similar rule holds for anisotropic inputs.
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Only the center of each incident patches is sampled, due to
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the time required to interpolate incident positions.
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.PP
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In the second invocation form, an input XML representation
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is resampled to produce the desired Klems matrix representation.
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This is primarily used to convert a tensor tree representation
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into a matrix for annual daylighting simulations.
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Any components in the input are reproduced on output, and inline
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geometric descriptions are passed unchanged.
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.PP
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In the third invocation form,
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.I bsdf2klems
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takes a functional specification of a BSDF.
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The named function should accept 6 parameters corresponding to the
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normalized incident and exiting vectors, respectively.
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By convention, these vectors point away from the surface, and a positive
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Z-component corresponds to the front side.
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The Y-component corresponds to the "up" orientation of the surface,
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as specified in the eventual scene description that references the XML
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output.
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If the function only takes 3 parameters, then the variables "Dx", "Dy",
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and "Dz" will be assigned to the reverse of the outgoing direction at
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each evaluation.
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(I.e., the vector will point into the surface and
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Dz will be negative on the front side.)\0
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This simplifies conversion of functional BSDF specifications using the
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legacy material primitives "plasfunc", "metfunc", and "transfunc".
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.PP
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The function is defined by one or more
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.I \-e
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and
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.I \-f
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options, and should obey both Helmholtz reciprocity and
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integrate to less than 1 over each projected incident hemisphere
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for energy conservation.
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.PP
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Similar to the
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.I genBSDF(1)
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command,
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the
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.I \+backward
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option (default) specifies that rays arriving from the front side of
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the surface will be tested for reflection and transmission.
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If both forward and backward (front and back) distributions are needed, the
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.I \+forward
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option may be given.
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To turn off the backward components, use the
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.I \-backward
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option.
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Computing both incident hemispheres takes about twice as long as one, but
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is recommended when rays will be impinging from either side.
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.SH EXAMPLE
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To take two components of an intermediate BSDF representation and create
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a full Klems matrix representation:
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.IP "" .2i
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bsdf2klems transmitted.sir reflected.sir > combined.xml
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.PP
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To reduce a tensor tree representation into a half-Klems matrix representation:
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.IP "" .2i
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bsdf2klems -h ttree.xml > klems_half.xml
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.PP
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To create a low-res BSDF corresponding to a one-sided,
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isotropic Phong distribution:
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.IP "" .2i
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bsdf2klems -e 'phong(ix,iy,iz,ox,oy,oz) = if(iz, .1+((iz+oz)/sqrt((ix+ox)^2+(iy+oy)^2+(iz+oz)^2))^50, 0)' phong > phong.xml
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.SH AUTHOR
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Greg Ward
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.SH "SEE ALSO"
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bsdf2ttree(1), dctimestep(1), icalc(1), gendaymtx(1), genklemsamp(1),
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genskyvec(1), mkillum(1), genBSDF(1), pkgBSDF(1), rcontrib(1), rtrace(1)
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