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.\" RCSid $Id: bsdf2ttree.1,v 1.8 2020/05/18 20:08:57 greg Exp $
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.TH BSDF2TTREE 1 4/24/2013 RADIANCE
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.SH NAME
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bsdf2ttree - generate XML tensor tree description of a BSDF
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.SH SYNOPSIS
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.B bsdf2ttree
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[
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.B "\-pC"
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][
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.B "{+|-}a"
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][
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.B "\-g Nlog2"
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][
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.B "\-t pctcull"
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][
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.B "\-n nss"
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][
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.B "\-s thresh"
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][
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.B "\-l maxlobes"
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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 bsdf2ttree
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.B "\-t{3|4}"
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[
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.B "\-pC"
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][
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.B "{+|-}a"
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][
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.B "\-g Nlog2"
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][
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.B "\-t pctcull"
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][
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.B "\-n nss"
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][
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.B "\-s thresh"
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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 Bsdf2ttree
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produces a tensor tree 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) or
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a functional description (in the second 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
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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 \-a
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option turns off reciprocity averaging for isotropic scattering or anisotropic reflection.
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Normally on (+a), this ensures that the tensor BRDF obeys Helmholtz reciprocity.
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However, in certain rare cases, reciprocity averaging can cause unwanted noise in the output.
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.PP
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The maximum resolution of the tensor tree may be controlled by the
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.I \-g
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option, which defaults to a value of 6.
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This corresponds to a peak resolution of 2^6 (64) in each dimension.
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Due to memory and time constraints, it is not recommended to set
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.I \-g
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higher than 7, which corresponds to a 128x128x128x128 initial sampling,
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or 268 million values.
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.PP
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The initial sampling is pared down by the percentage specified with the
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.I \-t
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option, which defaults to 90.
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Setting this parameter to -1 turns culling off entirely, which may be
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useful for comparisons.
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.PP
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The
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.I \-n
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option controls the number of super-samples to send in patches whose
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difference to its neighbors exceeds some threshold.
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The default number of super-samples is 256.
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The difference threshold for super-sampling is controlled by the
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.I \-s
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option, and defaults to 0.35.
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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 tensor tree 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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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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Parameter options may be altered between input files, in case a different
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resolution or culling percentage is indicated for transmission versus
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reflection for example.
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.PP
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In the second invocation form,
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.I bsdf2ttree
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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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The variable and function definitions in each
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.I \-f source
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file are read and compiled from the RADIANCE library where it is found.
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If the
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.I \-t3
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option is specified, the defined function is assumed to be isotropic.
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If the
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.I \-t4
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option is given, the function is assumed to be anisotropic.
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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 high-resolution tensor tree with 85% culling on transmission and 95%
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culling on reflection:
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.IP "" .2i
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bsdf2ttree -g 7 -t 85 transmitted.sir -t 95 reflected.sir > combined.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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bsdf2ttree -g 5 -t3 -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 ENVIRONMENT
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RAYPATH the directories to check for auxiliary files.
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.SH AUTHOR
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Greg Ward
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.SH "SEE ALSO"
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bsdf2klems(1), icalc(1), genBSDF(1), pkgBSDF(1), rcontrib(1),
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rfluxmtx(1), wrapBSDF(1)
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