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.\" RCSid $Id: bsdf2ttree.1,v 1.1 2013/04/25 00:43:20 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 "\-g Nlog2" |
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][ |
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.B "\-t pctcull |
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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 bsdf2ttree |
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.B "\-t{3|4}" |
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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 "{+|-}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 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 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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.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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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: |
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.IP "" .2i |
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bsdf2ttree -g 7 -t 85 transmitted.sir 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 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) |
