man/man1/pabopto2bsdf.1

.\" RCSid "$Id: pabopto2bsdf.1,v 1.1 2021/02/25 04:48:19 greg Exp $"
.TH PABOPTO2BSDF 1 2/24/2021 RADIANCE
.SH NAME
pabopto2bsdf - convert BSDF measurements to a scattering interpolant representation
.SH SYNOPSIS
.B pabopto2bsdf
[
.B \-t
][
.B "\-n nproc"
][
.B "\-s symmetry"
]
.B "meas1 meas2 .."
.SH DESCRIPTION
.I Pabopto2bsdf
takes two or more pab-opto
.I Mountain
files, each corresponding
to a different incident beam angle, and produces a
Scattering Interpolant Representation (SIR)
on the standard output for further processing.
The binary SIR contains a Radial Basis Function fitting
each incident BSDF data file
and a "transport plan" between neighboring RBF
directions in a spherical Delaunay mesh.
.PP
The SIR provides a complete 4-dimensional
BSDF description that may be resampled for other
formats such as Klems and tensor tree.
However, a separate run of
.I pabopto2bsdf
is needed to produce an SIR for each
incident and scattered hemisphere pair.
At most, there will be 4 such hemisphere pairs for
front reflection, back reflection, front transmission,
and back transmission.
Theoretically, only one transmission direction is required,
but it is often safest to measure both if they
be used in a simulation.
(See
.I bsdf2klems(1)
and
.I bsdf2ttree(1)
for details.
The
.I bsdf2rad(1)
and
.I bsdfview(1)
tools are also useful for visualizaing SIR and XML files.)
.PP
The
.I pabopto2bsdf
.I \-t
option reverses the assumed sample orientation front-to-back,
and is discussed below under the "#intheta" header entry.
Multi-processing may be used to accelerate the program
on systems that support it via the
.I \-n
option.
.PP
BSDF symmetry may be specified with the
.I \-s
option, which is one of "isotropic", "quadrilateral",
"bilateral", "up", or "anisotropic".
Any of these may be abbreviated with as little as a single
letter, and case is ignored.
.PP
Normally,
.I pabopto2bsdf
will assume a BSDF symmetry from the incident phi angles provided.
If every input data file uses the same incident phi angle, the
BSDF is assumed to be "isotropic", or rotationally symmetric.
If input phi angles only cover one quarter of the incident hemisphere,
then the sample is assumed to have quadrilateral symmetry.
Similarly, half-hemisphere coverage implies "bilateral" symmetry,
although it is also compatible with "up" symmetry, which must be specified
on the command line.
The difference is crucial.
Similar to quadrilateral symmetry, bilateral symmetry is "mirrored,"
meaning that the sample material looks identical when viewed in a mirror.
However, "up" symmetry means that the sample looks the same when
rotated by 180-degree (upside-down), but does not look the same in a mirror.
The "up" symmetry was a late addition, and involves rotating and copying the
input data, treating the result as anisotropic.
It is therefore less efficient, and should only be used when necessary.
Finally, if the incident hemisphere is fully covered, the final BSDF
is anisotropic.
.PP
If a
.I \-s
symmetry option is specified and it does not agree with the input
data provided, an error message is issued and no output is produced.
Note that only the "up" and "bilateral" symmetry options have
identical input coverage, so this is the only time the
.I \-s
option must be specified if the default mirroring is not appropriate.
.PP
The
.I Mountain
software operates the pg2 goniophotometer to
capture BSDF scattering data in separate text files for each incident
angle, beginning with a header
whose lines each start with a pound sign ('#').
Some header settings require colons and others do not, as indicated below.
The
.i pabopto2bsdf
program understands the following lines from each header and ignores
the rest:
.TP
.BR #sample_name
A double-quoted string containing the name associated with this sample.
If input files contain different sample names, the one read
will be the sample name passed to the SIR output.
.TP
.BR #format:
The data format, typically one of "theta phi DSF" or "theta phi BSDF".
These differ only in their inclusion of a cosine factor.
The word "BRDF" or "BTDF" is accepted in place of "BSDF".
Any other specification or a format missing generates an error.
.TP
.BR #intheta
The incident theta (polar) angle in degrees, measured from the sample's
surface normal.
Theta values should be between 0 and 180, where values less than 90
are considered incident to the "front" side of the sample, and
theta values greater than 90 are incident to the "back" side in
the standard coordinate system.
Notions of "front" and "back" may be reversed using the
.I -t
option if desired.
.TP
.BR #inphi
The incident phi (azimuthal) angle in degrees counter-clockwise as
seen from the "front" side of the sample.
.TP
.BR #incident_angle
The incident theta and phi angles are each given in this header
line, offered as an alternative to separate "#intheta" and "#inphi"
angles.
The interpretation is the same as above.
.TP
.BR #upphi
If present, this phi angle that corresponds to
the sample "up" orientation.
By default, it is assumed to be 0, meaning that "up"
is phi=0.
To get the standard RADIANCE coordinates for BSDFs, "#upphi" should
be set to 90 (degrees).
.TP
.BR #colorimetry:
Two colorimetry values are currently understood: "CIE-Y" and "CIE-XYZ".
The default "CIE-Y" colorimetry
takes each DSF or BSDF value as photometric.
If "CIE-XYZ" is specified, then the DSF or BSDF values must be triplets
corresponding to CIE XYZ values.
Such files are typically produced by the
.I pabopto2xyz(1)
tool rather than
.I Mountain,
directly.
.PP
The BSDF scattering data follows the header in unspecified order,
where each line in the file
contains the scattered theta and phi angles measured in the same
coordinate system as incident theta and phi, followed by the DSF
or BSDF value, which may either be a single photometric quantity
for "CIE-Y" colorimetry or a triplet if the colorimetry is "CIE-XYZ".
A minimal incident BSDF data file might contain:
.sp
.nf
#incident_angle 82.5 180
#format: theta phi DSF
84.968 125.790 0.009744
84.889 125.610 0.007737
84.805 125.427 0.008569
 ...
.fi
.sp
The above header is equivalent to the more complete version below:
.sp
.nf
#format: theta phi DSF
#incident_angle 82.5 180
#intheta 82.5
#inphi 180
#upphi 0
#colorimetry: CIE-Y
84.968 125.790 0.009744
84.889 125.610 0.007737
84.805 125.427 0.008569
 ...
.fi
.sp
The ordering of the header and data lines is unimportant,
but all header lines must precede all data lines in each input file.
.SH EXAMPLE
To generate an SIR file from a collection of transmission measurements
of a material with 180-degree symmetry using 4 processes:
.IP "" .2i
pabopto2bsdf -n 4 -s up f*_Tvis.txt > front_trans.sir
.PP
To combine this with front reflection measurements into a Klems BSDF file:
.IP "" .2i
pabopto2bsdf -n 4 -s up f*_Rvis.txt > front_refl.sir
.br
bsdf2klems front_trans.sir front_refl.sir > Klems_bsdf.xml
.SH NOTES
If the BSDF is being mirrored and there is no measured theta=0 incident
angle data file, this part of the distribution is filled in
by a special procedure.
This is important because there is no way to extrapolate missing
data at normal incidence.
.PP
The BSDF is extrapolated past the last measured theta angles towards
grazing using a constant value plus a single Gaussian lobe if one can
be reasonably fit to the near-grazing data.
This lobe will always be in the mirror direction in the case of
reflection, or the "through" direction in the case
of transmission.
The magnitude and width of this lobe is stored in the output header,
along with the constant value.
Both the lobe and the constant are neutral values, even with CIE-XYZ
colorimetry.
.PP
While there is no explicit handling of infrared or solar radiometry,
any single-channel BSDF will be created the same, and the final XML
file generated by
.I bsdf2klems
or
.I bsdf2ttree
can be edited to specify a different radiometry.
The interpolation process in
.I pabopto2bsdf
is not affected by this.
.PP
The standard BSDF coordinates in RADIANCE have the theta=0 direction
corresponding to the front-side surface normal.
The phi=0 direction points to the right as seen from the front, and
phi=90 degrees corresponds to the "up" orientation for the sample.
The same theta and phi are used for incoming and scattered angles,
so theta=180 is the opposite side surface normal.
This differs from the WINDOW, which use separate
coordinate systems for the front and the back.
To confusing things further, notions of "front" and "back" are
opposite in WINDOW and RADIANCE.
In RADIANCE, the normal of a window surface usually faces the
interior of a space.
.PP
In the
.I genBSDF(1)
utility, the world coordinate system follows trigonometric
conventions with theta=0 aligning to the Z-axis,
the X-axis matches (theta,phi)=(90,0), and the Y-axis
is has (theta,phi)=(90,90).
The latter is thought of as the "up" direction for the sample.
This usually needs to be rotated into position, since most
RADIANCE models use the Z-axis as the world "up" direction.
.SH AUTHOR
Greg Ward
.SH "SEE ALSO"
bsdf2klems(1), bsdf2rad(1), bsdf2ttree(1), bsdfview(1), genBSDF(1),
pabopto2xyz(1)
Revision:	1.2
Committed:	Thu Feb 25 19:31:36 2021 UTC (4 years, 7 months ago) by greg
Branch:	MAIN
Changes since 1.1:	+71 -22 lines
Log Message:	docs: improved wording and added NOTES section
#	User	Rev	Content
1	greg	1.2	.\" RCSid "$Id: pabopto2bsdf.1,v 1.1 2021/02/25 04:48:19 greg Exp $"
2	greg	1.1	.TH PABOPTO2BSDF 1 2/24/2021 RADIANCE
3			.SH NAME
4	greg	1.2	pabopto2bsdf - convert BSDF measurements to a scattering interpolant representation
5	greg	1.1	.SH SYNOPSIS
6			.B pabopto2bsdf
7			[
8			.B \-t
9			][
10			.B "\-n nproc"
11			][
12			.B "\-s symmetry"
13			]
14			.B "meas1 meas2 .."
15			.SH DESCRIPTION
16			.I Pabopto2bsdf
17			takes two or more pab-opto
18			.I Mountain
19			files, each corresponding
20			to a different incident beam angle, and produces a
21			Scattering Interpolant Representation (SIR)
22			on the standard output for further processing.
23			The binary SIR contains a Radial Basis Function fitting
24			each incident BSDF data file
25			and a "transport plan" between neighboring RBF
26			directions in a spherical Delaunay mesh.
27			.PP
28			The SIR provides a complete 4-dimensional
29			BSDF description that may be resampled for other
30			formats such as Klems and tensor tree.
31			However, a separate run of
32			.I pabopto2bsdf
33			is needed to produce an SIR for each
34			incident and scattered hemisphere pair.
35			At most, there will be 4 such hemisphere pairs for
36			front reflection, back reflection, front transmission,
37			and back transmission.
38			Theoretically, only one transmission direction is required,
39	greg	1.2	but it is often safest to measure both if they
40			be used in a simulation.
41			(See
42	greg	1.1	.I bsdf2klems(1)
43			and
44			.I bsdf2ttree(1)
45			for details.
46			The
47			.I bsdf2rad(1)
48			and
49			.I bsdfview(1)
50	greg	1.2	tools are also useful for visualizaing SIR and XML files.)
51	greg	1.1	.PP
52			The
53			.I pabopto2bsdf
54			.I \-t
55			option reverses the assumed sample orientation front-to-back,
56			and is discussed below under the "#intheta" header entry.
57			Multi-processing may be used to accelerate the program
58			on systems that support it via the
59			.I \-n
60			option.
61			.PP
62			BSDF symmetry may be specified with the
63			.I \-s
64			option, which is one of "isotropic", "quadrilateral",
65			"bilateral", "up", or "anisotropic".
66			Any of these may be abbreviated with as little as a single
67			letter, and case is ignored.
68			.PP
69			Normally,
70			.I pabopto2bsdf
71	greg	1.2	will assume a BSDF symmetry from the incident phi angles provided.
72	greg	1.1	If every input data file uses the same incident phi angle, the
73	greg	1.2	BSDF is assumed to be "isotropic", or rotationally symmetric.
74	greg	1.1	If input phi angles only cover one quarter of the incident hemisphere,
75			then the sample is assumed to have quadrilateral symmetry.
76			Similarly, half-hemisphere coverage implies "bilateral" symmetry,
77			although it is also compatible with "up" symmetry, which must be specified
78			on the command line.
79			The difference is crucial.
80			Similar to quadrilateral symmetry, bilateral symmetry is "mirrored,"
81			meaning that the sample material looks identical when viewed in a mirror.
82			However, "up" symmetry means that the sample looks the same when
83			rotated by 180-degree (upside-down), but does not look the same in a mirror.
84			The "up" symmetry was a late addition, and involves rotating and copying the
85			input data, treating the result as anisotropic.
86			It is therefore less efficient, and should only be used when necessary.
87	greg	1.2	Finally, if the incident hemisphere is fully covered, the final BSDF
88			is anisotropic.
89	greg	1.1	.PP
90			If a
91			.I \-s
92			symmetry option is specified and it does not agree with the input
93			data provided, an error message is issued and no output is produced.
94			Note that only the "up" and "bilateral" symmetry options have
95			identical input coverage, so this is the only time the
96			.I \-s
97	greg	1.2	option must be specified if the default mirroring is not appropriate.
98	greg	1.1	.PP
99			The
100			.I Mountain
101			software operates the pg2 goniophotometer to
102	greg	1.2	capture BSDF scattering data in separate text files for each incident
103			angle, beginning with a header
104	greg	1.1	whose lines each start with a pound sign ('#').
105			Some header settings require colons and others do not, as indicated below.
106			The
107			.i pabopto2bsdf
108			program understands the following lines from each header and ignores
109			the rest:
110			.TP
111			.BR #sample_name
112			A double-quoted string containing the name associated with this sample.
113			If input files contain different sample names, the one read
114			will be the sample name passed to the SIR output.
115			.TP
116			.BR #format:
117			The data format, typically one of "theta phi DSF" or "theta phi BSDF".
118			These differ only in their inclusion of a cosine factor.
119			The word "BRDF" or "BTDF" is accepted in place of "BSDF".
120			Any other specification or a format missing generates an error.
121			.TP
122			.BR #intheta
123			The incident theta (polar) angle in degrees, measured from the sample's
124			surface normal.
125			Theta values should be between 0 and 180, where values less than 90
126			are considered incident to the "front" side of the sample, and
127			theta values greater than 90 are incident to the "back" side in
128			the standard coordinate system.
129	greg	1.2	Notions of "front" and "back" may be reversed using the
130	greg	1.1	.I -t
131			option if desired.
132			.TP
133			.BR #inphi
134			The incident phi (azimuthal) angle in degrees counter-clockwise as
135			seen from the "front" side of the sample.
136			.TP
137			.BR #incident_angle
138			The incident theta and phi angles are each given in this header
139			line, offered as an alternative to separate "#intheta" and "#inphi"
140			angles.
141			The interpretation is the same as above.
142			.TP
143			.BR #upphi
144			If present, this phi angle that corresponds to
145			the sample "up" orientation.
146	greg	1.2	By default, it is assumed to be 0, meaning that "up"
147	greg	1.1	is phi=0.
148			To get the standard RADIANCE coordinates for BSDFs, "#upphi" should
149	greg	1.2	be set to 90 (degrees).
150	greg	1.1	.TP
151			.BR #colorimetry:
152	greg	1.2	Two colorimetry values are currently understood: "CIE-Y" and "CIE-XYZ".
153			The default "CIE-Y" colorimetry
154	greg	1.1	takes each DSF or BSDF value as photometric.
155			If "CIE-XYZ" is specified, then the DSF or BSDF values must be triplets
156			corresponding to CIE XYZ values.
157			Such files are typically produced by the
158			.I pabopto2xyz(1)
159			tool rather than
160			.I Mountain,
161			directly.
162			.PP
163			The BSDF scattering data follows the header in unspecified order,
164			where each line in the file
165			contains the scattered theta and phi angles measured in the same
166			coordinate system as incident theta and phi, followed by the DSF
167			or BSDF value, which may either be a single photometric quantity
168			for "CIE-Y" colorimetry or a triplet if the colorimetry is "CIE-XYZ".
169			A minimal incident BSDF data file might contain:
170			.sp
171			.nf
172			#incident_angle 82.5 180
173			#format: theta phi DSF
174			84.968 125.790 0.009744
175			84.889 125.610 0.007737
176			84.805 125.427 0.008569
177			...
178			.fi
179			.sp
180			The above header is equivalent to the more complete version below:
181			.sp
182			.nf
183			#format: theta phi DSF
184			#incident_angle 82.5 180
185			#intheta 82.5
186			#inphi 180
187			#upphi 0
188			#colorimetry: CIE-Y
189			84.968 125.790 0.009744
190			84.889 125.610 0.007737
191			84.805 125.427 0.008569
192			...
193			.fi
194			.sp
195			The ordering of the header and data lines is unimportant,
196			but all header lines must precede all data lines in each input file.
197			.SH EXAMPLE
198			To generate an SIR file from a collection of transmission measurements
199			of a material with 180-degree symmetry using 4 processes:
200			.IP "" .2i
201			pabopto2bsdf -n 4 -s up f*_Tvis.txt > front_trans.sir
202			.PP
203			To combine this with front reflection measurements into a Klems BSDF file:
204			.IP "" .2i
205			pabopto2bsdf -n 4 -s up f*_Rvis.txt > front_refl.sir
206			.br
207			bsdf2klems front_trans.sir front_refl.sir > Klems_bsdf.xml
208	greg	1.2	.SH NOTES
209			If the BSDF is being mirrored and there is no measured theta=0 incident
210			angle data file, this part of the distribution is filled in
211			by a special procedure.
212			This is important because there is no way to extrapolate missing
213			data at normal incidence.
214			.PP
215			The BSDF is extrapolated past the last measured theta angles towards
216			grazing using a constant value plus a single Gaussian lobe if one can
217			be reasonably fit to the near-grazing data.
218			This lobe will always be in the mirror direction in the case of
219			reflection, or the "through" direction in the case
220			of transmission.
221			The magnitude and width of this lobe is stored in the output header,
222			along with the constant value.
223			Both the lobe and the constant are neutral values, even with CIE-XYZ
224			colorimetry.
225			.PP
226			While there is no explicit handling of infrared or solar radiometry,
227			any single-channel BSDF will be created the same, and the final XML
228			file generated by
229			.I bsdf2klems
230			or
231			.I bsdf2ttree
232			can be edited to specify a different radiometry.
233			The interpolation process in
234			.I pabopto2bsdf
235			is not affected by this.
236			.PP
237			The standard BSDF coordinates in RADIANCE have the theta=0 direction
238			corresponding to the front-side surface normal.
239			The phi=0 direction points to the right as seen from the front, and
240			phi=90 degrees corresponds to the "up" orientation for the sample.
241			The same theta and phi are used for incoming and scattered angles,
242			so theta=180 is the opposite side surface normal.
243			This differs from the WINDOW, which use separate
244			coordinate systems for the front and the back.
245			To confusing things further, notions of "front" and "back" are
246			opposite in WINDOW and RADIANCE.
247			In RADIANCE, the normal of a window surface usually faces the
248			interior of a space.
249			.PP
250			In the
251			.I genBSDF(1)
252			utility, the world coordinate system follows trigonometric
253			conventions with theta=0 aligning to the Z-axis,
254			the X-axis matches (theta,phi)=(90,0), and the Y-axis
255			is has (theta,phi)=(90,90).
256			The latter is thought of as the "up" direction for the sample.
257			This usually needs to be rotated into position, since most
258			RADIANCE models use the Z-axis as the world "up" direction.
259	greg	1.1	.SH AUTHOR
260			Greg Ward
261			.SH "SEE ALSO"
262	greg	1.2	bsdf2klems(1), bsdf2rad(1), bsdf2ttree(1), bsdfview(1), genBSDF(1),
263			pabopto2xyz(1)