man/man1/pabopto2bsdf.1

.\" RCSid "$Id: pabopto2bsdf.1,v 1.6 2021/04/05 20:08:41 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 "\-g angle | 'A'"
]
.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 are to
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
If a
.I \-g
option is present, it will cull scattered measurements that are nearer
to grazing than the given angle in degrees.
If the word "auto" (which can be abbreviated as 'a' or 'A') is given
instead of an angle, then the near-grazing angle will be determined
by the lowest incident angle measurement present in the input data.
This is sometimes necessary to eliminate noise and edge effects that
some measurements exhibit near grazing.
.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 final sample name read
will be the one 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
corresponds to (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:	1.7
Committed:	Wed Apr 14 15:23:12 2021 UTC (4 years, 1 month ago) by greg
Branch:	MAIN
Changes since 1.6:	+3 -3 lines
Log Message:	docs(pabopto2bsdf): missing word
#	User	Rev	Content
1	greg	1.7	.\" RCSid "$Id: pabopto2bsdf.1,v 1.6 2021/04/05 20:08:41 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	greg	1.5	][
14	greg	1.6	.B "\-g angle \| 'A'"
15	greg	1.1	]
16			.B "meas1 meas2 .."
17			.SH DESCRIPTION
18			.I Pabopto2bsdf
19			takes two or more pab-opto
20			.I Mountain
21			files, each corresponding
22			to a different incident beam angle, and produces a
23			Scattering Interpolant Representation (SIR)
24			on the standard output for further processing.
25			The binary SIR contains a Radial Basis Function fitting
26			each incident BSDF data file
27			and a "transport plan" between neighboring RBF
28			directions in a spherical Delaunay mesh.
29			.PP
30			The SIR provides a complete 4-dimensional
31			BSDF description that may be resampled for other
32			formats such as Klems and tensor tree.
33			However, a separate run of
34			.I pabopto2bsdf
35			is needed to produce an SIR for each
36			incident and scattered hemisphere pair.
37			At most, there will be 4 such hemisphere pairs for
38			front reflection, back reflection, front transmission,
39			and back transmission.
40			Theoretically, only one transmission direction is required,
41	greg	1.3	but it is often safest to measure both if they are to
42	greg	1.2	be used in a simulation.
43			(See
44	greg	1.1	.I bsdf2klems(1)
45			and
46			.I bsdf2ttree(1)
47			for details.
48			The
49			.I bsdf2rad(1)
50			and
51			.I bsdfview(1)
52	greg	1.2	tools are also useful for visualizaing SIR and XML files.)
53	greg	1.1	.PP
54			The
55			.I pabopto2bsdf
56			.I \-t
57			option reverses the assumed sample orientation front-to-back,
58			and is discussed below under the "#intheta" header entry.
59			Multi-processing may be used to accelerate the program
60			on systems that support it via the
61			.I \-n
62			option.
63			.PP
64			BSDF symmetry may be specified with the
65			.I \-s
66			option, which is one of "isotropic", "quadrilateral",
67			"bilateral", "up", or "anisotropic".
68			Any of these may be abbreviated with as little as a single
69			letter, and case is ignored.
70			.PP
71			Normally,
72			.I pabopto2bsdf
73	greg	1.2	will assume a BSDF symmetry from the incident phi angles provided.
74	greg	1.1	If every input data file uses the same incident phi angle, the
75	greg	1.2	BSDF is assumed to be "isotropic", or rotationally symmetric.
76	greg	1.1	If input phi angles only cover one quarter of the incident hemisphere,
77			then the sample is assumed to have quadrilateral symmetry.
78			Similarly, half-hemisphere coverage implies "bilateral" symmetry,
79			although it is also compatible with "up" symmetry, which must be specified
80			on the command line.
81			The difference is crucial.
82			Similar to quadrilateral symmetry, bilateral symmetry is "mirrored,"
83			meaning that the sample material looks identical when viewed in a mirror.
84			However, "up" symmetry means that the sample looks the same when
85			rotated by 180-degree (upside-down), but does not look the same in a mirror.
86			The "up" symmetry was a late addition, and involves rotating and copying the
87			input data, treating the result as anisotropic.
88			It is therefore less efficient, and should only be used when necessary.
89	greg	1.2	Finally, if the incident hemisphere is fully covered, the final BSDF
90			is anisotropic.
91	greg	1.1	.PP
92			If a
93			.I \-s
94			symmetry option is specified and it does not agree with the input
95			data provided, an error message is issued and no output is produced.
96			Note that only the "up" and "bilateral" symmetry options have
97			identical input coverage, so this is the only time the
98			.I \-s
99	greg	1.2	option must be specified if the default mirroring is not appropriate.
100	greg	1.1	.PP
101	greg	1.5	If a
102			.I \-g
103			option is present, it will cull scattered measurements that are nearer
104			to grazing than the given angle in degrees.
105			If the word "auto" (which can be abbreviated as 'a' or 'A') is given
106			instead of an angle, then the near-grazing angle will be determined
107			by the lowest incident angle measurement present in the input data.
108			This is sometimes necessary to eliminate noise and edge effects that
109			some measurements exhibit near grazing.
110			.PP
111	greg	1.1	The
112			.I Mountain
113			software operates the pg2 goniophotometer to
114	greg	1.2	capture BSDF scattering data in separate text files for each incident
115			angle, beginning with a header
116	greg	1.1	whose lines each start with a pound sign ('#').
117			Some header settings require colons and others do not, as indicated below.
118			The
119			.i pabopto2bsdf
120			program understands the following lines from each header and ignores
121			the rest:
122			.TP
123			.BR #sample_name
124			A double-quoted string containing the name associated with this sample.
125	greg	1.7	If input files contain different sample names, the final sample name read
126			will be the one passed to the SIR output.
127	greg	1.1	.TP
128			.BR #format:
129			The data format, typically one of "theta phi DSF" or "theta phi BSDF".
130			These differ only in their inclusion of a cosine factor.
131			The word "BRDF" or "BTDF" is accepted in place of "BSDF".
132			Any other specification or a format missing generates an error.
133			.TP
134			.BR #intheta
135			The incident theta (polar) angle in degrees, measured from the sample's
136			surface normal.
137			Theta values should be between 0 and 180, where values less than 90
138			are considered incident to the "front" side of the sample, and
139			theta values greater than 90 are incident to the "back" side in
140			the standard coordinate system.
141	greg	1.2	Notions of "front" and "back" may be reversed using the
142	greg	1.1	.I -t
143			option if desired.
144			.TP
145			.BR #inphi
146			The incident phi (azimuthal) angle in degrees counter-clockwise as
147			seen from the "front" side of the sample.
148			.TP
149			.BR #incident_angle
150			The incident theta and phi angles are each given in this header
151			line, offered as an alternative to separate "#intheta" and "#inphi"
152			angles.
153			The interpretation is the same as above.
154			.TP
155			.BR #upphi
156			If present, this phi angle that corresponds to
157			the sample "up" orientation.
158	greg	1.2	By default, it is assumed to be 0, meaning that "up"
159	greg	1.1	is phi=0.
160			To get the standard RADIANCE coordinates for BSDFs, "#upphi" should
161	greg	1.2	be set to 90 (degrees).
162	greg	1.1	.TP
163			.BR #colorimetry:
164	greg	1.2	Two colorimetry values are currently understood: "CIE-Y" and "CIE-XYZ".
165			The default "CIE-Y" colorimetry
166	greg	1.1	takes each DSF or BSDF value as photometric.
167			If "CIE-XYZ" is specified, then the DSF or BSDF values must be triplets
168			corresponding to CIE XYZ values.
169			Such files are typically produced by the
170			.I pabopto2xyz(1)
171			tool rather than
172			.I Mountain,
173			directly.
174			.PP
175			The BSDF scattering data follows the header in unspecified order,
176			where each line in the file
177			contains the scattered theta and phi angles measured in the same
178			coordinate system as incident theta and phi, followed by the DSF
179			or BSDF value, which may either be a single photometric quantity
180			for "CIE-Y" colorimetry or a triplet if the colorimetry is "CIE-XYZ".
181			A minimal incident BSDF data file might contain:
182			.sp
183			.nf
184			#incident_angle 82.5 180
185			#format: theta phi DSF
186			84.968 125.790 0.009744
187			84.889 125.610 0.007737
188			84.805 125.427 0.008569
189			...
190			.fi
191			.sp
192			The above header is equivalent to the more complete version below:
193			.sp
194			.nf
195			#format: theta phi DSF
196			#incident_angle 82.5 180
197			#intheta 82.5
198			#inphi 180
199			#upphi 0
200			#colorimetry: CIE-Y
201			84.968 125.790 0.009744
202			84.889 125.610 0.007737
203			84.805 125.427 0.008569
204			...
205			.fi
206			.sp
207			The ordering of the header and data lines is unimportant,
208			but all header lines must precede all data lines in each input file.
209			.SH EXAMPLE
210			To generate an SIR file from a collection of transmission measurements
211			of a material with 180-degree symmetry using 4 processes:
212			.IP "" .2i
213			pabopto2bsdf -n 4 -s up f*_Tvis.txt > front_trans.sir
214			.PP
215			To combine this with front reflection measurements into a Klems BSDF file:
216			.IP "" .2i
217			pabopto2bsdf -n 4 -s up f*_Rvis.txt > front_refl.sir
218			.br
219			bsdf2klems front_trans.sir front_refl.sir > Klems_bsdf.xml
220	greg	1.2	.SH NOTES
221			If the BSDF is being mirrored and there is no measured theta=0 incident
222			angle data file, this part of the distribution is filled in
223			by a special procedure.
224			This is important because there is no way to extrapolate missing
225			data at normal incidence.
226			.PP
227			The BSDF is extrapolated past the last measured theta angles towards
228			grazing using a constant value plus a single Gaussian lobe if one can
229			be reasonably fit to the near-grazing data.
230			This lobe will always be in the mirror direction in the case of
231			reflection, or the "through" direction in the case
232			of transmission.
233			The magnitude and width of this lobe is stored in the output header,
234			along with the constant value.
235			Both the lobe and the constant are neutral values, even with CIE-XYZ
236			colorimetry.
237			.PP
238			While there is no explicit handling of infrared or solar radiometry,
239			any single-channel BSDF will be created the same, and the final XML
240			file generated by
241			.I bsdf2klems
242			or
243			.I bsdf2ttree
244			can be edited to specify a different radiometry.
245			The interpolation process in
246			.I pabopto2bsdf
247			is not affected by this.
248			.PP
249			The standard BSDF coordinates in RADIANCE have the theta=0 direction
250			corresponding to the front-side surface normal.
251			The phi=0 direction points to the right as seen from the front, and
252			phi=90 degrees corresponds to the "up" orientation for the sample.
253			The same theta and phi are used for incoming and scattered angles,
254			so theta=180 is the opposite side surface normal.
255			This differs from the WINDOW, which use separate
256			coordinate systems for the front and the back.
257			To confusing things further, notions of "front" and "back" are
258			opposite in WINDOW and RADIANCE.
259			In RADIANCE, the normal of a window surface usually faces the
260			interior of a space.
261			.PP
262			In the
263			.I genBSDF(1)
264			utility, the world coordinate system follows trigonometric
265			conventions with theta=0 aligning to the Z-axis,
266			the X-axis matches (theta,phi)=(90,0), and the Y-axis
267	greg	1.4	corresponds to (theta,phi)=(90,90).
268	greg	1.2	The latter is thought of as the "up" direction for the sample.
269			This usually needs to be rotated into position, since most
270			RADIANCE models use the Z-axis as the world "up" direction.
271	greg	1.1	.SH AUTHOR
272			Greg Ward
273			.SH "SEE ALSO"
274	greg	1.2	bsdf2klems(1), bsdf2rad(1), bsdf2ttree(1), bsdfview(1), genBSDF(1),
275	greg	1.3	pabopto2xyz(1)