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 ago) by greg
Branch:	MAIN
Changes since 1.6:	+3 -3 lines
Log Message:	docs(pabopto2bsdf): missing word
#	Content
1	.\" RCSid "$Id: pabopto2bsdf.1,v 1.6 2021/04/05 20:08:41 greg Exp $"
2	.TH PABOPTO2BSDF 1 2/24/2021 RADIANCE
3	.SH NAME
4	pabopto2bsdf - convert BSDF measurements to a scattering interpolant representation
5	.SH SYNOPSIS
6	.B pabopto2bsdf
7	[
8	.B \-t
9	][
10	.B "\-n nproc"
11	][
12	.B "\-s symmetry"
13	][
14	.B "\-g angle \| 'A'"
15	]
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	but it is often safest to measure both if they are to
42	be used in a simulation.
43	(See
44	.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	tools are also useful for visualizaing SIR and XML files.)
53	.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	will assume a BSDF symmetry from the incident phi angles provided.
74	If every input data file uses the same incident phi angle, the
75	BSDF is assumed to be "isotropic", or rotationally symmetric.
76	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	Finally, if the incident hemisphere is fully covered, the final BSDF
90	is anisotropic.
91	.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	option must be specified if the default mirroring is not appropriate.
100	.PP
101	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	The
112	.I Mountain
113	software operates the pg2 goniophotometer to
114	capture BSDF scattering data in separate text files for each incident
115	angle, beginning with a header
116	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	If input files contain different sample names, the final sample name read
126	will be the one passed to the SIR output.
127	.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	Notions of "front" and "back" may be reversed using the
142	.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	By default, it is assumed to be 0, meaning that "up"
159	is phi=0.
160	To get the standard RADIANCE coordinates for BSDFs, "#upphi" should
161	be set to 90 (degrees).
162	.TP
163	.BR #colorimetry:
164	Two colorimetry values are currently understood: "CIE-Y" and "CIE-XYZ".
165	The default "CIE-Y" colorimetry
166	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	.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	corresponds to (theta,phi)=(90,90).
268	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	.SH AUTHOR
272	Greg Ward
273	.SH "SEE ALSO"
274	bsdf2klems(1), bsdf2rad(1), bsdf2ttree(1), bsdfview(1), genBSDF(1),
275	pabopto2xyz(1)