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Revision 1.1 by greg, Thu Feb 25 04:48:19 2021 UTC vs.
Revision 1.6 by greg, Mon Apr 5 20:08:41 2021 UTC

# Line 1 | Line 1
1   .\" RCSid "$Id$"
2   .TH PABOPTO2BSDF 1 2/24/2021 RADIANCE
3   .SH NAME
4 < pabopto2bsdf - convert pab-opto BSDF measurements to scattering interpolant representation
4 > pabopto2bsdf - convert BSDF measurements to a scattering interpolant representation
5   .SH SYNOPSIS
6   .B pabopto2bsdf
7   [
# Line 10 | Line 10 | pabopto2bsdf - convert pab-opto BSDF measurements to s
10   .B "\-n nproc"
11   ][
12   .B "\-s symmetry"
13 + ][
14 + .B "\-g angle | 'A'"
15   ]
16   .B "meas1 meas2 .."
17   .SH DESCRIPTION
# Line 36 | Line 38 | 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 both directions
42 < will be used in a simulation.
43 < .PP
42 < See
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)
# Line 48 | Line 49 | The
49   .I bsdf2rad(1)
50   and
51   .I bsdfview(1)
52 < tools are also useful for visualizaing SIR and XML files.
52 > tools are also useful for visualizaing SIR and XML files.)
53   .PP
54   The
55   .I pabopto2bsdf
# Line 69 | Line 70 | letter, and case is ignored.
70   .PP
71   Normally,
72   .I pabopto2bsdf
73 < attempts to deduce BSDF symmetry from the incident phi angles
73 < provided.
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", meaning rotationally symmetric.
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,
# Line 83 | Line 83 | Similar to quadrilateral symmetry, bilateral symmetry
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 – Although bilateral symmetry is a superset of "up" symmetry,
87 – we assume the former when provided only half of the input hemisphere.
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 BSDF is also anisotropic.
89 > Finally, if the incident hemisphere is fully covered, the final BSDF
90 > is anisotropic.
91   .PP
92   If a
93   .I \-s
# Line 97 | Line 96 | data provided, an error message is issued and no outpu
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 inappropriate.
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 files for each incident
115 < angle in a text file beginning with a header
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
# Line 129 | Line 138 | Theta values should be between 0 and 180, where values
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 sample "front" and "back" may be reversed using the
141 > Notions of "front" and "back" may be reversed using the
142   .I -t
143   option if desired.
144   .TP
# Line 146 | Line 155 | The interpretation is the same as above.
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 degrees, meaning that "up"
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.
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 colorimetry of "CIE-Y", which may be left unspecified,
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.
# Line 208 | Line 217 | To combine this with front reflection measurements int
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), pabopto2xyz(1)
274 > bsdf2klems(1), bsdf2rad(1), bsdf2ttree(1), bsdfview(1), genBSDF(1),
275 > pabopto2xyz(1)

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