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Revision 1.1 by greg, Thu Feb 25 04:48:19 2021 UTC vs.
Revision 1.2 by greg, Thu Feb 25 19:31:36 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 36 | Line 36 | 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 < but it is often safest to measure both if both directions
40 < will be used in a simulation.
41 < .PP
42 < See
39 > but it is often safest to measure both if they
40 > be used in a simulation.
41 > (See
42   .I bsdf2klems(1)
43   and
44   .I bsdf2ttree(1)
# Line 48 | Line 47 | The
47   .I bsdf2rad(1)
48   and
49   .I bsdfview(1)
50 < tools are also useful for visualizaing SIR and XML files.
50 > tools are also useful for visualizaing SIR and XML files.)
51   .PP
52   The
53   .I pabopto2bsdf
# Line 69 | Line 68 | letter, and case is ignored.
68   .PP
69   Normally,
70   .I pabopto2bsdf
71 < attempts to deduce BSDF symmetry from the incident phi angles
73 < provided.
71 > will assume a BSDF symmetry from the incident phi angles provided.
72   If every input data file uses the same incident phi angle, the
73 < BSDF is assumed to be "isotropic", meaning rotationally symmetric.
73 > BSDF is assumed to be "isotropic", or rotationally symmetric.
74   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,
# Line 83 | Line 81 | Similar to quadrilateral symmetry, bilateral symmetry
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.
86 – Although bilateral symmetry is a superset of "up" symmetry,
87 – we assume the former when provided only half of the input hemisphere.
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 < Finally, if the incident hemisphere is fully covered, the BSDF is also anisotropic.
87 > Finally, if the incident hemisphere is fully covered, the final BSDF
88 > is anisotropic.
89   .PP
90   If a
91   .I \-s
# Line 97 | Line 94 | data provided, an error message is issued and no outpu
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 < option must be specified if the default mirroring is inappropriate.
97 > option must be specified if the default mirroring is not appropriate.
98   .PP
99   The
100   .I Mountain
101   software operates the pg2 goniophotometer to
102 < capture BSDF scattering data in separate files for each incident
103 < angle in a text file beginning with a header
102 > capture BSDF scattering data in separate text files for each incident
103 > angle, beginning with a header
104   whose lines each start with a pound sign ('#').
105   Some header settings require colons and others do not, as indicated below.
106   The
# Line 129 | Line 126 | Theta values should be between 0 and 180, where values
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 < Notions of sample "front" and "back" may be reversed using the
129 > Notions of "front" and "back" may be reversed using the
130   .I -t
131   option if desired.
132   .TP
# Line 146 | Line 143 | The interpretation is the same as above.
143   .BR #upphi
144   If present, this phi angle that corresponds to
145   the sample "up" orientation.
146 < By default, it is assumed to be 0 degrees, meaning that "up"
146 > By default, it is assumed to be 0, meaning that "up"
147   is phi=0.
148   To get the standard RADIANCE coordinates for BSDFs, "#upphi" should
149 < be set to 90.
149 > be set to 90 (degrees).
150   .TP
151   .BR #colorimetry:
152 < Two colorimetry values are currently understood, "CIE-Y" and "CIE-XYZ".
153 < The default colorimetry of "CIE-Y", which may be left unspecified,
152 > Two colorimetry values are currently understood: "CIE-Y" and "CIE-XYZ".
153 > The default "CIE-Y" colorimetry
154   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.
# Line 208 | Line 205 | To combine this with front reflection measurements int
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 + .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   .SH AUTHOR
260   Greg Ward
261   .SH "SEE ALSO"
262 < bsdf2klems(1), bsdf2rad(1), bsdf2ttree(1), bsdfview(1), pabopto2xyz(1)
262 > bsdf2klems(1), bsdf2rad(1), bsdf2ttree(1), bsdfview(1), genBSDF(1),
263 > pabopto2xyz(1)

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