man/man1/gensky.1

.\" RCSid "$Id"
.TH GENSKY 1 4/24/98 RADIANCE
.SH NAME
gensky - generate a RADIANCE description of the sky
.SH SYNOPSIS
.B "gensky month day time"
[
.B options
]
.br
.B "gensky -ang altitude azimuth"
[
.B options
]
.br
.B "gensky -defaults"
.SH DESCRIPTION
.I Gensky
produces a RADIANCE scene description for the CIE standard
sky distribution at the given month, day and time.
By default, the time is interpreted as local standard
time on a 24-hour clock.
The time value may be given either as decimal hours, or using a
colon to separate hours and minutes.
If the time is immediately followed (no white space)
by a North American or European time zone designation,
then this determines the standard meridian, which may
be specified alternatively with the
.I \-m
option.
The following time zones are understood, with their corresponding
hour differences from Greenwich Mean Time:
.sp .5
.nf
Standard time:
YST   PST   MST   CST   EST   GMT
 9     8     7     6     5     0

CET   EET   AST   GST   IST   JST  NZST
-1    -2    -3    -4    -5.5  -9   -12

Daylight savings time:
YDT   PDT   MDT   CDT   EDT   BST
 8     7     6     5     4     -1

CEST  EEST  ADT   GDT   IDT   JDT  NZDT
 -2    -3   -4    -5   -6.5   -10  -13
.fi
.PP
If the time is preceded by a plus sign ('+'), then it is interpreted
as local solar time instead.
It is very important to specify the correct latitude and longitude
(unless local solar time is given) using the
.I \-a
and
.I \-o
options to get the correct solar angles.
.PP
The second form gives the solar angles explicitly.
The altitude is measured in degrees above the horizon, and the
azimuth is measured in degrees west of South.
.PP
The third form prints the default option values.
.PP
The output sky distribution is given as a brightness function,
.I skyfunc.
Its value is in watts/steradian/meter2.
The x axis points east,
the y axis points north, and the z axis corresponds to the zenith.
The actual material and surface(s) used for the sky is left
up to the user.
For a hemispherical blue sky, the description might be:
.sp
.nf
!gensky 4 1 14

skyfunc glow skyglow
0
0
4 .9 .9 1 0

skyglow source sky
0
0
4 0 0 1 180
.fi
.sp
Often,
.I skyfunc
will actually be used to characterize the light coming in from
a window.
.PP
In addition to the specification of
a sky distribution function,
.I gensky
suggests an ambient value in a comment at the beginning of the
description to use with the
.I \-av
option of the RADIANCE rendering programs.
(See rview(1) and rpict(1).)
This value is the cosine-weighted radiance of the sky in
watts/steradian/meter2.
.PP
.I Gensky
supports the following options.
.TP 10n
.BR \-s
Sunny sky without sun.
The sky distribution will correspond to a standard CIE clear day.
.TP
.BR \+s
Sunny sky with sun.
In addition to the sky distribution function, a source
description of the sun is generated.
.TP
.BR \-c
Cloudy sky.
The sky distribution will correspond to a standard CIE overcast day.
.TP
.BR \-i
Intermediate sky without sun.
The sky will correspond to a standard CIE intermediate day.
.TP
.BR \+i
Intermediate sky with sun.
In addition to the sky distribution, a (somewhat subdued) sun
is generated.
.TP
.BR \-u
Uniform cloudy sky.
The sky distribution will be completely uniform.
.TP
.BI -g \ rfl
Average ground reflectance is
.I rfl.
This value is used to compute
.I skyfunc
when Dz is negative.
Ground plane brightness is the same for
.I \-s
as for
.I \+s.
(Likewise for
.I \-i
and
.I \+i,
but see the
.I \-r
option below.)
.TP
.BI -b \ brt
The zenith brightness is
.I brt.
Zenith radiance (in watts/steradian/meter2) is normally computed
from the sun angle and sky turbidity (for sunny sky).
It can be given directly instead, using this option.
.TP
.BI -B \ irrad
Same as
.I \-b,
except zenith brightness is computed from the horizontal
diffuse irradiance (in watts/meter2).
.TP
.BI -r \ rad
The solar radiance is
.I rad.
Solar radiance (in watts/steradian/meter2) is normally computed from
the solar altitude.
This option may be used to override the default calculation.
If a value of zero is given, no sun description is produced, and the
contribution of direct solar to ground brightness is neglected.
.TP
.BI -R \ irrad
Same as
.I \-r,
except solar radiance is computed from the horizontal direct
irradiance (in watts/meter2).
.TP
.BI -t \ trb
The turbidity factor is
.I trb.
Greater turbidity factors
correspond to greater atmospheric scattering.
A turbidity factor of 1.0 indicates an ideal clear atmosphere (i.e.
a completely dark sky).
Values less than 1.0 are physically impossible.
.PP
The following options do not apply when the solar
altitude and azimuth are given explicitly.
.TP
.BI -a \ lat
The site latitude is
.I lat
degrees north.
(Use negative angle for south latitude.)
This is used in the calculation of sun angle.
.TP
.BI -o \ lon
The site longitude is
.I lon
degrees west.
(Use negative angle for east longitude.)
This is used in the calculation of solar time and sun angle.
Be sure to give the corresponding standard meridian also!
If solar time is given directly, then this option has no effect.
.TP
.BI -m \ mer
The site standard meridian is
.I mer
degrees west of Greenwich.
(Use negative angle for east.)
This is used in the calculation of solar time.
Be sure to give the correct longitude also!
If solar time is given directly, then this option has no effect.
.SH EXAMPLE
To produce a sunny sky for July 4th at 2:30pm Eastern daylight time at a
site latitude of 42 degrees, 89 degrees west longitude:
.IP "" .2i
gensky 7 4 14:30EDT +s -a 42 -o 89
.PP
To produce a sunny sky distribution for a specific sun position but
without the sun description:
.IP "" .2i
gensky -ang 23 -40 -s
.SH FILES
/usr/local/lib/ray/skybright.cal
.SH AUTHOR
Greg Ward
.SH "SEE ALSO"
rpict(1), rview(1), xform(1)
Revision:	1.1
Committed:	Tue Mar 11 19:20:21 2003 UTC (22 years, 7 months ago) by greg
Branch:	MAIN
CVS Tags:	rad3R5
Log Message:	Added documentation to repository
#	User	Rev	Content
1	greg	1.1	.\" RCSid "$Id"
2			.TH GENSKY 1 4/24/98 RADIANCE
3			.SH NAME
4			gensky - generate a RADIANCE description of the sky
5			.SH SYNOPSIS
6			.B "gensky month day time"
7			[
8			.B options
9			]
10			.br
11			.B "gensky -ang altitude azimuth"
12			[
13			.B options
14			]
15			.br
16			.B "gensky -defaults"
17			.SH DESCRIPTION
18			.I Gensky
19			produces a RADIANCE scene description for the CIE standard
20			sky distribution at the given month, day and time.
21			By default, the time is interpreted as local standard
22			time on a 24-hour clock.
23			The time value may be given either as decimal hours, or using a
24			colon to separate hours and minutes.
25			If the time is immediately followed (no white space)
26			by a North American or European time zone designation,
27			then this determines the standard meridian, which may
28			be specified alternatively with the
29			.I \-m
30			option.
31			The following time zones are understood, with their corresponding
32			hour differences from Greenwich Mean Time:
33			.sp .5
34			.nf
35			Standard time:
36			YST PST MST CST EST GMT
37			9 8 7 6 5 0
38
39			CET EET AST GST IST JST NZST
40			-1 -2 -3 -4 -5.5 -9 -12
41
42			Daylight savings time:
43			YDT PDT MDT CDT EDT BST
44			8 7 6 5 4 -1
45
46			CEST EEST ADT GDT IDT JDT NZDT
47			-2 -3 -4 -5 -6.5 -10 -13
48			.fi
49			.PP
50			If the time is preceded by a plus sign ('+'), then it is interpreted
51			as local solar time instead.
52			It is very important to specify the correct latitude and longitude
53			(unless local solar time is given) using the
54			.I \-a
55			and
56			.I \-o
57			options to get the correct solar angles.
58			.PP
59			The second form gives the solar angles explicitly.
60			The altitude is measured in degrees above the horizon, and the
61			azimuth is measured in degrees west of South.
62			.PP
63			The third form prints the default option values.
64			.PP
65			The output sky distribution is given as a brightness function,
66			.I skyfunc.
67			Its value is in watts/steradian/meter2.
68			The x axis points east,
69			the y axis points north, and the z axis corresponds to the zenith.
70			The actual material and surface(s) used for the sky is left
71			up to the user.
72			For a hemispherical blue sky, the description might be:
73			.sp
74			.nf
75			!gensky 4 1 14
76
77			skyfunc glow skyglow
78			0
79			0
80			4 .9 .9 1 0
81
82			skyglow source sky
83			0
84			0
85			4 0 0 1 180
86			.fi
87			.sp
88			Often,
89			.I skyfunc
90			will actually be used to characterize the light coming in from
91			a window.
92			.PP
93			In addition to the specification of
94			a sky distribution function,
95			.I gensky
96			suggests an ambient value in a comment at the beginning of the
97			description to use with the
98			.I \-av
99			option of the RADIANCE rendering programs.
100			(See rview(1) and rpict(1).)
101			This value is the cosine-weighted radiance of the sky in
102			watts/steradian/meter2.
103			.PP
104			.I Gensky
105			supports the following options.
106			.TP 10n
107			.BR \-s
108			Sunny sky without sun.
109			The sky distribution will correspond to a standard CIE clear day.
110			.TP
111			.BR \+s
112			Sunny sky with sun.
113			In addition to the sky distribution function, a source
114			description of the sun is generated.
115			.TP
116			.BR \-c
117			Cloudy sky.
118			The sky distribution will correspond to a standard CIE overcast day.
119			.TP
120			.BR \-i
121			Intermediate sky without sun.
122			The sky will correspond to a standard CIE intermediate day.
123			.TP
124			.BR \+i
125			Intermediate sky with sun.
126			In addition to the sky distribution, a (somewhat subdued) sun
127			is generated.
128			.TP
129			.BR \-u
130			Uniform cloudy sky.
131			The sky distribution will be completely uniform.
132			.TP
133			.BI -g \ rfl
134			Average ground reflectance is
135			.I rfl.
136			This value is used to compute
137			.I skyfunc
138			when Dz is negative.
139			Ground plane brightness is the same for
140			.I \-s
141			as for
142			.I \+s.
143			(Likewise for
144			.I \-i
145			and
146			.I \+i,
147			but see the
148			.I \-r
149			option below.)
150			.TP
151			.BI -b \ brt
152			The zenith brightness is
153			.I brt.
154			Zenith radiance (in watts/steradian/meter2) is normally computed
155			from the sun angle and sky turbidity (for sunny sky).
156			It can be given directly instead, using this option.
157			.TP
158			.BI -B \ irrad
159			Same as
160			.I \-b,
161			except zenith brightness is computed from the horizontal
162			diffuse irradiance (in watts/meter2).
163			.TP
164			.BI -r \ rad
165			The solar radiance is
166			.I rad.
167			Solar radiance (in watts/steradian/meter2) is normally computed from
168			the solar altitude.
169			This option may be used to override the default calculation.
170			If a value of zero is given, no sun description is produced, and the
171			contribution of direct solar to ground brightness is neglected.
172			.TP
173			.BI -R \ irrad
174			Same as
175			.I \-r,
176			except solar radiance is computed from the horizontal direct
177			irradiance (in watts/meter2).
178			.TP
179			.BI -t \ trb
180			The turbidity factor is
181			.I trb.
182			Greater turbidity factors
183			correspond to greater atmospheric scattering.
184			A turbidity factor of 1.0 indicates an ideal clear atmosphere (i.e.
185			a completely dark sky).
186			Values less than 1.0 are physically impossible.
187			.PP
188			The following options do not apply when the solar
189			altitude and azimuth are given explicitly.
190			.TP
191			.BI -a \ lat
192			The site latitude is
193			.I lat
194			degrees north.
195			(Use negative angle for south latitude.)
196			This is used in the calculation of sun angle.
197			.TP
198			.BI -o \ lon
199			The site longitude is
200			.I lon
201			degrees west.
202			(Use negative angle for east longitude.)
203			This is used in the calculation of solar time and sun angle.
204			Be sure to give the corresponding standard meridian also!
205			If solar time is given directly, then this option has no effect.
206			.TP
207			.BI -m \ mer
208			The site standard meridian is
209			.I mer
210			degrees west of Greenwich.
211			(Use negative angle for east.)
212			This is used in the calculation of solar time.
213			Be sure to give the correct longitude also!
214			If solar time is given directly, then this option has no effect.
215			.SH EXAMPLE
216			To produce a sunny sky for July 4th at 2:30pm Eastern daylight time at a
217			site latitude of 42 degrees, 89 degrees west longitude:
218			.IP "" .2i
219			gensky 7 4 14:30EDT +s -a 42 -o 89
220			.PP
221			To produce a sunny sky distribution for a specific sun position but
222			without the sun description:
223			.IP "" .2i
224			gensky -ang 23 -40 -s
225			.SH FILES
226			/usr/local/lib/ray/skybright.cal
227			.SH AUTHOR
228			Greg Ward
229			.SH "SEE ALSO"
230			rpict(1), rview(1), xform(1)