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gensky (1)
NAME
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gensky - generate a RADIANCE description of the sky |
SYNOPSIS
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gensky month day time [ options ]
gensky -ang altitude azimuth [ options ]
gensky -defaults |
DESCRIPTION
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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 -m
option. The following time zones are understood, with their
corresponding hour differences from Greenwich Mean
Time: |
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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
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 -a and -o options to get the correct solar angles. |
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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. |
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The third form prints the default option
values. |
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The output sky distribution is given as a brightness
function, 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: |
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!gensky 4 1 14
skyfunc glow skyglow
0
0
4 .9 .9 1 0
skyglow source sky
0
0
4 0 0 1 180
Often, skyfunc will actually be used to characterize the light coming in from a window. |
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In addition to the specification of a sky distribution
function, gensky suggests an ambient value in a
comment at the beginning of the description to use with the
-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. |
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Gensky supports the following options. |
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-s Sunny sky without sun. The sky distribution will
correspond to a standard CIE clear day. |
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+s Sunny sky with sun. In addition to the sky
distribution function, a source description of the sun is
generated. |
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-c Cloudy sky. The sky distribution will correspond
to a standard CIE overcast day. |
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-i Intermediate sky without sun. The sky will
correspond to a standard CIE intermediate day. |
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+i Intermediate sky with sun. In addition to the sky
distribution, a (somewhat subdued) sun is
generated. |
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-u Uniform cloudy sky. The sky distribution will be
completely uniform. |
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Average ground reflectance is rfl. This value is used
to compute skyfunc when Dz is negative. Ground plane
brightness is the same for -s as for +s.
(Likewise for -i and +i, but see the -r
option below.) |
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The zenith brightness is 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. |
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Same as -b, except zenith brightness is computed from
the horizontal diffuse irradiance (in
watts/meter2). |
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The solar radiance is 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. |
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Same as -r, except solar radiance is computed from
the horizontal direct irradiance (in
watts/meter2). |
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The turbidity factor is 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. |
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The following options do not apply when the solar altitude
and azimuth are given explicitly. |
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The site latitude is lat degrees north. (Use negative
angle for south latitude.) This is used in the calculation
of sun angle. |
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The site longitude is 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. |
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The site standard meridian is 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. |
EXAMPLE
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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: |
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gensky 7 4 14:30EDT +s -a 42 -o 89 |
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To produce a sunny sky distribution for a specific sun
position but without the sun description: |
FILES
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/usr/local/lib/ray/skybright.cal |
AUTHOR
SEE ALSO
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