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gensky - generate a RADIANCE description of the sky
gensky month day hour [ options ]
gensky -ang altitude azimuth [ options ]
gensky -defaults
Gensky produces a RADIANCE scene description for the CIE
standard sky distribution at the given date and local
standard time. If the hour is preceded by a plus sign
('+'), then it is interpreted as local solar time instead of
standard time. 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. The third form prints the default option values.
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:
!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.
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.
Gensky supports the following options.
- -s
- Sunny sky without sun. The sky distribution will
correspond to a standard CIE clear day.
- +s
- Sunny sky with sun. In addition to the sky
distribution function, a source description of the
sun is generated.
- -c
- Cloudy sky. The sky distribution will correspond
to a standard CIE overcast day.
- -i
- Intermediate sky without sun. The sky will
correspond to a standard CIE intermediate day.
- +i
- Intermediate sky with sun. In addition to the sky
distribution, a (somewhat subdued) sun is
generated.
- -u
- Uniform cloudy sky. The sky distribution will be
completely uniform.
- -g rfl
- 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.)
- -b brt
- 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.
- -B irrad
- Same as -b, except zenith brightness is computed
from the horizontal diffuse irradiance (in
watts/meter2).
- -r rad
- 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.
- -R irrad
- Same as -r, except solar radiance is computed from
the horizontal direct irradiance (in
watts/meter2).
- -t trb
- 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.
The following options do not apply when the solar altitude
and azimuth are given explicitly.
- -a lat
-
The site latitude is lat degrees north. (Use negative
angle for south latitude.) This is used in the
calculation of sun angle.
- -o lon
-
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.
- -m mer
-
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.
To produce a sunny sky for July 4th at 2:30pm standard time
at a site latitude of 42 degrees, 108 degrees west
longitude, and a 110 degree standard meridian:
gensky 7 4 14.5 +s -a 42 -o 108 -m 110
To produce a sunny sky distribution for a specific sun
position but without the sun description:
gensky -ang 23 -40 -s
/usr/local/lib/ray/skybright.cal
Greg Ward
rpict(1), rview(1), xform(1)
Header and Footer
GENSKY(1) RADIANCE (5/2/95) GENSKY(1)
Page 1 (printed 7/17/96)
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