GENSKY(1) General Commands Manual GENSKY(1)
gensky - generate a RADIANCE description of the sky
gensky month day time [ options ]
gensky -ang altitude azimuth [ options ]
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 sepa-
rate 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:
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 cor-
rect latitude and longitude (unless local solar time is given) using the -a and -o options to get the correct solar angles.
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
4 .99 .99 1.1 0
skyglow source sky
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 rvu(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 tur-
bidity (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 contribu-
tion 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
-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 a time zone or solar time is given directly, then this option has no
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:
gensky 7 4 14:30EDT +s -a 42 -o 89
To produce a sunny sky distribution for a specific sun position but without the sun description:
gensky -ang 23 -40 -s
rpict(1), rvu(1), xform(1)