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 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:
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.
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
-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
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
a time zone or solar time is given directly, then this option
has no effect.
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)