Ubuntu Manpage: gensky - generate a RADIANCE description of the sky

Provided by: radiance_4R1+20120125-1.1_amd64

NAME

       gensky - generate a RADIANCE description of the sky

SYNOPSIS

       gensky month day time [ options ]
       gensky -ang altitude azimuth [ options ]
       gensky -defaults

DESCRIPTION

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:
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.

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 .99 .99 1.1 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 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 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 a time zone or solar time is given directly, then this
option has no effect.

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:

         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

FILES

       /usr/share/radiance/skybright.cal

AUTHOR