xenial (1) gensky.1.gz

Provided by: radiance_4R1+20120125-1.1_amd64 bug

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

       Greg Ward

SEE ALSO

       rpict(1), rvu(1), xform(1)