GENSKY(1)                                                            GENSKY(1)



NNAAMMEE
       gensky - generate a RADIANCE description of the sky

SSYYNNOOPPSSIISS
       ggeennsskkyy mmoonntthh ddaayy ttiimmee [ ooppttiioonnss ]
       ggeennsskkyy --aanngg aallttiittuuddee aazziimmuutthh [ ooppttiioonnss ]
       ggeennsskkyy --ddeeffaauullttss

DDEESSCCRRIIPPTTIIOONN
       _G_e_n_s_k_y  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, _s_k_y_f_u_n_c_.
       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, _s_k_y_f_u_n_c will actually be used to characterize the  light  coming
       in from a window.

       In addition to the specification of a sky distribution function, _g_e_n_s_k_y
       suggests an ambient value in a comment at the beginning of the descrip-
       tion  to  use  with  the _-_a_v 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.

       _G_e_n_s_k_y supports the following options.

       --ss        Sunny  sky without sun.  The sky distribution will correspond
                 to a standard CIE clear day.

       ++ss        Sunny sky with sun.  In  addition  to  the  sky  distribution
                 function, a source description of the sun is generated.

       --cc        Cloudy  sky.  The sky distribution will correspond to a stan-
                 dard CIE overcast day.

       --ii        Intermediate sky without sun.  The sky will correspond  to  a
                 standard CIE intermediate day.

       ++ii        Intermediate  sky with sun.  In addition to the sky distribu-
                 tion, a (somewhat subdued) sun is generated.

       --uu        Uniform cloudy sky.  The sky distribution will be  completely
                 uniform.

       --gg _r_f_l    Average  ground  reflectance  is  _r_f_l_.  This value is used to
                 compute _s_k_y_f_u_n_c 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.)

       --bb _b_r_t    The  zenith  brightness  is   _b_r_t_.    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.

       --BB _i_r_r_a_d  Same  as  _-_b_,  except  zenith brightness is computed from the
                 horizontal diffuse irradiance (in watts/meter2).

       --rr _r_a_d    The solar radiance is _r_a_d_.  Solar radiance  (in  watts/stera-
                 dian/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.

       --RR _i_r_r_a_d  Same  as _-_r_, except solar radiance is computed from the hori-
                 zontal direct irradiance (in watts/meter2).

       --tt _t_r_b    The turbidity factor is _t_r_b_.  Greater turbidity factors  cor-
                 respond  to greater atmospheric scattering.  A turbidity fac-
                 tor of 1.0 indicates an ideal clear atmosphere (i.e.  a  com-
                 pletely  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.

       --aa _l_a_t The site latitude is _l_a_t degrees north.  (Use negative angle for
              south latitude.)  This is used in the calculation of sun  angle.

       --oo _l_o_n The site longitude is _l_o_n 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.

       --mm _m_e_r The  site  standard  meridian  is _m_e_r 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.

EEXXAAMMPPLLEE
       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

FFIILLEESS
       /usr/local/lib/ray/skybright.cal

AAUUTTHHOORR
       Greg Ward

SSEEEE AALLSSOO
       rpict(1), rvu(1), xform(1)



RADIANCE                            4/24/98                          GENSKY(1)
