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.\" RCSid "$Id: gensky.1,v 1.4 2004/01/01 19:31:44 greg Exp $" |
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.TH GENSKY 1 4/24/98 RADIANCE |
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.SH NAME |
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gensky - generate a RADIANCE description of the sky |
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.SH SYNOPSIS |
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.B "gensky month day time" |
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[ |
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.B options |
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] |
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.br |
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.B "gensky -ang altitude azimuth" |
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[ |
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.B options |
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] |
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.br |
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.B "gensky -defaults" |
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.SH DESCRIPTION |
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.I Gensky |
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produces a RADIANCE scene description for the CIE standard |
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sky distribution at the given month, day and time. |
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By default, the time is interpreted as local standard |
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time on a 24-hour clock. |
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The time value may be given either as decimal hours, or using a |
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colon to separate hours and minutes. |
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If the time is immediately followed (no white space) |
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by a North American or European time zone designation, |
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then this determines the standard meridian, which may |
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be specified alternatively with the |
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.I \-m |
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option. |
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The following time zones are understood, with their corresponding |
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hour differences from Greenwich Mean Time: |
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.sp .5 |
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.nf |
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Standard time: |
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YST PST MST CST EST GMT |
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9 8 7 6 5 0 |
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|
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CET EET AST GST IST JST NZST |
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-1 -2 -3 -4 -5.5 -9 -12 |
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|
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Daylight savings time: |
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YDT PDT MDT CDT EDT BST |
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8 7 6 5 4 -1 |
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|
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CEST EEST ADT GDT IDT JDT NZDT |
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-2 -3 -4 -5 -6.5 -10 -13 |
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.fi |
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.PP |
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If the time is preceded by a plus sign ('+'), then it is interpreted |
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as local solar time instead. |
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It is very important to specify the correct latitude and longitude |
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(unless local solar time is given) using the |
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.I \-a |
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and |
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.I \-o |
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options to get the correct solar angles. |
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.PP |
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The second form gives the solar angles explicitly. |
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The altitude is measured in degrees above the horizon, and the |
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azimuth is measured in degrees west of South. |
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.PP |
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The third form prints the default option values. |
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.PP |
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The output sky distribution is given as a brightness function, |
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.I skyfunc. |
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Its value is in watts/steradian/meter2. |
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The x axis points east, |
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the y axis points north, and the z axis corresponds to the zenith. |
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The actual material and surface(s) used for the sky is left |
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up to the user. |
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For a hemispherical blue sky, the description might be: |
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.sp |
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.nf |
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!gensky 4 1 14 |
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|
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skyfunc glow skyglow |
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0 |
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0 |
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4 .99 .99 1.1 0 |
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|
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skyglow source sky |
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0 |
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0 |
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4 0 0 1 180 |
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.fi |
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.sp |
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Often, |
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.I skyfunc |
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will actually be used to characterize the light coming in from |
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a window. |
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.PP |
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In addition to the specification of |
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a sky distribution function, |
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.I gensky |
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suggests an ambient value in a comment at the beginning of the |
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description to use with the |
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.I \-av |
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option of the RADIANCE rendering programs. |
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(See rvu(1) and rpict(1).) |
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This value is the cosine-weighted radiance of the sky in |
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watts/steradian/meter2. |
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.PP |
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.I Gensky |
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supports the following options. |
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.TP 10n |
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.BR \-s |
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Sunny sky without sun. |
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The sky distribution will correspond to a standard CIE clear day. |
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.TP |
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.BR \+s |
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Sunny sky with sun. |
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In addition to the sky distribution function, a source |
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description of the sun is generated. |
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.TP |
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.BR \-c |
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Cloudy sky. |
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The sky distribution will correspond to a standard CIE overcast day. |
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.TP |
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.BR \-i |
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Intermediate sky without sun. |
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The sky will correspond to a standard CIE intermediate day. |
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.TP |
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.BR \+i |
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Intermediate sky with sun. |
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In addition to the sky distribution, a (somewhat subdued) sun |
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is generated. |
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.TP |
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.BR \-u |
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Uniform cloudy sky. |
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The sky distribution will be completely uniform. |
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.TP |
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.BI -g \ rfl |
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Average ground reflectance is |
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.I rfl. |
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This value is used to compute |
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.I skyfunc |
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when Dz is negative. |
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Ground plane brightness is the same for |
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.I \-s |
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as for |
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.I \+s. |
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(Likewise for |
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.I \-i |
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and |
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.I \+i, |
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but see the |
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.I \-r |
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option below.) |
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.TP |
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.BI -b \ brt |
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The zenith brightness is |
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.I brt. |
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Zenith radiance (in watts/steradian/meter2) is normally computed |
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from the sun angle and sky turbidity (for sunny sky). |
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It can be given directly instead, using this option. |
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.TP |
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.BI -B \ irrad |
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Same as |
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.I \-b, |
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except zenith brightness is computed from the horizontal |
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diffuse irradiance (in watts/meter2). |
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.TP |
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.BI -r \ rad |
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The solar radiance is |
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.I rad. |
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Solar radiance (in watts/steradian/meter2) is normally computed from |
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the solar altitude. |
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This option may be used to override the default calculation. |
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If a value of zero is given, no sun description is produced, and the |
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contribution of direct solar to ground brightness is neglected. |
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.TP |
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.BI -R \ irrad |
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Same as |
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.I \-r, |
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except solar radiance is computed from the horizontal direct |
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irradiance (in watts/meter2). |
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.TP |
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.BI -t \ trb |
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The turbidity factor is |
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.I trb. |
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Greater turbidity factors |
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correspond to greater atmospheric scattering. |
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A turbidity factor of 1.0 indicates an ideal clear atmosphere (i.e. |
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a completely dark sky). |
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Values less than 1.0 are physically impossible. |
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.PP |
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The following options do not apply when the solar |
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altitude and azimuth are given explicitly. |
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.TP |
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.BI -a \ lat |
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The site latitude is |
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.I lat |
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degrees north. |
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(Use negative angle for south latitude.) |
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This is used in the calculation of sun angle. |
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.TP |
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.BI -o \ lon |
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The site longitude is |
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.I lon |
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degrees west. |
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(Use negative angle for east longitude.) |
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This is used in the calculation of solar time and sun angle. |
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Be sure to give the corresponding standard meridian also! |
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If solar time is given directly, then this option has no effect. |
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.TP |
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.BI -m \ mer |
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The site standard meridian is |
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.I mer |
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degrees west of Greenwich. |
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(Use negative angle for east.) |
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This is used in the calculation of solar time. |
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Be sure to give the correct longitude also! |
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If a time zone or solar time is given directly, then this option has no effect. |
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.SH EXAMPLE |
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To produce a sunny sky for July 4th at 2:30pm Eastern daylight time at a |
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site latitude of 42 degrees, 89 degrees west longitude: |
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.IP "" .2i |
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gensky 7 4 14:30EDT +s -a 42 -o 89 |
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.PP |
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To produce a sunny sky distribution for a specific sun position but |
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without the sun description: |
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.IP "" .2i |
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gensky -ang 23 -40 -s |
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.SH FILES |
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/usr/local/lib/ray/skybright.cal |
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.SH AUTHOR |
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Greg Ward |
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.SH "SEE ALSO" |
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rpict(1), rvu(1), xform(1) |