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This package calculates the colors of sky as detailed in:

http://www.cs.utah.edu/vissim/papers/sunsky/sunsky.pdf

A Practical Analytic Model for Daylight

A. J. Preetham, Peter Shirley, Brian Smits

- Function:
**solar-hour***julian-day hour* -
Returns the solar-time in hours given the integer

`julian-day`in the range 1 to 366, and the local time in hours.To be meticulous, subtract 4 minutes for each degree of longitude west of the standard meridian of your time zone.

- Function:
**solar-declination***julian-day*

- Function:
**solar-polar***declination latitude solar-hour* Returns a list of

`theta_s`, the solar angle from the zenith, and`phi_s`, the solar azimuth. 0 <=`theta_s`measured in degrees.`phi_s`is measured in degrees from due south; west of south being positive.

In the following procedures, the number 0 <= `theta_s` <= 90 is
the solar angle from the zenith in degrees.

Turbidity is a measure of the fraction of scattering due to haze as opposed to molecules. This is a convenient quantity because it can be estimated based on visibility of distant objects. This model fails for turbidity values less than 1.3.

_______________________________________________________________ 512|-: | | * pure-air | 256|-:** | | : ** exceptionally-clear | 128|-: * | | : ** | 64|-: * | | : ** very-clear | 32|-: ** | | : ** | 16|-: *** clear | | : **** | 8|-: **** | | : **** light-haze | 4|-: **** | | : ****** | 2|-: ******** haze thin-| | : *********** fog | 1|-:----------------------------------------------------*******--| |_:____.____:____.____:____.____:____.____:____.____:____.____:_| 1 2 4 8 16 32 64 Meterorological range (km) versus Turbidity

- Function:
**sunlight-spectrum***turbidity theta_s* Returns a vector of 41 values, the spectrum of sunlight from 380.nm to 790.nm for a given

`turbidity`and`theta_s`.

- Function:
**sunlight-chromaticity***turbidity theta_s* Given

`turbidity`and`theta_s`,`sunlight-chromaticity`

returns the CIEXYZ triple for color of sunlight scaled to be just inside the RGB709 gamut.

- Function:
**zenith-xyy***turbidity theta_s* Returns the xyY (chromaticity and luminance) at the zenith. The Luminance has units kcd/m^2.

- Function:
**overcast-sky-color-xyy***turbidity theta_s* `turbidity`is a positive real number expressing the amount of light scattering. The real number`theta_s`is the solar angle from the zenith in degrees.`overcast-sky-color-xyy`

returns a function of one angle`theta`, the angle from the zenith of the viewing direction (in degrees); and returning the xyY value for light coming from that elevation of the sky.

- Function:
**clear-sky-color-xyy***turbidity theta_s phi_s* - Function:
**sky-color-xyy***turbidity theta_s phi_s* `turbidity`is a positive real number expressing the amount of light scattering. The real number`theta_s`is the solar angle from the zenith in degrees. The real number`phi_s`is the solar angle from south.`clear-sky-color-xyy`

returns a function of two angles,`theta`and`phi`which specify the angles from the zenith and south meridian of the viewing direction (in degrees); returning the xyY value for light coming from that direction of the sky.`sky-color-xyY`

calls`overcast-sky-color-xyY`

for`turbidity`<= 20; otherwise the`clear-sky-color-xyy`

function.

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