======  Preetham(SunSky)  ======


Preetham (1999) analytic sky model.

Implements the Preetham et al. sky luminance model via Perez functions (zenithY/x/y + five Perez coefficients per channel). Physically attenuated solar spectrum is computed via Rayleigh scattering, aerosol attenuation, and ozone / water-vapor absorption.

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This shader is added twice, once for backwards compatibility  ("SunSky") and once with the proper scientific name ("Preetham")