/*
 * Copyright (C) 2003 Fabien Chereau
 *
 * This program is free software; you can redistribute it and/or
 * modify it under the terms of the GNU General Public License
 * as published by the Free Software Foundation; either version 2
 * of the License, or (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program; if not, write to the Free Software
 * Foundation, Inc., 51 Franklin Street, Suite 500, Boston, MA  02110-1335, USA.
 */

#ifndef _SKYBRIGHT_HPP_
#define _SKYBRIGHT_HPP_

//! @class Skybright
//! Compute the luminance of the sky according to some parameters like sun moon position
//! or time or altitude etc...
// GZ: tentative additions to the documentation.
//! This class makes use of the 1998 sky brightness model by Bradley Schaefer.
//! Further reading:
//! B. Schaefer: Sky&Telescope 4/1987
//! B. Schaefer: Astronomy and the Limits of Vision. Vistas in Astronomy 36, 311-361, 1993.
//! B. Schaefer: To the Visual Limits. Sky&Telescope 5/1998 57-60.
//! B. Schaefer: Archaeoastronomy XV, 2000.
//! TASKS TO IMPROVE:
//! Some components of the Schaefer model are strongly simplified. E.g.,
//! - setDate should be taken always in the Gregorian Calendar, or circumvented altogether:
//! - RA takes the right ascension of the sun. --> Could be replaced with the proper value!
//! - bNightTerm includes an 11.0-year solar cycle, and will be erratic a few cycles from 1992. --> At least find a more accurate average period.
//! Extinction could make use of the K computed in SetLocation. But first re-verify that
//! the code here really follows the Schaefer model (with obvious amendments).
class Skybright
{
public:
	//! Constructor
	Skybright();

	//! Set the sky date to use for atmosphere computation
	//! @param year the year in YYYY format
	//! @param month the month: 1=Jan, 12=Dec
	//! @param moonPhase the moon phase in radian 0=Full Moon, PI/2=First Quadrant/Last Quadran, PI=No Moon
	void setDate(const int year, const int month, const float moonPhase);

	//! Set the position parameters to use for atmosphere computation
	//! @param latitude observer latitude in radian
	//! @param altitude observer altitude in m
	//! @param temperature temperature in deg. C
	//! @param relativeHumidity air humidity in %
	void setLocation(const float latitude, const float altitude, const float temperature=15.f, const float relativeHumidity=40.f);

	//! Set the moon and sun zenith angular distance (cosin given) and precompute what can be
	//! This function has to be called once before any call to getLuminance()
	//! @param cosDistMoonZenith cos(angular distance between moon and zenith)
	//! @param cosDistSunZenith cos(angular distance between sun and zenith)
	void setSunMoon(const float cosDistMoonZenith, const float cosDistSunZenith);

	//! Compute the luminance at the given position
	//! @param cosDistMoon cos(angular distance between moon and the position)
	//! @param cosDistSun cos(angular distance between sun  and the position)
	//! @param cosDistZenith cos(angular distance between zenith and the position)
	float getLuminance(float cosDistMoon, const float cosDistSun, const float cosDistZenith) const;

private:
	float airMassMoon;  // Air mass for the Moon
	float airMassSun;   // Air mass for the Sun
	float magMoon;      // Moon magnitude
	float RA;           // Extremely crude estimate of Right Ascension of the Sun!
	float K;            // Summary extinction coefficient
	float C3;           // Term for moon brightness computation
	float C4;           // Term for sky brightness computation
	float SN;           // Snellen Ratio (20/20=1.0, good 20/10=2.0)

	// Optimisation variables
	float bNightTerm;
	float bMoonTerm1;
	float bTwilightTerm;
};

#endif // _SKYBRIGHT_HPP_