/*
 * Copyright (C) 2021  Christopher J. Howard
 *
 * This file is part of Antkeeper source code.
 *
 * Antkeeper source code 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 3 of the License, or
 * (at your option) any later version.
 *
 * Antkeeper source code 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 Antkeeper source code.  If not, see <http://www.gnu.org/licenses/>.
 */

#ifndef ANTKEEPER_PHYSICS_LIGHT_LUMINOSITY_HPP
#define ANTKEEPER_PHYSICS_LIGHT_LUMINOSITY_HPP

#include <cmath>

namespace physics {
namespace light {

/// Luminous efficiency functions.
namespace luminosity {

/**
 * Fitted Gaussian approximation to the CIE 1931 standard observer photopic luminosity function.
 *
 * @param lambda Wavelength of light, in nanometers.
 * @return Luminous efficiency, on `[0, 1]`.
 *
 * @see Wyman, C., Sloan, P.J., & Shirley, P. (2013). Simple Analytic Approximations to the CIE XYZ Color Matching Functions.
 */
template <class T>
T photopic(T lambda)
{
	const T t0 = (lambda - T(568.8)) * ((lambda < T(568.8)) ? T(0.0213) : T(0.0247));
	const T t1 = (lambda - T(530.9)) * ((lambda < T(530.9)) ? T(0.0613) : T(0.0322));
	
	const T y0 = T(0.821) * std::exp(T(-0.5) * t0 * t0);
	const T y1 = T(0.286) * std::exp(T(-0.5) * t1 * t1);
	
	return y0 + y1;
}

} // namespace luminosity

} // namespace light
} // namespace physics

#endif // ANTKEEPER_PHYSICS_LIGHT_LUMINOSITY_HPP