/*

Copyright (C) 2017-2026 Topological Manifold

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 3 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, see <http://www.gnu.org/licenses/>.

*/

#pragma once

#include <src/com/exponent.h>

#include <cstddef>

#include <type_traits>

namespace ns::sampling

{

template <std::size_t N, typename T>

T reflected_pdf(const T pdf, const T cosine)

{

static_assert(N >= 2);

static_assert(std::is_floating_point_v<T>);

// reflected_pdf<2> * pow(sin(angle) / sin(reflected), N - 2)

// reflected_pdf<2> = pdf / 2

// cos(angle) = cosine

// cos(reflected) = 2 * square(cosine) - 1

// sin(angle) = sqrt(1 - square(cosine))

// sin(reflected) = sqrt(1 - square(2 * square(cosine) - 1))

// sin(reflected) = sqrt(4 * square(cosine) * (1 - square(cosine)))

// sin_ratio = 1 / sqrt(4 * square(cosine)) = 1 / (2 * cosine)

// pdf / (2 * pow(2 * cosine, N - 2))

// pdf / (pow(2, N - 1) * pow(cosine, N - 2))

if (cosine > 0)

{

return pdf / ((1 << (N - 1)) * power<N - 2>(cosine));

}

return 0;

}

template <std::size_t N, typename T>

T area_pdf_to_solid_angle_pdf(const T pdf, const T cosine, const T distance)

{

static_assert(N >= 2);

static_assert(std::is_floating_point_v<T>);

if (cosine > 0)

{

return pdf * power<N - 1>(distance) / cosine;

}

return 0;

}

template <std::size_t N, typename T>

T solid_angle_pdf_to_area_pdf(const T pdf, const T cosine, const T distance)

{

static_assert(N >= 2);

static_assert(std::is_floating_point_v<T>);

if (cosine > 0)

{

return pdf * cosine / power<N - 1>(distance);

}

return 0;

}

}