// Copyright (c) the JPEG XL Project Authors. All rights reserved.

//

// Use of this source code is governed by a BSD-style

// license that can be found in the LICENSE file.

#include "lib/jxl/enc_photon_noise.h"

#include <algorithm>

#include "lib/jxl/cms/opsin_params.h"

namespace jxl {

namespace {

// Assumes a daylight-like spectrum.

// https://www.strollswithmydog.com/effective-quantum-efficiency-of-sensor/#:~:text=11%2C260%20photons/um%5E2/lx-s

constexpr float kPhotonsPerLxSPerUm2 = 11260;

// Order of magnitude for cameras in the 2010-2020 decade, taking the CFA into

// account.

constexpr float kEffectiveQuantumEfficiency = 0.20;

// TODO(sboukortt): reevaluate whether these are good defaults, notably whether

// it would be worth making read noise higher at lower ISO settings.

constexpr float kPhotoResponseNonUniformity = 0.005;

constexpr float kInputReferredReadNoise = 3;

// Assumes a 35mm sensor.

constexpr float kSensorAreaUm2 = 36000.f * 24000;

template <typename T>

inline constexpr T Square(const T x) {

  return x * x;

}

template <typename T>

inline constexpr T Cube(const T x) {

  return x * x * x;

}

}  // namespace

NoiseParams SimulatePhotonNoise(const size_t xsize, const size_t ysize,

                                const float iso) {

  const float kOpsinAbsorbanceBiasCbrt =

      std::cbrt(jxl::cms::kOpsinAbsorbanceBias[1]);

  // Focal plane exposure for 18% of kDefaultIntensityTarget, in lx·s.

  // (ISO = 10 lx·s ÷ H)

  const float h_18 = 10 / iso;

  const float pixel_area_um2 = kSensorAreaUm2 / (xsize * ysize);

  const float electrons_per_pixel_18 = kEffectiveQuantumEfficiency *

                                       kPhotonsPerLxSPerUm2 * h_18 *

                                       pixel_area_um2;

  NoiseParams params;

  for (size_t i = 0; i < NoiseParams::kNumNoisePoints; ++i) {

    const float scaled_index = i / (NoiseParams::kNumNoisePoints - 2.f);

    // scaled_index is used for XYB = (0, 2·scaled_index, 2·scaled_index)

    const float y = 2 * scaled_index;

    // 1 = default intensity target

    const float linear = std::max(0.f, Cube(y - kOpsinAbsorbanceBiasCbrt) +

                                           jxl::cms::kOpsinAbsorbanceBias[1]);

    const float electrons_per_pixel = electrons_per_pixel_18 * (linear / 0.18f);

    // Quadrature sum of read noise, photon shot noise (sqrt(S) so simply not

    // squared here) and photo response non-uniformity.

    // https://doi.org/10.1117/3.725073

    // Units are electrons rms.

    const float noise =

        std::sqrt(Square(kInputReferredReadNoise) + electrons_per_pixel +

                  Square(kPhotoResponseNonUniformity * electrons_per_pixel));

    const float linear_noise = noise * (0.18f / electrons_per_pixel_18);

    const float opsin_derivative =

        (1.f / 3) /

        Square(std::cbrt(linear - jxl::cms::kOpsinAbsorbanceBias[1]));

    const float opsin_noise = linear_noise * opsin_derivative;

    // TODO(sboukortt): verify more thoroughly whether the denominator is

    // correct.

    params.lut[i] =

        Clamp1(opsin_noise /

                   (0.22f             // norm_const

                    * std::sqrt(2.f)  // red_noise + green_noise

                    * 1.13f  // standard deviation of a plane of generated noise

                    ),

               0.f, 1.f);

  }

  return params;

}

}  // namespace jxl