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// 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"
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(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) + 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 - 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
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