enc_photon_noise.cc (3379B)
1 // Copyright (c) the JPEG XL Project Authors. All rights reserved. 2 // 3 // Use of this source code is governed by a BSD-style 4 // license that can be found in the LICENSE file. 5 6 #include "lib/jxl/enc_photon_noise.h" 7 8 #include <algorithm> 9 10 #include "lib/jxl/cms/opsin_params.h" 11 12 namespace jxl { 13 14 namespace { 15 16 // Assumes a daylight-like spectrum. 17 // https://www.strollswithmydog.com/effective-quantum-efficiency-of-sensor/#:~:text=11%2C260%20photons/um%5E2/lx-s 18 constexpr float kPhotonsPerLxSPerUm2 = 11260; 19 20 // Order of magnitude for cameras in the 2010-2020 decade, taking the CFA into 21 // account. 22 constexpr float kEffectiveQuantumEfficiency = 0.20; 23 24 // TODO(sboukortt): reevaluate whether these are good defaults, notably whether 25 // it would be worth making read noise higher at lower ISO settings. 26 constexpr float kPhotoResponseNonUniformity = 0.005; 27 constexpr float kInputReferredReadNoise = 3; 28 29 // Assumes a 35mm sensor. 30 constexpr float kSensorAreaUm2 = 36000.f * 24000; 31 32 template <typename T> 33 inline constexpr T Square(const T x) { 34 return x * x; 35 } 36 template <typename T> 37 inline constexpr T Cube(const T x) { 38 return x * x * x; 39 } 40 41 } // namespace 42 43 NoiseParams SimulatePhotonNoise(const size_t xsize, const size_t ysize, 44 const float iso) { 45 const float kOpsinAbsorbanceBiasCbrt = 46 std::cbrt(jxl::cms::kOpsinAbsorbanceBias[1]); 47 48 // Focal plane exposure for 18% of kDefaultIntensityTarget, in lx·s. 49 // (ISO = 10 lx·s ÷ H) 50 const float h_18 = 10 / iso; 51 52 const float pixel_area_um2 = kSensorAreaUm2 / (xsize * ysize); 53 54 const float electrons_per_pixel_18 = kEffectiveQuantumEfficiency * 55 kPhotonsPerLxSPerUm2 * h_18 * 56 pixel_area_um2; 57 58 NoiseParams params; 59 60 for (size_t i = 0; i < NoiseParams::kNumNoisePoints; ++i) { 61 const float scaled_index = i / (NoiseParams::kNumNoisePoints - 2.f); 62 // scaled_index is used for XYB = (0, 2·scaled_index, 2·scaled_index) 63 const float y = 2 * scaled_index; 64 // 1 = default intensity target 65 const float linear = std::max(0.f, Cube(y - kOpsinAbsorbanceBiasCbrt) + 66 jxl::cms::kOpsinAbsorbanceBias[1]); 67 const float electrons_per_pixel = electrons_per_pixel_18 * (linear / 0.18f); 68 // Quadrature sum of read noise, photon shot noise (sqrt(S) so simply not 69 // squared here) and photo response non-uniformity. 70 // https://doi.org/10.1117/3.725073 71 // Units are electrons rms. 72 const float noise = 73 std::sqrt(Square(kInputReferredReadNoise) + electrons_per_pixel + 74 Square(kPhotoResponseNonUniformity * electrons_per_pixel)); 75 const float linear_noise = noise * (0.18f / electrons_per_pixel_18); 76 const float opsin_derivative = 77 (1.f / 3) / 78 Square(std::cbrt(linear - jxl::cms::kOpsinAbsorbanceBias[1])); 79 const float opsin_noise = linear_noise * opsin_derivative; 80 81 // TODO(sboukortt): verify more thoroughly whether the denominator is 82 // correct. 83 params.lut[i] = 84 Clamp1(opsin_noise / 85 (0.22f // norm_const 86 * std::sqrt(2.f) // red_noise + green_noise 87 * 1.13f // standard deviation of a plane of generated noise 88 ), 89 0.f, 1.f); 90 } 91 92 return params; 93 } 94 95 } // namespace jxl