1// Copyright 2022 Google Inc. All Rights Reserved.
2//
3// Use of this source code is governed by a BSD-style license
4// that can be found in the COPYING file in the root of the source
5// tree. An additional intellectual property rights grant can be found
6// in the file PATENTS. All contributing project authors may
7// be found in the AUTHORS file in the root of the source tree.
8// -----------------------------------------------------------------------------
9//
10// Gamma correction utilities.
11
12#include "sharpyuv/sharpyuv_gamma.h"
13
14#include <assert.h>
15#include <math.h>
16
17#include "src/webp/types.h"
18
19// Gamma correction compensates loss of resolution during chroma subsampling.
20// Size of pre-computed table for converting from gamma to linear.
21#define GAMMA_TO_LINEAR_TAB_BITS 10
22#define GAMMA_TO_LINEAR_TAB_SIZE (1 << GAMMA_TO_LINEAR_TAB_BITS)
23static uint32_t kGammaToLinearTabS[GAMMA_TO_LINEAR_TAB_SIZE + 2];
24#define LINEAR_TO_GAMMA_TAB_BITS 9
25#define LINEAR_TO_GAMMA_TAB_SIZE (1 << LINEAR_TO_GAMMA_TAB_BITS)
26static uint32_t kLinearToGammaTabS[LINEAR_TO_GAMMA_TAB_SIZE + 2];
27
28static const double kGammaF = 1. / 0.45;
29#define GAMMA_TO_LINEAR_BITS 16
30
31static volatile int kGammaTablesSOk = 0;
32void SharpYuvInitGammaTables(void) {
33 assert(GAMMA_TO_LINEAR_BITS <= 16);
34 if (!kGammaTablesSOk) {
35 int v;
36 const double a = 0.09929682680944;
37 const double thresh = 0.018053968510807;
38 const double final_scale = 1 << GAMMA_TO_LINEAR_BITS;
39 // Precompute gamma to linear table.
40 {
41 const double norm = 1. / GAMMA_TO_LINEAR_TAB_SIZE;
42 const double a_rec = 1. / (1. + a);
43 for (v = 0; v <= GAMMA_TO_LINEAR_TAB_SIZE; ++v) {
44 const double g = norm * v;
45 double value;
46 if (g <= thresh * 4.5) {
47 value = g / 4.5;
48 } else {
49 value = pow(x: a_rec * (g + a), y: kGammaF);
50 }
51 kGammaToLinearTabS[v] = (uint32_t)(value * final_scale + .5);
52 }
53 // to prevent small rounding errors to cause read-overflow:
54 kGammaToLinearTabS[GAMMA_TO_LINEAR_TAB_SIZE + 1] =
55 kGammaToLinearTabS[GAMMA_TO_LINEAR_TAB_SIZE];
56 }
57 // Precompute linear to gamma table.
58 {
59 const double scale = 1. / LINEAR_TO_GAMMA_TAB_SIZE;
60 for (v = 0; v <= LINEAR_TO_GAMMA_TAB_SIZE; ++v) {
61 const double g = scale * v;
62 double value;
63 if (g <= thresh) {
64 value = 4.5 * g;
65 } else {
66 value = (1. + a) * pow(x: g, y: 1. / kGammaF) - a;
67 }
68 kLinearToGammaTabS[v] =
69 (uint32_t)(final_scale * value + 0.5);
70 }
71 // to prevent small rounding errors to cause read-overflow:
72 kLinearToGammaTabS[LINEAR_TO_GAMMA_TAB_SIZE + 1] =
73 kLinearToGammaTabS[LINEAR_TO_GAMMA_TAB_SIZE];
74 }
75 kGammaTablesSOk = 1;
76 }
77}
78
79static WEBP_INLINE int Shift(int v, int shift) {
80 return (shift >= 0) ? (v << shift) : (v >> -shift);
81}
82
83static WEBP_INLINE uint32_t FixedPointInterpolation(int v, uint32_t* tab,
84 int tab_pos_shift_right,
85 int tab_value_shift) {
86 const uint32_t tab_pos = Shift(v, shift: -tab_pos_shift_right);
87 // fractional part, in 'tab_pos_shift' fixed-point precision
88 const uint32_t x = v - (tab_pos << tab_pos_shift_right); // fractional part
89 // v0 / v1 are in kGammaToLinearBits fixed-point precision (range [0..1])
90 const uint32_t v0 = Shift(v: tab[tab_pos + 0], shift: tab_value_shift);
91 const uint32_t v1 = Shift(v: tab[tab_pos + 1], shift: tab_value_shift);
92 // Final interpolation.
93 const uint32_t v2 = (v1 - v0) * x; // note: v1 >= v0.
94 const int half =
95 (tab_pos_shift_right > 0) ? 1 << (tab_pos_shift_right - 1) : 0;
96 const uint32_t result = v0 + ((v2 + half) >> tab_pos_shift_right);
97 return result;
98}
99
100uint32_t SharpYuvGammaToLinear(uint16_t v, int bit_depth) {
101 const int shift = GAMMA_TO_LINEAR_TAB_BITS - bit_depth;
102 if (shift > 0) {
103 return kGammaToLinearTabS[v << shift];
104 }
105 return FixedPointInterpolation(v, tab: kGammaToLinearTabS, tab_pos_shift_right: -shift, tab_value_shift: 0);
106}
107
108uint16_t SharpYuvLinearToGamma(uint32_t value, int bit_depth) {
109 return FixedPointInterpolation(
110 v: value, tab: kLinearToGammaTabS,
111 tab_pos_shift_right: (GAMMA_TO_LINEAR_BITS - LINEAR_TO_GAMMA_TAB_BITS),
112 tab_value_shift: bit_depth - GAMMA_TO_LINEAR_BITS);
113}
114

source code of qtimageformats/src/3rdparty/libwebp/sharpyuv/sharpyuv_gamma.c