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// Copyright 2022 Google Inc. All Rights Reserved.
//
// Use of this source code is governed by a BSD-style license
// that can be found in the COPYING file in the root of the source
// tree. An additional intellectual property rights grant can be found
// in the file PATENTS. All contributing project authors may
// be found in the AUTHORS file in the root of the source tree.
// -----------------------------------------------------------------------------
//
// Gamma correction utilities.
#include "sharpyuv/sharpyuv_gamma.h"
#include <assert.h>
#include <math.h>
#include "src/webp/types.h"
// Gamma correction compensates loss of resolution during chroma subsampling.
// Size of pre-computed table for converting from gamma to linear.
#define GAMMA_TO_LINEAR_TAB_BITS 10
#define GAMMA_TO_LINEAR_TAB_SIZE (1 << GAMMA_TO_LINEAR_TAB_BITS)
static uint32_t kGammaToLinearTabS[GAMMA_TO_LINEAR_TAB_SIZE + 2];
#define LINEAR_TO_GAMMA_TAB_BITS 9
#define LINEAR_TO_GAMMA_TAB_SIZE (1 << LINEAR_TO_GAMMA_TAB_BITS)
static uint32_t kLinearToGammaTabS[LINEAR_TO_GAMMA_TAB_SIZE + 2];
static const double kGammaF = 1. / 0.45;
#define GAMMA_TO_LINEAR_BITS 16
static volatile int kGammaTablesSOk = 0;
void SharpYuvInitGammaTables(void) {
assert(GAMMA_TO_LINEAR_BITS <= 16);
if (!kGammaTablesSOk) {
int v;
const double a = 0.09929682680944;
const double thresh = 0.018053968510807;
const double final_scale = 1 << GAMMA_TO_LINEAR_BITS;
// Precompute gamma to linear table.
{
const double norm = 1. / GAMMA_TO_LINEAR_TAB_SIZE;
const double a_rec = 1. / (1. + a);
for (v = 0; v <= GAMMA_TO_LINEAR_TAB_SIZE; ++v) {
const double g = norm * v;
double value;
if (g <= thresh * 4.5) {
value = g / 4.5;
} else {
value = pow(a_rec * (g + a), kGammaF);
}
kGammaToLinearTabS[v] = (uint32_t)(value * final_scale + .5);
}
// to prevent small rounding errors to cause read-overflow:
kGammaToLinearTabS[GAMMA_TO_LINEAR_TAB_SIZE + 1] =
kGammaToLinearTabS[GAMMA_TO_LINEAR_TAB_SIZE];
}
// Precompute linear to gamma table.
{
const double scale = 1. / LINEAR_TO_GAMMA_TAB_SIZE;
for (v = 0; v <= LINEAR_TO_GAMMA_TAB_SIZE; ++v) {
const double g = scale * v;
double value;
if (g <= thresh) {
value = 4.5 * g;
} else {
value = (1. + a) * pow(g, 1. / kGammaF) - a;
}
kLinearToGammaTabS[v] =
(uint32_t)(final_scale * value + 0.5);
}
// to prevent small rounding errors to cause read-overflow:
kLinearToGammaTabS[LINEAR_TO_GAMMA_TAB_SIZE + 1] =
kLinearToGammaTabS[LINEAR_TO_GAMMA_TAB_SIZE];
}
kGammaTablesSOk = 1;
}
}
static WEBP_INLINE int Shift(int v, int shift) {
return (shift >= 0) ? (v << shift) : (v >> -shift);
}
static WEBP_INLINE uint32_t FixedPointInterpolation(int v, uint32_t* tab,
int tab_pos_shift_right,
int tab_value_shift) {
const uint32_t tab_pos = Shift(v, -tab_pos_shift_right);
// fractional part, in 'tab_pos_shift' fixed-point precision
const uint32_t x = v - (tab_pos << tab_pos_shift_right); // fractional part
// v0 / v1 are in kGammaToLinearBits fixed-point precision (range [0..1])
const uint32_t v0 = Shift(tab[tab_pos + 0], tab_value_shift);
const uint32_t v1 = Shift(tab[tab_pos + 1], tab_value_shift);
// Final interpolation.
const uint32_t v2 = (v1 - v0) * x; // note: v1 >= v0.
const int half =
(tab_pos_shift_right > 0) ? 1 << (tab_pos_shift_right - 1) : 0;
const uint32_t result = v0 + ((v2 + half) >> tab_pos_shift_right);
return result;
}
uint32_t SharpYuvGammaToLinear(uint16_t v, int bit_depth) {
const int shift = GAMMA_TO_LINEAR_TAB_BITS - bit_depth;
if (shift > 0) {
return kGammaToLinearTabS[v << shift];
}
return FixedPointInterpolation(v, kGammaToLinearTabS, -shift, 0);
}
uint16_t SharpYuvLinearToGamma(uint32_t value, int bit_depth) {
return FixedPointInterpolation(
value, kLinearToGammaTabS,
(GAMMA_TO_LINEAR_BITS - LINEAR_TO_GAMMA_TAB_BITS),
bit_depth - GAMMA_TO_LINEAR_BITS);
}
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