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