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128 lines
4.3 KiB
C++
128 lines
4.3 KiB
C++
/*
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* Copyright (c) 2020, the SerenityOS developers.
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*
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* SPDX-License-Identifier: BSD-2-Clause
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*/
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#pragma once
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#include "Color.h"
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#include <math.h>
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#include <xmmintrin.h>
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#include <AK/SIMD.h>
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#define GAMMA 2.2
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// Most computer graphics are stored in the sRGB color space, which stores something close to
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// the square root of the display intensity of each color channel. This is problematic for most
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// operations that we want to perform on colors, since they typically assume that color scales
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// linearly (e.g. rgb(127, 0, 0) is half as bright as rgb(255, 0, 0)). This causes incorrect
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// results that look more gray than they should, to fix this we have to convert colors to the linear
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// color space before performing these operations, then convert back before displaying.
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//
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// Conversion between linear and sRGB spaces are somewhat expensive to do on the CPU, so we instead
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// interpret sRGB colors as gamma2.2 colors, which are close enough in most cases to be indistinguishable.
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// Gamma 2.2 colors follow the simple rule of `display_intensity = pow(stored_intensity, 2.2)`.
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// This module implements some fast color space transforms between the gamma2.2 and linear color spaces, plus
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// some common primitive operations like blending.
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//
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// For a more in-depth overview of how gamma-adjustment works, check out:
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// https://blog.johnnovak.net/2016/09/21/what-every-coder-should-know-about-gamma/
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namespace Gfx {
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using AK::SIMD::f32x4;
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#ifndef NO_FPU
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# ifdef __SSE__
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// Transform f32x4 from gamma2.2 space to linear space
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// Assumes x is in range [0, 1]
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// FIXME: Remove this hack once clang-11 is available as the default in Github Actions.
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// This is apparently sometime mid-December. https://github.com/actions/virtual-environments/issues/2130
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# if !defined(__clang__) || __clang_major__ >= 11
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constexpr f32x4 gamma_to_linear4(f32x4 x)
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# else
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inline f32x4 gamma_to_linear4(f32x4 x)
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# endif
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{
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return (0.8f + 0.2f * x) * x * x;
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}
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// Transform f32x4 from linear space to gamma2.2 space
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// Assumes x is in range [0, 1]
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inline f32x4 linear_to_gamma4(f32x4 x)
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{
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// Source for approximation: https://mimosa-pudica.net/fast-gamma/
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constexpr float a = 0.00279491f;
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constexpr float b = 1.15907984f;
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float c = (b / sqrtf(1.0f + a)) - 1;
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return ((b * __builtin_ia32_rsqrtps(x + a)) - c) * x;
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}
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// Linearize v1 and v2, lerp them by mix factor, then convert back.
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// The output is entirely v1 when mix = 0 and entirely v2 when mix = 1
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inline f32x4 gamma_accurate_lerp4(f32x4 v1, f32x4 v2, float mix)
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{
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return linear_to_gamma4(gamma_to_linear4(v1) * (1 - mix) + gamma_to_linear4(v2) * mix);
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}
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# endif
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// Transform scalar from gamma2.2 space to linear space
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// Assumes x is in range [0, 1]
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constexpr float gamma_to_linear(float x)
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{
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return (0.8f + 0.2f * x) * x * x;
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}
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// Transform scalar from linear space to gamma2.2 space
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// Assumes x is in range [0, 1]
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inline float linear_to_gamma(float x)
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{
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// Source for approximation: https://mimosa-pudica.net/fast-gamma/
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constexpr float a = 0.00279491;
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constexpr float b = 1.15907984;
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float c = (b / sqrtf(1 + a)) - 1;
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return ((b / __builtin_sqrtf(x + a)) - c) * x;
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}
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// Linearize v1 and v2, lerp them by mix factor, then convert back.
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// The output is entirely v1 when mix = 0 and entirely v2 when mix = 1
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inline float gamma_accurate_lerp(float v1, float v2, float mix)
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{
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return linear_to_gamma(gamma_to_linear(v1) * (1 - mix) + gamma_to_linear(v2) * mix);
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}
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// Convert a and b to linear space, blend them by mix factor, then convert back.
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// The output is entirely a when mix = 0 and entirely b when mix = 1
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inline Color gamma_accurate_blend(Color a, Color b, float mix)
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{
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# ifdef __SSE__
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f32x4 ac = {
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(float)a.red(),
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(float)a.green(),
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(float)a.blue(),
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};
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f32x4 bc = {
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(float)b.red(),
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(float)b.green(),
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(float)b.blue(),
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};
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f32x4 out = 255.f * gamma_accurate_lerp4(ac * (1.f / 255.f), bc * (1.f / 255.f), mix);
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return Color(out[0], out[1], out[2]);
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# else
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return {
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static_cast<u8>(255.f * gamma_accurate_lerp(a.red() / 255.f, b.red() / 255.f, mix)),
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static_cast<u8>(255.f * gamma_accurate_lerp(a.green() / 255.f, b.green() / 255.f, mix)),
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static_cast<u8>(255.f * gamma_accurate_lerp(a.blue() / 255.f, b.blue() / 255.f, mix)),
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};
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# endif
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}
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#endif
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}
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