gba_shader.cpp

// SOURCE: https://github.com/nba-emu/NanoBoyAdvance/blob/master/src/platform/core/src/device/shader/color_agb.glsl.hpp
// GODBOLT: https://godbolt.org/z/1cfaedavK

#include <cstdint>
#include <cmath>
#include <algorithm>

uint32_t gba_shader(uint16_t bgr555) {
    constexpr auto darken_screen = 1.0;
    constexpr auto target_gamma = 2.2;
    constexpr auto display_gamma = 2.2;
    constexpr auto lum = 0.94;
    constexpr auto rr = 0.82;
    constexpr auto rg = 0.125;
    constexpr auto rb = 0.195;
    constexpr auto gr = 0.24;
    constexpr auto gg = 0.665;
    constexpr auto gb = 0.075;
    constexpr auto br = -0.06;
    constexpr auto bg = 0.21;
    constexpr auto bb = 0.73;

    const auto R = (bgr555 >> 0) & 31;
    const auto G = (bgr555 >> 5) & 31;
    const auto B = (bgr555 >> 10) & 31;

    const auto lr1 = std::pow(R / 31.0, target_gamma + darken_screen);
    const auto lg1 = std::pow(G / 31.0, target_gamma + darken_screen);
    const auto lb1 = std::pow(B / 31.0, target_gamma + darken_screen);

    // apply lum and clamp
    const auto lr2 = std::clamp(lr1 * lum, 0.0, 1.0);
    const auto lg2 = std::clamp(lg1 * lum, 0.0, 1.0);
    const auto lb2 = std::clamp(lb1 * lum, 0.0, 1.0);

    // colour
    const auto lr3 = std::pow(rr * lr2 + gr * lg2 + br * lb2, 1.0 / display_gamma);
    const auto lg3 = std::pow(rg * lr2 + gg * lg2 + bg * lb2, 1.0 / display_gamma);
    const auto lb3 = std::pow(rb * lr2 + gb * lg2 + bb * lb2, 1.0 / display_gamma);

    const uint8_t rout = lr3 * 255;
    const uint8_t gout = lg3 * 255;
    const uint8_t bout = lb3 * 255;

    return (rout << 24) | (gout << 16) | (bout << 8);
}