Butt generation

sf::Image img;
img.create(181 * 2, 181 * 2);
const auto img_s = img.getSize();
const auto center = img_s / 2u;

for(auto abs_y = 0u; abs_y < img_s.y; ++abs_y)
  for(auto abs_x = 0u; abs_x < img_s.x; ++abs_x) {
    const auto y = static_cast<signed int>(abs_y) - static_cast<signed int>(center.y);
    const auto x = static_cast<signed int>(abs_x) - static_cast<signed int>(center.x);
    const auto r = std::sqrt(x * x + y * y);
    const auto g = (std::abs(std::atan2(y, x)) / (2 * std::acos(0))) * 255;
    const auto luma = static_cast<std::uint8_t>(std::sqrt(r * r + g * g));
    img.setPixel(abs_x, abs_y, {luma, luma, luma});
  }

img.saveToFile("output.png");