対称定規

# include <Siv3D.hpp>

void Main()
{
    Window::Resize(1280, 720);

    const int32 N = 12;

    Image image(Window::Size(), Color(20, 40, 60));

    DynamicTexture texture(image);

    Mouse::SetTransform(Mat3x2::Translate(Window::Center()));

    while (System::Update())
    {
        if (Input::MouseL.pressed)
        {
            for (auto i : step(N))
            {
                Circular cs[2] = { Input::MouseL.clicked ? Mouse::Pos() : Mouse::PreviousPos(), Mouse::Pos() };

                for (auto& c : cs)
                {
                    c.theta = i % 2 ? -c.theta - TwoPi / N * (i - 1) : c.theta + TwoPi / N * i;
                }

                Line(cs[0], cs[1]).moveBy(Window::Center()).overwrite(image, 2, HSV(System::FrameCount(), 0.5, 1.0));
            }

            texture.tryFill(image);
        }
        else if (Input::MouseR.clicked)
        {
            image.fill(Color(20, 40, 60));

            texture.fill(image);
        }

        texture.draw();
    }
}