zsbzsb.cpp

#include <memory>
#include <SFML/Graphics.hpp>

int main()
{
	sf::RenderWindow window{{800, 600}, "Backface Calculations for zsbzsb"};
	sf::CircleShape light_point(5.f);
	light_point.setOrigin({5.f, 5.f});
	light_point.setPosition(static_cast<sf::Vector2f>(window.getSize())/2.f);

	sf::CircleShape* circle_octogon = new sf::CircleShape(50.f, 8);
	sf::VertexArray octogon(sf::TrianglesFan, 8);
	for(int i{0}; i<circle_octogon->getPointCount(); ++i)
	{
		octogon[i] = sf::Vertex(circle_octogon->getPoint(i), sf::Color::Green);
	}
	delete circle_octogon;
	auto default_octogon = octogon;
	sf::VertexArray lines(sf::Lines, 4);

	while(window.isOpen())
	{
		sf::Event event;
		while(window.pollEvent(event))
		{
			switch(event.type)
			{
				case sf::Event::Closed:
					window.close();
			}
		}

		auto mouse_pos = static_cast<sf::Vector2f>(sf::Mouse::getPosition(window));

		// Generate lines
		{
			auto light_pos = light_point.getPosition();
			auto vec_perp = mouse_pos-light_pos;
			std::swap(vec_perp.x, vec_perp.y);
			vec_perp.x = -vec_perp.x;
			lines[0] = sf::Vertex( vec_perp+mouse_pos, sf::Color::Blue);
			lines[1] = sf::Vertex(-vec_perp+mouse_pos, sf::Color::Blue);
			lines[2] = sf::Vertex( light_pos, sf::Color::Red);
			lines[3] = sf::Vertex( mouse_pos, sf::Color::Red);
		}
		// Calculate backface vectors
		{
			for(int i{0}; i < default_octogon.getVertexCount(); ++i)
			{
				//Reset shape
				octogon[i] = default_octogon[i];
				//Translate shape
				octogon[i].position += mouse_pos - sf::Vector2f{50.f, 50.f};

				//lines[0] and lines[1] are our perpendicular line
				//Get the side of the vertex
				bool side = std::signbit(
					(lines[1].position.x-lines[0].position.x)*(octogon[i].position.y-lines[0].position.y)
					-
					(lines[1].position.y-lines[0].position.y)*(octogon[i].position.x-lines[0].position.x)
				);

				// If side is the same, set backface vector to red
				if(side == std::signbit(mouse_pos.x))
					octogon[i].color = sf::Color::Red;

			}
		}

		window.clear();
		window.draw(octogon);
		window.draw(light_point);
		window.draw(lines);
		window.display();
	}
	return 0;
}