How to simulate a lighter effect like Canvas globalCompositeOperation does? - c++

Question

Every end of year I make a fireworks display Canvas effect (Javascript), the last was:
http://js1k.com/2011-dysentery/demo/955
This effect is simple: a lot of circles filled with createRadialGradient() method, with some transparency. The secret, is the property:
globalCompositeOperation = "lighter";
When used, all the colors are mixed giving the sensation of light. This year, I want to make the same, but in SFML/C++. I'm begginer in SFML and have no idea where to start making this effect. Can anyone show me how to simulate Canvas globalCompositeOperation() in SFML?

Answer 1

You can get this effect with a sf::Blend::Mode. See this example:
#include <SFML/Graphics.hpp>
int main()
{
sf::RenderWindow window(sf::VideoMode(400, 400), "Blend::Mode Sample");
sf::Shape square = sf::Shape::Rectangle(100, 100, 200, 200, sf::Color(250, 30, 30));
sf::Shape circle1 = sf::Shape::Circle(100, 100, 50, sf::Color(30, 30, 250));
sf::Shape circle2 = sf::Shape::Circle(300, 100, 50, sf::Color(30, 30, 250));
sf::Shape circle3 = sf::Shape::Circle(100, 300, 50, sf::Color(30, 30, 250));
sf::Shape circle4 = sf::Shape::Circle(300, 300, 50, sf::Color(30, 30, 250));
circle1.SetBlendMode(sf::Blend::Alpha);
circle2.SetBlendMode(sf::Blend::Add);
circle3.SetBlendMode(sf::Blend::Multiply);
circle4.SetBlendMode(sf::Blend::None);
while (window.IsOpened())
{
sf::Event event;
while (window.PollEvent(event))
if (event.Type == sf::Event::Closed)
window.Close();
window.Clear();
window.Draw(square);
window.Draw(circle1);
window.Draw(circle2);
window.Draw(circle3);
window.Draw(circle4);
window.Display();
}
}
The output:
I think the effect you want would be SetBlendMode(sf::Blend::Add).