C++ SFML，轨道运行

Question

I recently started to learn SFML, and I have a question, how to make what would be the second body moving in an orbit, help please.

#include <SFML/Graphics.hpp>
using namespace sf;
int main()
{
   RenderWindow window(VideoMode(800, 600), "Hello, world!");

    CircleShape shape(50.f);
    shape.setFillColor(Color::Black);
    shape.setPosition(400,300);
    shape.setOrigin(50,50);

    CircleShape shape2(10.f);
    shape2.setFillColor(Color::Black);
    shape2.setPosition(700,500);
    shape2.setOrigin(10,10);

    while (window.isOpen())
    {
        Event event;
        while (window.pollEvent(event)) 
        {       
            if (event.type == sf::Event::Closed)        
                window.close();

        }
        window.clear(Color::White);
        window.draw(shape);
        window.draw(shape2);
        window.display();
    }
    return 0;
}

Answer 1

Well... I will not post full solution. It would not be educational to give you complete code. But I will give you some hints :) .

1. Your world update should happen in the loop. In the while loop. You have two there. Which one do you think is the one which updates your world?
2. Circle equation in Cartesian coordinate system is: (xa)^2 + (yb)^2 = r^2
3. In the loop from 1 you should use equation from 2 to update coordinates of second object (shape2).
4. To perform action from point 3 you have two possibilities: function setPosition and function move , both members of class CircleShape .

If you have further questions ask them in the comments.

For your future questions on stack: give us proof that you put some effort in resolving problem. If question looks like the one I'm answering now we have the idea that you did not think about it just posted it here and you are waiting for someone to write your code for you.

Answer 2

The easiest and i think most precise way would be to use two perpendicular sine waves with the origin being your well origin and the amplitude being the distance for which you want to orbit.

#include <cmath> -- sin(), cos functions

As you probably know sin goes from 0 to 1 then -1 and it repeats. cos is identical but shifted. If you use both for the position then you're left with the sine and cosine values of a given number. If you use one for 'x' and the other for 'y' you're left with circular motion (perpendicular waves). Try out the code below:

int main()
{

    CircleShape shape(50.f);
    shape.setFillColor(Color::Black);
    shape.setPosition(400,300);
    shape.setOrigin(50,50);

    CircleShape shape2(10.f);
    shape2.setFillColor(Color::Black);
    shape2.setPosition(700,500);
    shape2.setOrigin(10,10);

    CircleShape shape3(10.f);
    shape3.setFillColor(Color::Black);
    shape3.setPosition(700,500);
    shape3.setOrigin(10,10);

    CircleShape shape4(10.f);
    shape4.setFillColor(Color::Black);
    shape4.setPosition(700,500);
    shape4.setOrigin(10,10);


    float speed = 0.01;
    float distance = 100;
    // create the window
    sf::RenderWindow window(sf::VideoMode(800, 600), "My window");
    window.setFramerateLimit(60);
    float counter = 0;
    // run the program as long as the window is open
    while (window.isOpen())
    {
        // check all the window's events that were triggered since the last iteration of the loop
        sf::Event event;
        while (window.pollEvent(event))
        {
            // "close requested" event: we close the window
            if (event.type == sf::Event::Closed)
                window.close();
        }
        counter += speed;
        // clear the window with black color
        window.clear(Color::White);
        shape2.setPosition(shape.getPosition().x+distance*sin(counter),shape.getPosition().y+distance*cos(counter));
        shape3.setPosition(shape.getPosition().x+distance*sin(counter),shape.getPosition().y+distance*sin(counter));
        shape4.setPosition(shape.getPosition().x+2*distance*sin(counter),shape.getPosition().y+distance*cos(counter));

        // draw everything here...
        // window.draw(...);
        window.draw(shape);
        window.draw(shape2);
        window.draw(shape3);
        window.draw(shape4);
        // end the current frame
        window.display();
    }
    return 0;
}

As you can see shape2 is oribitng shape1. Shape2 and shape3's position is dependant on the position of shape1 (our origin) as well as the sine and cosine values generated. I marked distinct places in the code with variables to show you the possibilities you can have with this setup. Let's go through the setposition function:

For both x and y we need a base position from which the shape will orbit. we use shape1's position. The sine and cos need some type of number input from which to generate a value. I just used a normal iterator that grows every frame by 'speed' amount. the more it is the faster the sine will repeat thus faster orbiting around the shape1. The next part is the addition of the sine and cos values to the origin. this is where things get fun. we're essentially adding the sine value to the position to move it by that much in any direction. 'Distance' is how much we multiply the sine cos values. Thus bigger distance = further away.

Note:

• using sine for x and cos for y you get a normal circular orbit.
• using sine for both x and y you get a 3D-looking orbit to and from the screen (shape3)
• multiplying sine and cos value additions by a non identical number will result in eliptical rotations (shape4)