When moving a circle in sfml c++ a stuttery contrail glitches behind the ball

Question

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

using namespace std;
using namespace sf;

int main()
{

    RenderWindow window(VideoMode(800, 600), "Lolyan", Style::Default);

    Event event;

    CircleShape circle(30.0f, 30.0f);
    circle.setFillColor(Color::Blue);
    circle.setPosition(40, 530);

    window.setFramerateLimit(200);

    window.draw(circle);
    window.display();

    int velocityX = 0, velocityY = 0;
    Vector2i mousePosition;

    while (window.isOpen()) {


        circle.move(velocityX / 2, velocityY / 2);


        window.draw(circle);


        window.display();


        while (window.pollEvent(event)) {

            window.clear(Color::Black);



            if (event.type == Event::Closed) {
                window.close();
            }


            else if (event.type == Event::KeyPressed ) {

                cout << velocityX << endl;

                switch (event.key.code) {

                case Keyboard::A:
                    velocityX = -5;
                    break;

                case Keyboard::D:
                    velocityX = 5;
                    break;

                case Keyboard::S:
                    velocityY = 5;
                    break;

                case Keyboard::W:
                    velocityY = -5;
                    break;
                }

            }


            else if (event.type == Event::MouseButtonPressed) {
                circle.setFillColor(Color::Red);
            }


            else if (event.type == Event::MouseButtonReleased) {
                circle.setFillColor(Color::Blue);

            }

            else if (event.type == Event::MouseMoved) {
                mousePosition = Mouse::getPosition(window);
                circle.setPosition(mousePosition.x - 30, mousePosition.y - 30);

            }

            else if (event.type == Event::KeyReleased) {
                velocityX = 0;
                velocityY = 0;
            }






            window.display();

        }


    }

    return 0;
}

So this is not the first method I tried to move the ball. The first method is every event when a key is pressed the x or y changed, but with this method the movement wasn't as smooth as I want so I tried this method where when example: I press w the velocity (velocityY) becomes 5 and every frame of the game the position changes. And it works smoothly but a glitchy contrail runs before the ball disapering and reappearing sometimes. I found out to that when i move the mouse in the window (knowing that I implemented an event that takes mouse movement) the ball moves very smoothly.

You are clearing the window inside the event loop. It might be better to move it outside. Put it immediately before you start drawing the circle. — Galik
Also you call window.display() in the event loop and also outside. You should call that only once. windoe.clear(); /* draw window stuff */ window.display();. — Galik
It is probably best to work from an example in the instructions and modify that: sfml-dev.org/documentation/2.5.1/… — Galik
thanks @Galik as you said I was forgetting window.clear() Thanks again. — Nicolas Azzi

alseether alseether · Accepted Answer · 2019-03-13T12:37:58

Extending the answer in comments, a proper implementation of a game loop with sfml should look like follows:

while (window.isOpen())
{
   // Event processing
   sf::Event event;
   while (window.pollEvent(event))
   {
       // Request for closing the window
       if (event.type == sf::Event::Closed)
           window.close();
   }

   // Clear the whole window before rendering a new frame
   window.clear();

   // Draw some graphical entities
   window.draw(sprite);
   window.draw(circle);
   window.draw(text);

   // End the current frame and display its contents on screen
   window.display();
}

^{Code extracted from SFML tutorials}

But, since you're working with physics, I recommend you this answer about setting a constant game tick, to avoid that kind of glitches when elements collide.

When moving a circle in sfml c++ a stuttery contrail glitches behind the ball

1 Answers