Movement of sprite using fixed time step

Question

I have this loop:

sf::Clock clock;
sf::Time timeSinceLastUpdate = sf::Time::Zero;
while (mWindow.isOpen())
{
    processEvents();
    timeSinceLastUpdate += clock.restart();

    while (timeSinceLastUpdate > TimePerFrame)
    {
        timeSinceLastUpdate -= TimePerFrame;
        processEvents();
        update(TimePerFrame);
    }
    render();
}

and in update function I do this:

object.speed = object.speed * TimePerFrame.asSeconds();

and then I run a method in which I do:

sprite.move(cos(sprite.getRotation()*PI / 180) * speed, sin(sprite.getRotation()*PI / 180) * speed);

But the problem is that the sprite doesn't move. When I don't multiply speed by TimePerFrame.asSeconds(), it does move. How should I fix it so everything is correct? Where and how to use TimePerFrame variable?

Answer 1

object.speed = object.speed * TimePerFrame.asSeconds();

This line is reducing the speed every frame until it is 0.

You need to scale the movement distance by delta time whenever you move the object. Try this:

float distance = speed * TimePerFrame.asSeconds(); 
sprite.move(cos(sprite.getRotation()*PI / 180) * distance, sin(sprite.getRotation()*PI / 180) * distance);