Smooth movement of player SFML

Question

void update(RenderWindow& window)
{
    if (Keyboard::isKeyPressed(Keyboard::W))
    {
        dy = -0.3;
    }
    if (Keyboard::isKeyPressed(Keyboard::A))
    {
        dx = -0.3;
    }
    if (Keyboard::isKeyPressed(Keyboard::S))
    {
        dy = 0.3;
    }
    if (Keyboard::isKeyPressed(Keyboard::D))
    {
        dx = 0.3;
    }
    
    x += dx;
    y += dy;

    dx = dy = 0;

    EntitySprite.setPosition(x, y);

    window.draw(EntitySprite);         
}

When the movement is described in such a code, the player moves angularly: either to the left, or up, or to the right, or down, or diagonally at an angle of 45 degrees by pressing two buttons together, for example S and D. Can this angle be made smoother so that the movement itself was carried out not only to the left, right, diagonally, etc.? My knowledge of geometry is not enough here, so I ask for your help).

Answer 1

In your code, the user input directly modifies the player position. This is probably the reason why the player movement looks so abrupt to you. Technically, in your code, the user input is what is determining the player velocity at any given moment.

A more realistic approach would be to let the player have a velocity property instead – that represents the player position's rate of change – and then have the player position updated only through this velocity, not directly from the user input. Instead, the velocity would be what is directly modified by the input, but not entirely determined by the current input as it will also depend on its previous value.

Following this approach, the user input is used to calculate the player acceleration in each call to update(). This acceleration – the rate of change of the velocity – is used to update the player velocity directly. Finally, the player velocity is, in turn, used to update the player position.

The following code implements this approach by introducing the velocity_ data member and the acceleration local variable:

void update(RenderWindow& window) {
    sf::Vector2f acceleration;

    // adjust this at will
    const float dAcc = 0.3f;

    // set acceleration
    if (Keyboard::isKeyPressed(Keyboard::W))
      acceleration.y -= dAcc;
    if (Keyboard::isKeyPressed(Keyboard::A))
      acceleration.x -= dAcc;
    if (Keyboard::isKeyPressed(Keyboard::S))
      acceleration.y += dAcc;
    if (Keyboard::isKeyPressed(Keyboard::D))
      acceleration.x += dAcc;

    // update velocity through acceleration
    velocity_ += acceleration;
    
    // update position through velocity
    x += velocity_.x;
    y += velocity_.y;

    // apply damping to the velocity
    velocity_ = 0.99f * velocity_;

    EntitySprite.setPosition(x, y);

    window.draw(EntitySprite);
};

This way, the player possesses some kind of inertia, and its movement looks smoother.

Note that you may want to have some damping for the velocity as in:

velocity_ = 0.99f * velocity_;

This will resemble the effect of drag forces.