How do I implement deceleration?

Question

I say "deceleration" because I'm not using acceleration at the moment; what I mean is moving the velocity back towards zero, eventually stopping.

I'm new to vectors and not so great with physics and such. How is "deceleration" normally handled?

---

What I have now works, but it seems kind of hackish.

update:function(Game, t, dt) {
    var speed = Game.Input.isKeyDown('shift') ? 8 : 4;

    if (Game.Input.isKeyDown('a')) {
        this.velocity.i -= speed;
    }
    else if (Game.Input.isKeyDown('d')) {
        this.velocity.i += speed;
    }
    else {
        if (Math.abs(this.velocity.i) > 3) {
            this.velocity.i += (this.velocity.i > 0) ? -speed : speed;
        }
        else {
            this.velocity.i = 0;
        }
    }

    if (Game.Input.isKeyDown('w')) {
        this.velocity.j -= speed;
    }
    else if (Game.Input.isKeyDown('s')) {
        this.velocity.j += speed;
    }
    else {
        if (Math.abs(this.velocity.j) > 3) {
            this.velocity.j += (this.velocity.j > 0) ? -speed : speed;
        }
        else {
            this.velocity.j = 0;
        }
    }

    this.updateVectors(dt);
}

I used 3 because anything lower exhibits weird behaviour, I'm guessing if I raised the speed then it would need to be changed.

Answer 1

Something as simple as

this.velocity.i *= 0.9;

works nicely.

Answer 2

In Pseudocode, I do variations of this:

Speed += ((MoveDirection * MaximumSpeed) - Speed) * AccelerationFactor

Where:

• Speed is the current speed the entity is travelling at on the current axis.
• MoveDirection is the direction the entity is trying to travel in on the current axis, 1 is forward, 0 is still and -1 is backwards. All values in between are allowed.
• MaximumSpeed is a constant determining the fastest that the entity can travel on the current axis.
• AccelerationFactor is a constant between 0 and 1 that represents the rate of acceleration and deceleration. 1 is instant, and 0 will never move.

Which handles both Acceleration and Deceleration nicely in a curved, rather than line. If you want different acceleration and deceleration rates you could do IF statements that determine whether the player is attempting to not move or move in the opposite direction.

Answer 3

The answers here (vel = vel * 0.9) are actually damping, not what I would consider 'deceleration'.

I often do deceleration like this:

if ( Game.Input.isKeyDown( "w" ) )
{
    this.velocity.i = Math.max( -WALKSPEED, this.velocity.i - WALKFORCE);
}
else if ( Game.Input.isKeyDown( "d" ) )
{
    this.velocity.i = Math.min( WALKSPEED, this.velocity.i + WALKFORCE);
}
else
{
    if (this.velocity.i < 0)
    {
        this.velocity.i = Math.min( 0, this.velocity.i + WALKFORCE);
    }
    else if (this.velocity.i > 0)
    {
        this.velocity.i = Math.max( 0, this.velocity.i - WALKFORCE);
    }
}

Some pros and cons vs. damping:

Pros:

• The speeding up acceleration and the slowing down acceleration are both linear, which gives a pleasant subtle 'game feel' that I think damping doesn't provide. This is the important part.
• The character comes to a predictable complete stop after a predictable number of iterations.

Cons:

• This is trickier to implement if you are using non-orthogonal motion (which it sounds like you are?) Basically, you have to get a force vector that is aligned with the velocity, compare lengths, and subtract or zero-out as approrpriate. (If you want that explained with code, just ask.)

Answer 4

Very simply put, in pseudo code:

if(no movement keys pressed) [Meaning we want to start to decelerate]
current speed *= 0.85 [Or some number between 0 and 1, the smaller the faster the deceleration]

However, you would need to check if(current speed < 0.001f) or something and set to 0.