Understanding the Seek steering behavior

Question

I'm currently studying steering behaviors. Have recently learned about Seek.

I have a question about this.

Seek is supposed to turn the agent slowly in the direction of it's target, and then move in that direction.

However, I don't understand how Seek turns the agent around slowly. It seems to me like it should make the agent change direction immediately.

This is the code for a Seek function:

Vector Seek(Point target){

    Vector toTarget = new Vector(target - agent.getPosition());
    Vector desiredVelocity = toTarget.normalize() * agent.getMaxVelocity();

    return desiredVelocity.subtract(agent.getVelocity());

}

And then here is the update() function of the agent.

void update(){

    Vector force = seek(target);
    Vector acceleration = force.divideBy(mass); // mass = 1.
    velocity = velocity.add(acceleration);
    position = position.add(velocity);

}

Anybody mind explaining to me where the gradual change of velocity takes place? Thanks

Answer 1

That function gives you a direction scaled such that it indeed performs an instantaneous velocity fix, if added to your velocity. You can think of this as the "steer direction". You can scale this direction by whatever factor you like. Maybe something like this to get started:

const dt = (1.0 / 60.0);
object.velocity += dt * object.Seek( targetPosition );

A scaling factor of 1 would provide an instant change, and lesser than 1 would apply a partial change.

If you want your steering behavior to preserve constant velocity, you'd probably want to just rotate your velocity vector towards the target, instead of adding/subtracting vectors.

In your example code acceleration is used as the seek vector, and scaled by a factor related to mass. The larger the mass of agent the greater the divisor of the steer vector. This is like scaling in the range of 0 to 1 with inverted mass:

velocity += direction * (1.0 / mass);

You can see from above that if your mass is 1, then an instant fix will occur.

However, steering with the rate of change dependent on mass might be silly for most games. Usually you just want to directly tune the scaling factor by hand as a designer, within a range of 0 to 1 (or negative, or higher than 1 if you want):

velocity += direction * scalingFactor;

Answer 2

This function appear strange because it does not actually compute force, it compute the delta in velocity (the change in velocity) which is needed in order for the agent to turn and pursue the target. It has nothing to do with force and mass and yet those are artificially squeezed in there for no reason.

If you want gradual change, compute the desired change (like you already do) and make partial changes each frame. This could be done in several ways, one is to cap the acceleration like you do with the velocity. The other is to compute the desired change and then multiply it by some fraction.

The change is gradual only when acceleration is bound or partial (e.g. 10% of the needed acceleration). Whenever acceleration is used to change velocity and velocity is not changed instantly to a new value, you get the desired gradual smoothness. In this case there does not appear to any gradual change. It appears that the change is instant. This is because the acceleration is not capped or multiplied by some fraction e.g. 0.05.

If you wish to make change smooth and gradual, simply multiply the acceleration by some fraction such as deltaTime (frame time) and perhaps cap it with some maximal value. Otherwise you are just setting the new velocity to the desired velocity for pursuit.

You can use:

acceleration = acceleration.normalize() * agent.getMaxAcceleration();
velocity = velocity.add(acceleration);

@RandyGaul was the first to answer correctly