2 objects following each other but distance pulls them apart

Basically I am making a game where the enemy comes after the hero when He is in sight. What I do to achieve this is

var distx = hero.px - px;
var disty = hero.py - py;

moveX += distx * .00005;
moveY += disty * .00005;

Now I had to multiply it by .00005 because anything higher, makes him come up on the hero pretty quick. I want the enemy to move slow so I multiplied it by .00005. My problem is the farther the distance they are, the faster the monster moves which makes it harder for the hero to get away. I know why this is, I just need to know how to resolve it.

I develop the game where if the monster is too far away, i remove him from the stage. but the problem is getting away. I want him to move at a constant speed no matter how far apart they are.

I also want to achieve this by not using any class properties. (i.e I want all varibles to be local). This method is in a loop as well

What am I doing wrong? and what are solutions.

Thanks guys


All you have to do is rate limit the velocity of the enemy.

// Calculate velocity just as you're doing it
var p = .00005;
var velx = distx * p;
var vely = disty * p;

// Rate limit that velocity
var maxVel = 5; // You will have to tune this value as your max speed
var vel = Math.sqrt(velx*velx + vely*vely);
if (vel > maxVel) {
    var r = maxVel / vel;
    velx *= r;
    vely *= r;

// Move the enemy
moveX += velx;
moveY += vely;

You have essentially implement only the "proportional" part of what control systems engineers (1) call a PID controller. Your desire is for the enemy to track the player, and the PID controller is a nice simple tunable (think enemy difficulties) tracking controller.

In a tracking PID controller, you first measure the error between your desired goal and the current state of affairs. (You do this now, and called the variables distx and disty.) In a PID controller, you keep a memory of past errors so that you can act differently based upon different perspectives of the error.

PID = function() {
    this.p = 0;
    this.i = 0;
    this.d = 0;

PID.prototype.reset = function(error) {
    this.error = error;
    this.integral = 0;
    this.differential = 0;

PID.prototype.step = function(error) {
    this.integral += error;
    this.differential = (error - this.error);
    this.error = error;
    var control = this.p * this.error +
                  this.i * this.integral +
                  this.d * this.differential;
    return control;

You create a PID controller for each of the controllable states for the object you are trying to control (the enemy). So your update logic will change to:

// distx and disty are calculated as normal
var velx = pidx.step(distx);
var vely = pidy.step(disty);

moveX += velx;
moveY += vely;

You can now get a variety of behaviors out of enemies by varying the p,i,d parameters of the PID object. To replicate your current AI, you would set:

pidx.p = 0.00005;
pidx.i = 0;
pidx.d = 0;
// same for pidy

While a more interesting AI can be achieved by bumping up the i parameter and reducing p even further:

pidx.p = 0.000005;
pidx.i = 0.0005;
pidx.d = 0;

Every so often you will want to clear the enemy's memory if he lasts for a very long time. Just call pidx.reset(distx) to reset that memory.

Also you will still want to rate-limit the velx, vely as given earlier in this answer.

What are the benefits of all this complexity? You get enemies that behave very naturally as compared to simpler maths. I also wanted to point out that you're moving along the right tracks with your distx * 0.00005 logic.

(1) Think of controls engineers as those who study the behavior of closed systems and use that knowledge to force the system to act in ways they desire.

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