# PlayfulJS

## Particle effects are easy

by Hunter Loftis

Almost every game uses particles to create engaging effects for fire, smoke, explosions, fabric, water, gunfire, and more. Learn how to use this simple yet powerful technique to build your own stunning visuals. [Demo] [Source]

### The humble particle

``````function Particle(x, y) {
this.x = this.oldX = x;
this.y = this.oldY = y;
}
``````

First, we define what a particle is: a current position (x, y), plus an old position (oldX, oldY). At first, they're the same thing because the particle isn't moving yet.

### Verlet integration

Now, let's teach our particle how to move with Verlet integration. That's just a fancy way to say that we're going to find out how fast our particle is moving by comparing its position this frame to its position last frame. Why? Because that gives us implicit velocity - any change to the particle's current position will automatically update its velocity.

``````Particle.prototype.integrate = function() {
var velocityX = this.x - this.oldX;
var velocityY = this.y - this.oldY;
this.oldX = this.x;
this.oldY = this.y;
this.x += velocityX;
this.y += velocityY;
};
``````

### Tracking

For this demo, let's push each particle towards the mouse on every frame. This will make a cloud of particles that tends to swarm around the cursor. Since we're using Verlet integration, shifting the particle's position in the direction of the mouse will automatically change its velocity as if it were pushed.

``````Particle.prototype.attract = function(x, y) {
var dx = x - this.x;
var dy = y - this.y;
var distance = Math.sqrt(dx * dx + dy * dy);  // Pythagorean theorum
this.x += dx / distance;
this.y += dy / distance;
};
``````

Since we're dividing the push by distance, closer particles will be more strongly attracted to the mouse than more distant particles.

### Putting it all together

Each frame we loop through all of our particles, attracting, integrating, and drawing as fast as possible for smooth animation.

``````for (var i = 0; i < particles.length; i++) {
particles[i].attract(mouse.x, mouse.y);
particles[i].integrate();
particles[i].draw();
}
``````

## Try it out

If an image is worth 1000 words, a 60fps Demo must be priceless.

## Get creative!

• Can you make the particles slow down over time? (damping)
• What happens if the attraction doesn't care about distance?
• Can you draw each particle in a different random color?
• What's it like with 20 particles? With 2,000?
• Can you make the particles be attracted to each other as well as the mouse?

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