Smoke Particle System by Daniel Shiffman.

A basic smoke effect using a particle system. Each particle is rendered as an alpha masked image.

ParticleSystem ps;
Random generator;

void setup() {

  size(640, 200);
  colorMode(RGB, 255, 255, 255, 100);

  // Using a Java random number generator for Gaussian random numbers
  generator = new Random();

  // Create an alpha masked image to be applied as the particle's texture
  PImage msk = loadImage("texture.gif");
  PImage img = new PImage(msk.width,msk.height);
  for (int i = 0; i < img.pixels.length; i++) img.pixels[i] = color(255);
  img.mask(msk);
  ps = new ParticleSystem(0, new PVector(width/2,height-20 ),img);

  smooth();
}

void draw() {
  background(75);

  // Calculate a "wind" force based on mouse horizontal position
  float dx = (mouseX - width/2) / 1000.0;
  PVector wind = new PVector(dx,0,0);
  displayVector(wind,width/2,50,500);
  ps.add_force(wind);
  ps.run();
  for (int i = 0; i < 2; i++) {
    ps.addParticle();
  }
}

 void displayVector(PVector v, float x, float y, float scayl) {
    pushMatrix();
    float arrowsize = 4;
    // Translate to location to render vector
    translate(x,y);
    stroke(255);
    // Call vector heading function to get direction (note that pointing up is a heading of 0) and rotate
    rotate(v.heading2D());
    // Calculate length of vector & scale it to be bigger or smaller if necessary
    float len = v.mag()*scayl;
    // Draw three lines to make an arrow (draw pointing up since we've rotate to the proper direction)
    line(0,0,len,0);
    line(len,0,len-arrowsize,+arrowsize/2);
    line(len,0,len-arrowsize,-arrowsize/2);
    popMatrix();
  } 













// A class to describe a group of Particles
// An ArrayList is used to manage the list of Particles 

class ParticleSystem {

  ArrayList particles;    // An arraylist for all the particles
  PVector origin;        // An origin point for where particles are birthed
  PImage img;
  
  ParticleSystem(int num, PVector v, PImage img_) {
    particles = new ArrayList();              // Initialize the arraylist
    origin = v.get();                        // Store the origin point
    img = img_;
    for (int i = 0; i < num; i++) {
      particles.add(new Particle(origin, img));    // Add "num" amount of particles to the arraylist
    }
  }

  void run() {
    // Cycle through the ArrayList backwards b/c we are deleting
    for (int i = particles.size()-1; i >= 0; i--) {
      Particle p = (Particle) particles.get(i);
      p.run();
      if (p.dead()) {
        particles.remove(i);
      }
    }
  }
  
  // Method to add a force vector to all particles currently in the system
  void add_force(PVector dir) {
    for (int i = particles.size()-1; i >= 0; i--) {
      Particle p = (Particle) particles.get(i);
      p.add_force(dir);
    }
  
  }  

  void addParticle() {
    particles.add(new Particle(origin,img));
  }

  void addParticle(Particle p) {
    particles.add(p);
  }

  // A method to test if the particle system still has particles
  boolean dead() {
    if (particles.isEmpty()) {
      return true;
    } else {
      return false;
    }
  }

}





// A simple Particle class, renders the particle as an image

class Particle {
  PVector loc;
  PVector vel;
  PVector acc;
  float timer;
  PImage img;

  // One constructor
  Particle(PVector a, PVector v, PVector l, PImage img_) {
    acc = a.get();
    vel = v.get();
    loc = l.get();
    timer = 100.0;
    img = img_;
  }

  // Another constructor (the one we are using here)
  Particle(PVector l,PImage img_) {
    acc = new PVector(0.0,0.0,0.0);
    float x = (float) generator.nextGaussian()*0.3f;
    float y = (float) generator.nextGaussian()*0.3f - 1.0f;
    vel = new PVector(x,y,0);
    loc = l.get();
    timer = 100.0;
    img = img_;
  }

  void run() {
    update();
    render();
  }
  
  // Method to apply a force vector to the Particle object
  // Note we are ignoring "mass" here
  void add_force(PVector f) {
    acc.add(f);
  }  

  // Method to update location
  void update() {
    vel.add(acc);
    loc.add(vel);
    timer -= 2.5;
    acc.mult(0);
  }

  // Method to display
  void render() {
    imageMode(CORNER);
    tint(255,timer);
    image(img,loc.x-img.width/2,loc.y-img.height/2);
  }

  // Is the particle still useful?
  boolean dead() {
    if (timer <= 0.0) {
      return true;
    } else {
      return false;
    }
  }
}