Smoke Particle System

/**
 * 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;

void setup() {
  size(640, 360);
  PImage* img = loadImage("texture.png");
  ps = new ParticleSystem(0, PVector(width/2, height-60), img);
}

void draw() {
  background(0);

  // Calculate a "wind" force based on mouse horizontal position
  float dx = map(mouseX, 0, width, -0.2, 0.2);
  PVector wind(dx, 0);
  ps->applyForce(wind);
  ps->run();
  for (int i = 0; i < 2; i++) {
    ps->addParticle();
  }

  // Draw an arrow representing the wind force
  drawVector(wind, PVector(width/2, 50, 0), 500);
}

// Renders a vector object 'v' as an arrow and a position 'loc'
void drawVector(PVector v, PVector loc, float scayl) {
  pushMatrix();
  float arrowsize = 4;
  // Translate to position to render vector
  translate(loc.x, loc.y);
  stroke(255);
  // Call vector heading function to get direction (note that pointing up is a heading of 0) and rotate
  rotate(v.heading());
  // 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 rotated to the proper direction)
  line(0, 0, len, 0);
  line(len, 0, len-arrowsize, +arrowsize/2);
  line(len, 0, len-arrowsize, -arrowsize/2);
  popMatrix();
}