canvas.js

(function() {
  // Create an array to store our particles
var particles = [];

// The amount of particles to render
var particleCount = 30;

var activeCount = 1;

// The maximum velocity in each direction
var maxVelocity = 2;

// The target frames per second (how often do	 we want to update / redraw the scene)
var targetFPS = 33;

// Set the dimensions of the canvas as variables so they can be used.
var canvasWidth = 400;
var canvasHeight = 400;

var redrawAlpha = 0.99;

// Create an image object (only need one instance)
var imageObj = new Image();

var frameCallback = null;

// Once the image has been downloaded then set the image on all of the particles
imageObj.onload = function() {
  particles.forEach(function(particle) {
    particle.setImage(imageObj);
  });
};

// Once the callback is arranged then set the source of the image
imageObj.src = "/Smoke10.png";

// A function to create a particle object.
function Particle(context) {

  // Set the initial x and y positions
  this.x = 0;
  this.y = 0;

  // Set the initial velocity
  this.xVelocity = 0;
  this.yVelocity = 0;

  // Set the radius
  this.radius = 5;

  // Store the context which will be used to draw the particle
  this.context = context;

  // The function to draw the particle on the canvas.
  this.draw = function() {

      // If an image is set draw it
      if(this.image){
          this.context.drawImage(this.image, this.x-128, this.y-128);
          // If the image is being rendered do not draw the circle so break out of the draw function
          return;
      }
      // Draw the circle as before, with the addition of using the position and the radius from this object.
      this.context.beginPath();
      this.context.arc(this.x, this.y, this.radius, 0, 2 * Math.PI, false);
      this.context.fillStyle = "rgba(0, 255, 255, 1)";
      this.context.fill();
      this.context.closePath();
  };

  // Update the particle.
  this.update = function() {
      // Update the position of the particle with the addition of the velocity.
      this.x += this.xVelocity;
      this.y += this.yVelocity;

      // Check if has crossed the right edge
      if (this.x >= canvasWidth) {
          this.xVelocity = -this.xVelocity;
          this.x = canvasWidth;
      }
      // Check if has crossed the left edge
      else if (this.x <= 0) {
          this.xVelocity = -this.xVelocity;
          this.x = 0;
      }

      // Check if has crossed the bottom edge
      if (this.y >= canvasHeight) {
          this.yVelocity = -this.yVelocity;
          this.y = canvasHeight;
      }

      // Check if has crossed the top edge
      else if (this.y <= 0) {
          this.yVelocity = -this.yVelocity;
          this.y = 0;
      }
  };

  // A function to set the position of the particle.
  this.setPosition = function(x, y) {
      this.x = x;
      this.y = y;
  };

  // Function to set the velocity.
  this.setVelocity = function(x, y) {
      this.xVelocity = x;
      this.yVelocity = y;
  };

  this.setImage = function(image){
      this.image = image;
  }
}

// A function to generate a random number between 2 values
function generateRandom(min, max){
  return Math.random() * (max - min) + min;
}

// The canvas context if it is defined.
var context;

// Initialise the scene and set the context if possible
function init() {
  var canvas = document.getElementById('myCanvas');
  if (canvas.getContext) {

      // Set the context variable so it can be re-used
      context = canvas.getContext('2d');

      // Create the particles and set their initial positions and velocities
      for(var i=0; i < particleCount; ++i){
          var particle = new Particle(context);

          // Set the position to be inside the canvas bounds
          particle.setPosition(generateRandom(0, canvasWidth), generateRandom(0, canvasHeight));

          // Set the initial velocity to be either random and either negative or positive
          particle.setVelocity(generateRandom(-maxVelocity, maxVelocity), generateRandom(-maxVelocity, maxVelocity));
          particles.push(particle);
      }
  }
  else {
      alert("Please use a modern browser");
  }
}

// The function to draw the scene
function draw() {
  // Clear the drawing surface and fill it with a black background
  context.globalAlpha = redrawAlpha;
  context.fillStyle = "rgba(0, 0, 0, 0.5)";
  context.fillRect(0, 0, 400, 400);

  context.globalAlpha = 1;
  // Go through all of the particles and draw them.
  particles.slice(0, activeCount).forEach(function(particle) {
      particle.draw();
  });
}

// Update the scene
function update() {
  particles.slice(0, activeCount).forEach(function(particle) {
      particle.update();
  });
}

// Initialize the scene
init();

// If the context is set then we can draw the scene (if not then the browser does not support canvas)
if (context) {
  setInterval(function() {
      // Update the scene befoe drawing
      if (frameCallback) {
        frameCallback();
      }
      update();

      // Draw the scene
      draw();
  }, 1000 / targetFPS);
}

window.setActiveCount = function(c) {
  activeCount = c;
}
window.setRedrawAlpha = function(a) {
  redrawAlpha = a;
}
window.setFrameCallback = function(f) {
  frameCallback = f;
}

}());