Snowflakes canvas generator

holidays.js

// as seen at http://gtaforums.com/

$j(document).ready(function() {
	window.disableSnow = false;
	
	//canvas init
	var canvas = document.getElementById("canvas");
	var ctx = canvas.getContext("2d");
	
	//canvas dimensions
	$branding = $j("#branding");
	var W = $branding.innerWidth();
	var H = $branding.innerHeight();
	canvas.width = W;
	canvas.height = H;
	
	//snowflake particles
	var mp = 25; //max particles
	var particles = [];
	for(var i = 0; i < mp; i++)
	{
		particles.push({
			x: Math.random()*W, //x-coordinate
			y: Math.random()*H, //y-coordinate
			r: Math.random()*4+1, //radius
			d: Math.random()*mp //density
		})
	}
	
	//Lets draw the flakes
	function draw()
	{
		ctx.clearRect(0, 0, W, H);
		
		ctx.fillStyle = "rgba(255, 255, 255, 0.8)";
		ctx.beginPath();
		for(var i = 0; i < mp; i++)
		{
			var p = particles[i];
			ctx.moveTo(p.x, p.y);
			ctx.arc(p.x, p.y, p.r, 0, Math.PI*2, true);
		}
		ctx.fill();
		update();
	}
	
	//Function to move the snowflakes
	//angle will be an ongoing incremental flag. Sin and Cos functions will be applied to it to create vertical and horizontal movements of the flakes
	var angle = 0;
	function update()
	{
		if(window.disableSnow) {
			return;
		}
		
		angle += 0.01;
		for(var i = 0; i < mp; i++)
		{
			var p = particles[i];
			//Updating X and Y coordinates
			//We will add 1 to the cos function to prevent negative values which will lead flakes to move upwards
			//Every particle has its own density which can be used to make the downward movement different for each flake
			//Lets make it more random by adding in the radius
			p.y += Math.cos(angle+p.d) + 1 + p.r/2;
			p.x += Math.sin(angle) * 2;
			
			//Sending flakes back from the top when it exits
			//Lets make it a bit more organic and let flakes enter from the left and right also.
			if(p.x > W+5 || p.x < -5 || p.y > H)
			{
				if(i%3 > 0) //66.67% of the flakes
				{
					particles[i] = {x: Math.random()*W, y: -10, r: p.r, d: p.d};
				}
				else
				{
					//If the flake is exitting from the right
					if(Math.sin(angle) > 0)
					{
						//Enter from the left
						particles[i] = {x: -5, y: Math.random()*H, r: p.r, d: p.d};
					}
					else
					{
						//Enter from the right
						particles[i] = {x: W+5, y: Math.random()*H, r: p.r, d: p.d};
					}
				}
			}
		}
	}
	
	//animation loop
	setInterval(draw, 33);
		
	// Disable when scrolling or out of view
	(function() {
		var bottomOfCanvasPosition = $branding.offset().top + $branding.outerHeight();

	    var timer;
	    $window = $j(window);
	    $window.bind('scroll',function () {
	    	window.disableSnow = true;
	        clearTimeout(timer);
	        timer = setTimeout( refresh , 300 );
	    });
	    var refresh = function () { 
	    	if($window.scrollTop() > bottomOfCanvasPosition) {
				window.disableSnow = true;
			} else {
				window.disableSnow = false;
			}
	    };
	})();

});