eyeofmidas.com/htdocs/js/starfield.js

function Starfield() {
	var stars = [];
	var offset = { x: 0, y: 0 };
	var targetOffset = { x: 0, y: 0 };
	var velocity = { x: -1, y: -1 };
	var size = { width: 300, height: 300 };
	var debug = false;
	var depth = 0;

	this.init = function (canvas, fieldDepth, fieldVelocity) {
		size.width = canvas.width;
		size.height = canvas.height;
		depth = fieldDepth;
		offset.x = (size.width / 2) / depth;
		offset.y = (size.height / 2) / depth;
		targetOffset.x = (size.width / 2) / depth;
		targetOffset.y = (size.height / 2) / depth;

		velocity.x = fieldVelocity.x;
		velocity.y = fieldVelocity.y;

		var starDensity = Math.floor((size.width * size.height) / 10000);

		for (var i = 0; i < starDensity; i++) {
			stars.push(new Star());
		}

		for (var i = 0, star = stars[0]; star = stars[i]; i++) {
			star.init(canvas);
		}
	};
	this.update = function (canvas) {
		offset.x += (0.01 / (depth + 1)) * (targetOffset.x - offset.x)
		offset.y += (0.01 / (depth + 1)) * (targetOffset.y - offset.y)

		if (offset.x > size.width) {
			offset.x -= size.width;
			targetOffset.x = offset.x;
		}
		if (offset.y > size.height) {
			offset.y -= size.height;
			targetOffset.y = offset.y;
		}

		if (offset.x < 0) {
			offset.x += 2 * size.width;
			targetOffset.x = offset.x;
		}
		if (offset.y < 0) {
			offset.y += 2 * size.height;
			targetOffset.y = offset.y;
		}
		for (var i = 0, star = stars[0]; star = stars[i]; i++) {
			star.update(canvas);
		}
	};

	this.draw = function (context) {

		var xoffset = { x: offset.x - size.width, y: offset.y };
		var yoffset = { x: offset.x, y: offset.y - size.height };
		var fulloffset = { x: xoffset.x, y: yoffset.y };

		if (debug) {
			context.strokeStyle = "#00FF00";
			context.beginPath();
			context.rect(offset.x, offset.y, size.width, size.height);
			context.rect(xoffset.x, xoffset.y, size.width, size.height);
			context.rect(yoffset.x, yoffset.y, size.width, size.height);
			context.rect(fulloffset.x, fulloffset.y, size.width, size.height);
			context.stroke();
		}

		context.fillStyle = "#FFFFFF";
		for (var i = 0, star = stars[0]; star = stars[i]; i++) {
			star.draw(context, offset);
			star.draw(context, xoffset);
			star.draw(context, yoffset);
			star.draw(context, fulloffset);
		}
	};

	this.mouseMove = function (canvas, x, y) {
		if (depth != 0) {
			targetOffset.x = x / depth;
			targetOffset.y = y / depth;
		}
	};
}