2D shadows in LibGDX

shadows.java

	static Vector2 L = new Vector2();
	static Vector2 N = new Vector2();
	static Vector2 tmp = new Vector2();
	static Vector2[] points = new Vector2[] {
		new Vector2(), new Vector2(), new Vector2(), new Vector2()
	};
	
	static Vector2[] tmpBoundary = new Vector2[] {
		new Vector2(), new Vector2()
	};
	
	
	/**
	 * For the given light position (x, y with lower-left origin), determine the two
	 * "shadow boundary" points for the given rectangle (also uses lower-left origin). 
	 * These points are the two that will need to be extended/projected from the light vector,
	 * in order to construct the shadow geometry. If the light is inside the rectangle, or could
	 * not find two shadow boundary points, null is returned.
	 * 
	 * To reduce garbage, this method returns a static array
	 * with vectors shared across the various shadowBoundary methods.
	 * 
	 * @param lightPos
	 * @param rect
	 * @return
	 */
	public static Vector2[] shadowBoundary(Vector2 lightPos, Rectangle r) {
		//corners, clockwise 
		points[0].set(r.x, r.y);
		points[1].set(r.x + r.width, r.y);
		points[2].set(r.x + r.width, r.y + r.height);
		points[3].set(r.x, r.y + r.height);
		
		int off = 0;
		boolean lastFace = true;
		//go through each point + 1
		for (int i=0; i<points.length + 1; i++) {
			Vector2 p = points[i % points.length];
			Vector2 prev = points[i==0 ? points.length-1 : i-1];
			
			//line to point
			//light to point vec, normalized
			L.set(p).sub(lightPos).nor();
			
			//normalized direction of edge
			N.set(p).sub(prev).nor();
			
			//dot product 
			float dot = N.dot(L);
			boolean face = dot < 0;
			
			//skip the first index, we'll come back to it later
			if (i!=0) {
				if (face != lastFace) {
					if (off >= tmpBoundary.length)
						return tmpBoundary;
					tmpBoundary[off].set(p);
					off++;
				}
			} 
			lastFace = face;
		}
		return off==0 ? null : tmpBoundary;
	}