lengstrom

function intersect(x1, y1, x2, y2, x3, y3, x4, y4){
	var a1, a2, b1, b2, c1, c2;
	var r1, r2 , r3, r4;
	var denom, offset, num;

	// Compute a1, b1, c1, where line joining points 1 and 2
	// is "a1 x + b1 y + c1 = 0".
	a1 = y2 - y1;
	b1 = x1 - x2;
	c1 = (x2 * y1) - (x1 * y2);