main/src/georegression/geometry/UtilLine2D_I32.java
public static double acuteAngle( LineSegment2D_I32 line0 , LineSegment2D_I32 line1 ) {
		int dx0 = line0.b.x - line0.a.x;
		int dy0 = line0.b.y - line0.a.y;
		int dx1 = line1.b.x - line1.a.x;
		int dy1 = line1.b.y - line1.a.y;

		double bottom = Math.sqrt(dx0*dx0 + dy0*dy0) * Math.sqrt(dx1*dx1 + dy1*dy1);
		return Math.acos((dx0*dx1 + dy0*dy1)/bottom);
	}
public static double acuteAngle( LineSegment2D_I32 line0 , LineSegment2D_I32 line1 ) {
		int dx0 = line0.b.x - line0.a.x;
		int dy0 = line0.b.y - line0.a.y;
		int dx1 = line1.b.x - line1.a.x;
		int dy1 = line1.b.y - line1.a.y;

		double bottom = Math.sqrt(dx0*dx0 + dy0*dy0) * Math.sqrt(dx1*dx1 + dy1*dy1);
		return Math.acos((dx0*dx1 + dy0*dy1)/bottom);
	}
Computes the acute angle between the two lines.  Does not check for intersection

@param line0 First line
@param line1 Second line
@return Acute angle in radians