main/src/georegression/geometry/UtilVector2D_F64.java
public static double acute( Vector2D_F64 a , Vector2D_F64 b ) {
		double dot = a.dot(b);

		double value = dot/(a.norm()*b.norm());
		if( value > 1.0 )
			value = 1.0;
		else if( value < -1.0 )
			value = -1.0;

		return Math.acos( value );
	}
public static double acute( Vector2D_F64 a , Vector2D_F64 b ) {
		double dot = a.dot(b);

		double value = dot/(a.norm()*b.norm());
		if( value > 1.0 )
			value = 1.0;
		else if( value < -1.0 )
			value = -1.0;

		return Math.acos( value );
	}
Returns the acute angle between the two vectors.  Computed using the dot product.
@param a vector
@param b vector
@return acute angle