This paper presents a line extraction method to process images taken inside a machine tool. Instead of using real background images, our approach utilizes a virtual background image. This approach solves the problem of absence of real background images due to a dynamic background. In order to only extract lines of the object, all corners are detected from the real image first. Then, those corners generated from the background are filtered through composite background subtraction. Afterwards, a hypothesis of a line exists between any two corners is made. All the hypothetical lines are mapped back to the original real image to test for their existence. Those lines caused by noises, such as reflections or scratches, can be then eliminated. Our experimental results prove the feasibility and effectiveness of this proposed method..