The idea of ray tracing comes from as early as the 16th century when it was described by Albrecht Dürer, who is credited for its invention. In "Four Books on Measurement", he described an apparatus called a "Dürer's door" using a thread attached to the end of a stylus that an assistant moves along the contours of the object to draw. The thread passes through the door's frame and then through a hook on the wall. The thread forms a ray and the hook acts as the center of projection and corresponds to the camera position in raytracing.