Patent US 4,561,017

A computer graphic input device is disclosed for imaging the points of contact between objects and a surface. The input device is defined by a transparent material with one surface thereof defining a drawing surface. The index of refraction of the material is greater than the index of refraction of the medium on the upper side of the drawing surface. In the preferred embodiment, the transparent material is prism shaped in configuration. A camera is mounted on the lower side of the drawing surface in a manner to only capture light rays which have been totally internally reflected from the drawing surface. By this arrangement, objects located on the upper side of the drawing surface will be imaged by the camera only when and where the object is in direct contact with the drawing surface. The size and shape of the input pattern is digitized, processed and displayed on a monitor. The subject apparatus can be used to produce graphic effects achieved by various natural drawing implements, such as a brush.