A New Light Pen With Subpixel Accuracy

Abstract

A new light pen system using a real-time algorithm for computing the centroid of the intensity pattern seen by a photosensor is described. This new light pen achieves an accuracy of better than one-quarter of a pixel on a cathode-ray-tube screen with 1K × 1K resolution. This corresponds to a capability of resolving 0.004 inch on a 15-inch screen, which is an improvement by a factor of at least 50 over the conventional light pens available today. The transistor-transistor logic hardware implementation of the algorithm is described in detail. The central part of the hardware is a real-time, moment-generating circuit that implements an efficient moment calculation algorithm reported earlier. Issues such as complexity of the hardware compared with the conventional techniques, and the possibility of implementing the algorithm in a single complementary metal-oxide semiconductor chip are addressed. Some line and curve drawing results sketched by the new light pen are presented and compared to similar drawings obtained by a conventional light pen.