Three-dimensional data input by tablet

A large-area multiple-pen tablet system for three-dimensional data input is described. The large tablet area provides space for simultaneous use of several views of the three-dimensional object being digitized. The multiple pens enable the user to indicate a single point simultaneously in two such views, thus defining the three-dimensional position of the point. Five significant techniques are outlined. First, the large-area digitizing surface with multiple pens has proved to be an instrument very different from the more familiar single-pen small tablets. Second, a pair of two-dimensional positions is converted into a four-dimensional space and then back to three dimensions. Third, the specification of view areas, viewing directions, view positions, and coordinate axis is accomplished by giving examples directly in the viewing space rather than by specifying abstract viewing parameters. Fourth, an attitude about coordinate conversion using the inverse of a basis matrix is used throughout which automatically compensates for any tilt in the views on the tablet surface and any nonperpendicularity of the tablet axis. Fifth, the mathematics of converting from pairs of perspective views or pairs of photographs, while not new, is formulated simply in an appendix with several examples.