Sketchpad, A Man-Machine Graphical Communication System

by Ivan Edward Sutherland

Submitted to the Department of Electrical Engineering on January 7, 1963, in partial fulfillment of the requirements for the degree of the Doctor of Philosophy.

Abstract

The Sketchpad system uses drawing as a novel communication medium for a computer. The system contains input, output, and computation programs which enable it to interpret information drawn directly on a computer display. It has been used to draw electrical, mechanical, scientific, mathematical, and animated drawings; it is a general purpose system. Sketchpad has shown the most usefulness as an aid to the understanding of processes, such as the motion of linkages, which can be described with pictures. Sketchpad also makes it easy to draw highly repetitive or highly accurate drawings and to change drawings previously drawn with it. The many drawings in this thesis were all made with Sketchpad.

A Sketchpad user sketches directly on a computer display with a “light pen”. The light pen is used both to position parts of the drawing on the display and to point to them to change them. A set of push buttons controls the changes to be made such as “erase”, or “move”. Except for legends, no written language is used.

Information sketched can include straight line segments and circle arcs. Arbitrary symbols may be defined from any collection of line segments, circle arcs, and previously defined symbols. A user may define and use as many symbols as he wishes. Any change in the definition of a symbol is at once seen wherever that symbol appears.

Sketchpad stores explicit information about the topology of a drawing. If the user moves one vertex of a polygon, both adjacent sides will be moved. If the users moves a symbol, all lines attached to that symbol will automatically move to stay attached to it. The topological connections of the drawing are automatically indicated by the user as he sketches. Since sketchpad is able to accept topological information from a human being in a picture language perfectly natural to the human, it can be used as an input program for computation programs which require topological data, e.g., circuit simulators.

Sketchpad itself is able to move parts of the drawings around to meet new conditions which the user may apply to them. The user indicates conditions with the light pen and push buttons. For example, to make two lines parallel, he successfully points to the lines with the light pen and presses a button. The conditions themselves are displayed on the drawing so that they may be erased or changed with the light pen language. Any combinations of conditions can be defined as a composite condition and applied in one step.

It is easy to add entirely new types of conditions to Sketchpad’s vocabulary. Since the conditions can involve anything computable, Sketchpad can be used for a very wide range of problems. For example, Sketchpad has been used to find the distribution of forces in the members of truss bridges drwan with it.

Sketchpad drawings are stored in the computer in a specially designed “ring” structure. The ring structure features rapid processing of topological information with no searching at all. The basic operations used in Sketchpad for manipulating the ring structure are described.

Thesis Supervisor: Claude E. Shannon

Title: Donner Professor of Science