Reading #4: Sutherland. Sketchpad: A Man-Made Graphical Communication System (Sutherland)

Summary

Sketchpad by Ivan Sutherland is a device which allows users to draw different shapes and diagrams from primary shapes, such as lines and circles. Compound shapes can be built upon simpler shapes. Later, they can become a template of which multiple copies can be reproduced and these templates can be stored on a storage device. Sketchpad is controlled with a light pen and an array of buttons. Buttons are used to issue commands, such as draw polygon after which the user can enter the polygon vertices using a light pen. It is also possible to enforce constraints on shapes and sketchpad will try to satisfy those constraints if possible. For instance, it is possible to force all the polygon vertices to lie on a circle or force two lines to become parallel. The shapes are stored in their hierarchical form and the screen is refreshed by redrawing each shape one after another. Two scan conversion formulas for circle and line is also introduced in the paper.

The constraint satisfaction mechanism works as follows: Initially it builds a graph where constrained variables are nodes and edges mean the two variables are related to each other in a constraint. It seeks to find a free variable which can be directly assigned a value that satisfies the constraint. This variable is then removed from the graph and this procedure is repeated until all the constraints are satisfied. If the aforementioned procedure did not work for a set of constraints, a relaxation will be performed on the constrained values to gradually minimize the error of constraint satisfaction.

As a conclusion, the author introduces various applications of this system. For instance, it is possible to draw a bridge and let the sketchpad calculate different force components on each part or design a circuit using reusable drawings of different elements. The author also brought up the possibility of simulating and debugging the electrical circuits and 3d drawing as future avenues of expanding Sketchpad.

Discussion

Sketchpad project amazingly utilizes object oriented ideas such as creating a template shape for storage and creating instances thereof and a lot of innovations. Moreover, its means of interaction with user and aiding him in design by satisfying constraints is also impressive. On the whole, Sketchpad accomplished a fine job in its time.