"Sketch Pad: A Man-machine Graphical Communication System"

by Sutherland

Comments
1. Akshay's blog

Summary

The author first showed us an example of using light pen to draw a hexagon as well as to point to parts of the existing hexagon and apply certain "constraint" (corners on a circle) and "definition copying" (sides of equal length) to it to make it regular. And thereafter, this hexagon was served as a "subpicture", and by attaching a numerous of such subpictures together, a pattern was produced. What's worth mentioning is that Sketchpad stores how various parts of the drawing are related and the structure of the subpictures used.

Then he explained the implementation of Sketchpad. The data storage structures used: n-component objects are stored in ring structures, in which members can be added, removed or modified. Additionally, generic headings of the rings are collected together as generic types, which may in turn gathered under super-generic blocks to form a hierarchical structure. A light pen is used in the system, the user should touch existing line or spot on the display to initialize, and when pointing to existing object, a "pseudo pen location" is displayed to indicate "aim at" if the exact position falls within a distance to the object. The system can display drawing of a range of magnification of 2000 and compute which part is to be shown on the display, and positions of spots on lines and circles are computed with difference equations. Sketchpad can also display digits and texts. And for easy manipulation, abstractions such as constraints and scalar value can even be displayed. Recursive functions are used when dealing with deleting, merging and display of instances. Attachers (points, lines, cirles, etc.) allow objects to integrate. Attachers will recursively merge objects of like type when copying occurs, and when copying instances, the copy losses all identity as a unit. Moreover, copying allows duplication of not only objects, but constraints as well. As for constraints, Sketchpad applies the fast one pass method before trying the slow but reliable relaxation method.

In the last part, examples (patterns, linkages, dimension lines, bridges, artistic drawings, circuit diagrams) were illustrated and conclusions were drawn.


Discussion

This paper shows the first pen-based computer interaction system in the world, as early as 1963. It must be an amazing work at that year. The system is really versatile, the hierarchical structure of generic type enables drawing of objects composed of simpler components that are already known. And by copying and attachers, users can produce complicated drawings which has thousands of parts. The function of constraints is actually quite strong, you can define certain properties of objects and relation between them, so I'd say it's really the highlight of this system.

In term of sketch recognition, however, Sketchpad is nothing more than a computer-assisted drawing pad, users have to push buttons and turn knob from time to time to tell the computer what is to be drawn and what effects they are expecting, drawing with Sketchpad is a totally manipulated process rather than automatic analysis and understanding by the computer itself.