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

COMMENTS:

The Amazing Drew-Drew Logsdon!

SUMMARY:

This paper describes Ivan Sutherland’s Sketchpad system, the first pen-based computer system. Sutherland begins by describing an example of how sketchpad is used. A light pen, a box of buttons and a bank of toggle switches are used to create a simple shape drawing. The shape can then be printed or “inked” on paper using a PACE plotter.

Sutherland gives an overview of how sketchpad can be used to alter and/or move shapes on the display. Sketchpad features the following capabilities: subpicture (an image made up of smaller images repeated), constraint (relationships between lines) and definition copying (copying attributes of one shape to another). Sutherland discusses some possible applications for sketchpad including creating circuit diagrams or highly repetitive drawings.

Sutherland then goes into the details of how Sketchpad works. It uses a ring structure of pointers to keep track of elements. The light pen’s optics are used to identify spots within the pen’s field of view on the display. Spots are tagged with the address of the element they represent. The tags also identify elements in view of the light pen. Sutherland explains how various elements are created in the display. For example, lines and circles are generated using difference equations. Text is added to the display from special tables containing the line and circle segments used to create the letters and numbers.

Recursion is used to implement many of Sketchpad’s functions such as deletion and merging. The draw and copy functions are the result manipulation with the light pen in conjunction with the button box which creates the ring structure of parts required for the drawing.

Finally, Sutherland discusses some of the drawings that can be done using Sketchpad, such as patterns which show shapes repeated many times and dimension lines that show absolute scale. He also mentions how it can be used to create non-technical, artistic drawings. The paper’s future work includes improvements to sketch pad that would allow conversion from photographs to line drawings and the application of one shape’s attributes to another.

DISCUSSION:

I think the level of detail Sutherland goes into when describing the introductory example goes to show just how novel the pen-based interface was at that time. Many of the functions that Sutherland described involved interactions that we may consider fairly intuitive today. I also thought it was interesting how Sutherland made a point of describing behaviors that we may take for granted in today’s drawing applications. For example, having line segments of a shape translate as a unit when the shape is moved.

Some areas where I thought there might be some difficulty was in the function of the light pen. Since the system determines the pseudo location of the pen when it’s aimed at a part of the drawing, how does it distinguish the selection of parts that are close in proximity? Maybe there was a button for that too?

One thing I found very interesting was the fact that the hexagonal lattice design would have taken almost two days to create using drafting prior to Sketchpad. And, I was really curious about what that PACE plotter looked like, but I couldn’t find an image of one online. Does anyone else have a reference?