A Technical History of Apple's Operating Systems - Mac OS X Internals

16 Chapter 1 A Technical History of Apple’s Operating Systems
The primary input device of Sketchpad was the light pen: a hand held photo-
diode device with the shape and dimensions of a pen. The light pen was connected
to the computer by a coaxial cable, through which it communicated to the computer
when its field-of-view encompassed a spot on the TX-2’s display. The pen’s barrel
could be rotated to vary the distance between the photodiode and the lens at the
pen’s tip, thus adjusting the field-of-view’s size.
As the user drew directly on the screen using the light pen, Sketchpad inter-
preted the drawing. Using straight-line segments and circle arcs as basic drawable
shapes, the user could draw more complex shapes. The system treated on-screen
shapes as objects 11 that could be operated upon. For example, transforms such as
rotation, scaling, and translation could be applied to existing drawings. Once drawn,
shapes could be saved, and reused as primitive units. 12 The light pen was used in
conjunction with push-button controls. For example, the draw control created a new
line segment or arc, with the drawn shape’s end-point remaining attached to the pen.
Other examples of controls included circle center, move, delete, and instance.
Sketchpad was demonstrated by creating electrical, mathematical, mechanical,
scientific, and even artistic drawings. Moreover, arbitrary mathematical conditions,
which could be the result of complex computations, could be applied to drawings.
Sketchpad would automatically satisfy the conditions and alter the drawings to pic-
torially display the results. It had various other mathematical abilities that made it a
powerful graphical input program—for example, it allowed sub-pictures within pic-
tures, with no intrinsic limit on the nesting level.
Sketchpad drawings, or rather, their topologies, were stored in special files
using data structures optimized for fast editing of the drawings.
Sutherland received the Turing award in 1988 for his pioneering contribu-
11 Sketchpad could be seen as an object-oriented graphics editor, and a precursor to object-oriented
programming.
12 Thus, Sketchpad provided the first example of the Prototype design pattern.