![]() To draw a circle we place the light pen where the center is to be and press the button ‘circle center,’ leaving behind a center point. Whereas Alberti’s armature gently corrected the erratic radius of our hand, Sketchpad effectively ignores it and uses the light pen’s location only to decide how much arc to draw. By never registering the wobble of the hand-drawn circumference on screen, Sutherland sought to correct error before it could even be recognized by the human eye. Sutherland’s extraordinarily prescient and elegant program, Sketchpad: a Man-machine Graphical Communication System, inherited everything the US air defense system SAGE had recently invented, and added almost everything else required to make the CAD interface through which so much design is now thought, developed and represented, including the physical mechanics of drawing something-a cursor and hand-held input, first in the form of a touchscreen “light pen” that would later become a mouse-as well as deleting it. Whereas Alberti devised a way to draw curved perfection by erecting an approximating join-the-dots scaffold, Sutherland did away with the stepping-stone dots altogether. Sutherland, under Claude Shannon’s wily guidance, radically augmented the corrective capacity of the algorithm at work in an exercise that is not difficult for machines but exceptionally tricky for humans. Crucially, in both, one does not have to be able to draw a circle to draw a circle. ![]() When in 1962 Ivan Sutherland designed the first drafting program that would allow us, amongst other things, to draw better circles, he was in many ways simply providing an update to Leon Battista Alberti’s circle-drawing system issued some five hundred years earlier in De Pictura. If to err is human, to design corrective systems is all the more so.
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