Tetris, Torture, and the Gorilla-Arm Problem
<p>Every time computing technology changes, we learn something new about the affordances of being human.  I got my most recent lesson when I discovered DroidTris, a clone of the popular falling-blocks game for Android.  As it turns out, a touchscreen version of Tetris works really, <em>really</em> well &#8211; and in doing so, it sheds new light on one of the classic ergonomic blunders of computing history.</p>
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<p>The earliest ancestor of today&#8217;s handheld touch screens was the light pen, originally invented on the Whirlwind project in 1952. A light pen is nothing but a photodiode on a stick that you can use to designate a spot on a monitor; as Wikipedia notes, &#8220;a light pen works by sensing the sudden small change in brightness of a point on the screen when the electron gun  refreshes that spot. By noting exactly where the scanning has reached at that moment, the X,Y position of the pen can be resolved.&#8221;  Several early attempts at hypertext editing systems in the 1960s used light pens, but the device was overshadowed by Doug Engelbart&#8217;s invention of the mouse.</p>
<p>Touch-screens proper were invented in 1971, and enjoyed a brief round of hype in the early 1980s.  For a time it appeared that this was an input style that might displace the mouse and make computing interfaces really natural &#8211; after all, what could be simpler than just pointing at what you want?  But it didn&#8217;t happen.  The technology claimed a couple of niches in places like restaurant point-of-sale systems, where it&#8217;s still widely used today, but it never achieved mass consumer adoption.</p>
<p>The light pen and touch screens both fell victim to the same problem, which was really about the way the host displays were mounted.  Humans aren&#8217;t designed to hold their arms at or above shoulder height for any longer than it takes to complete a burst-exertion maneuver.  In fact, forcing people to do this with the threat of beatings or electrocution was a form of torture favored by the North Vietnamese that left  former presidential candidate John McCain (among others) with crippling injuries.  In less extreme settings, the discomfort caused by using a light pen or touch surface mounted at or  above shoulder height is called the &#8220;gorilla-arm problem&#8221;, and neatly explains why mice and trackballs won.</p>
<p>The hint that restaurant point-of-sale systems give us about touch screens is that display mounting matters.  Take your vertically-mounted torture device, drop it two feet and rotate it to face upwards, and the gorilla-arm problem vanishes; suddenly touch manipulation is actually useful.  Especially for short command sequences that don&#8217;t involve repetitive motion.</p>
<p>Now fast-forward to my Android phone.  I try out DroidTris and I discover that being able to literally swipe and drag the falling piece any direction but up is terrific &#8211; in act, I can play competently at significantly higher speeds than I  ever manged with arrow keys.  Er, but wait &#8211; this isn&#8217;t short bursty command sequences like a restaurant point-of-sale system, it&#8217;s rapid precise motions for long periods.   Where&#8217;d my ergonomic problem go?</p>
<p>Answer: <em>I can position the touch surface where I want it with my other hand</em>.  We&#8217;ve just rediscovered the ergonomics of scribbling on a hand-held notepad with a pencil or pen.  In practice, doing this involves occasional changes of the relative position of the writing surface to avoid repetitively stressing the same small bunches of muscle fibers in the arm.  People who scribble on notepads are unaware they&#8217;re doing this, but in combination with keeping the pad below shoulder height this is what banishes gorilla arm.</p>
<p>So, indeed, the ancient failures of light pens and touch screens were all about the tacit assumption that the touch surface needed to be mounted in  a fixed vertical position at eye height, well above shoulder level.  That&#8217;s a valuable lesson.</p>
