GO-21 Size History

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<center>SciRPN</center>
<center>2008 - 2027</center>



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As a college student in the early 1970s I was vaguely competent with a slide rule calculator, it got me through my chemistry, physics and mathematics courses. Yet I yearned for the fancy new HP-35 calculator that the astronauts were using; trouble was, it was way out of my price range, so no fancy new electronic gizmo for me.
When the HP-21 appeard in 1976 I eagerly bought one and was in calculator heaven for many, many, years. Eventually the rechargable batteries failed and would no longer hold a charge, so I chanced it and removed the original batteries, replaced them with 2 AA batteries and eureka, the calculator worked (notwithstanding that the voltage was slightly off, as I discovered years later). Still, I worried my beloved HP-21 was on its last legs, and I do not do hardware so repairing it was out of the question ... but I do do software!!! If I could capture the look, feel and features of an HP-21 on an iPhone I could mothball the real hardware.

Thus GO-21 was born: coding began in late 2008 with iOS 2, and version 1.0 debuted in the new App Store in the summer of 2009, on an iPhone 3G running iOS 3. An interesting problem I had to deal with was how to squeeze a 5.1" tall HP-21 calculator into a 3.5" diagonal iPhone display. I soon discovered there are tradeoffs between authenticity, aspect ratio of the displayed view and a touch user interface.
I initially displayed the entire HP-21 in its proper aspect ratio, but it was too tall and skinny for the fat-ish iPhone screen area of 320 x 480 pixels. Worse, the calculator buttons were really tiny, esssentially impossible to reliability touch. Back then the minimum touch target size was 64 x 64 pixels, and the calculator buttons were much smaller than that. I could make them wider (but not taller), although the calculator's aspect ratio suffered and it looked too fat.

To make the touch targets as large as possible I decided to chop off the top, bottom and sides of the calculator, leaving only the display and buttons. This had the desired zoom effect and did indeed enlarge the touch halos, although the aspect ratio changed noticably. But at least the calculator was usable for, well, calculating. You can see the result below, along with GO-21 running on various other iPod, iPhone and iPad models, as well as Macintosh and Apple TV.

On a 3.5" display a realistic aspect ratio is sacrificed for improved user interaction.<a href="#fn1" id="ref1">1</a> On the 4" and 4.7" displays it was possible to improve the aspect ratio while maintaining useable touch targets. It was not until the iPad was released that the entire HP-21 could be displayed at essentially full size and proper aspect ratio. The iPhone would not have that capability until the 5.5" iPhone 6 Plus arrived years later.

Beginning in 2024 GO-21 provides a Magnify Calculator option on iPhones with displays of at least 5.4", which creates a zoom-in effect that simulates the chopped calculator look from 2009, distorted aspect ratio and all.

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<center><h2>The HP-21 Calculator on an iPhone, and Beyond</h2></center>

GO-21 brings the classic 1975 HP-21 scientific calculator to your Apple devices, including its signature RPN (Reverse Polish Notation) style of entering data and performing calculations.

Development commenced in late 2008 using the iPhone OS 2 SDK. Version 1.0 of the App, for the iPhone 3G running iPhone OS 3, debuted on the new App Store in the summer of 2009.

The hardest part of building the App was squeezing a 5.1" tall HP-21 into a squat 3.5" diagonal iPhone display — there were tradeoffs between authenticity, aspect ratio of the calculator and a touch user interface.

Possible scenarios were:

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<li>Place the entire HP-21 in its correct aspect ratio. The calculator looked authentic although shrunken, with unused space on either side, and the buttons were tiny and impossible to touch reliably.
<li>Stretch the calculator sideways into the unused area, making the buttons wider (although not taller), but touches were still too unreliable and the calculator's aspect ratio became unacceptably distorted.
<li>Crop the calculator bezel (the option chosen), effectively zooming in so buttons were large enough to tap effectively. The aspect ratio was slightly off, but reliable typing came first and perfect looks second.
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This is how the App's aspect ratio adapts to a device's screen size:

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<li>3.5-inch displays rely on the fully cropped calculator view .

<li>4.0-inch and 4.7-inch displays are taller and permit a slightly more accurate aspect ratio while maintaining reliable touch targets.

<li> All iPads display the entire calculator and its bezel in the proper aspect ratio.

<li>iPhones with a display 5.4-inches or greater also show the HP-21 and bezel at the proper aspect ratio.

(Incidentally, these iPhones have an Accesibility option Magnify Calculator that brings back the fully cropped and zoomed-in look for eyes that prefer the larger appearance.)
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GO-21 has always run on every iPhone, iPod Touch, and iPad, and now it's available for Mac and Apple TV as well.

<center>HP-21 and iOS Devices Shown to Scale</center>

<center>macOS and tvOS Devices Shown to Scale</center>

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