Tilting your thumb to press more of it against the touch screen (figure C) lets Fat Thumb parse it as a different input from normal thumb presses.

Pinch-to-zoom gestures are difficult to manage with one hand.

These heat maps show the different kinds of thumb presses that the designers tested.

These heat maps show the different kinds of thumb presses that the designers tested.

Fat Thumb’s designers are working to refine its ergonomics.

When a user’s second hand is occupied with another task, two-handed touch-screen gestures become awkward or impossible.

Co.Design

Fat Thumb: A One-Handed Alternative To Pinch-To-Zoom

An experimental smartphone interaction lets users pan and zoom without using two hands.

Apple may be fighting tooth and nail to patent "pinch-to-zoom," but sometimes I wonder if I’d even miss it if it were gone. The two-handed interaction is great on a tablet but damn annoying on my iPhone, which I almost always hold with one hand and operate with one thumb. "Tap to zoom" is much more elegant and useful, but it only goes one way. If I want to zoom back out and reorient on, say, a Google Map, I’ve got to stop what I’m doing, grip the phone with one hand, and start making little crab motions with the fingers of my other hand.

So I was excited to try Fat Thumb, an experimental iPhone interaction that lets me do everything that pinch-to-zoom does, but with one digit. Fat Thumb senses touch pressure--technically, how much area the pad of your thumb is taking up on the capacitive screen during a press. Using the tip of your thumb (i.e., normal pressure) allows you to pan around an image, just like you normally do. But pressing and "dragging" your thumb invokes a zooming function: up to zoom in, down to zoom out.

"I was getting frustrated with always making sure that I have two hands available to zoom in and out," says Sebastian Boring, lead author of a research paper about Fat Thumb from the Department of Computer Science at University of Calgary. "One [solution] is, of course, to use physical or visual buttons. However, that always forces people to break out of the interaction. We figured out how to detect the [thumb’s] contact size [on the screen], designed a simple mode switch between panning and zooming a map."

Fat Thumb’s gesture takes some getting used to, but once I calibrated the app and gave it some practice, I didn’t even have to take my thumb off the screen while fluently maneuvering between pan and zoom modes. The interaction also cannily respects Fitts’s Law: Pressing your thumb into the screen and pushing it back and forth is a much cruder motion than lightly moving the tip around, so it makes sense that the zooming function (which requires a "ballpark" level of accuracy) is mapped to the hard-press gesture. Fat Thumb’s zooming function quickly gets you to the right "altitude level," at which point you can go back to the more precise, traditional thumb movement to pan and orient the map.

So is it a pinch-to-zoom killer? Not quite. "Fat Thumb was never designed to replace two-handed gestures. In fact, I believe that two-handed gestures are more precise than single-handed ones--at least in our context," Boring says. Instead, it’s "a fallback solution, with the neat side effect that it does not interfere with existing techniques like the pinch-to-zoom gesture. Crucial interactions such as the simple pan-and-zoom interaction should--in our opinion--at least have such an alternative, as the interaction context could be that we only have one finger available."

Fat Thumb is only a demo app for research purposes at the moment. Apple took ages just to implement copy-and-paste, so I wouldn’t hold your breath for Fat Thumb-like functionality to make its appearance anytime soon. But the researchers say that they would be "quite happy" "if phone manufacturers add a map application that makes use of the presented technique"--so maybe we’ll see something like it in the next flagship Android device, or even the open-source Ubuntu smartphone.

[Read the Fat Thumb research paper here.]

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4 Comments

  • Adam Roades

    I'm glad to see Apple finally catching up to Android. As other commenters noted, the double-tap to zoom in/out has been part of Google Maps for some time. I'm looking forward to Google implementing the feature in other apps like Chrome.