Touch screen dead zone.

While chasing down a usability bug, I discovered something interesting about the hardware for the Google developer phone, which I also suspect plagues the release G1 and G2 phones. The problem is an issue with the touch screen technology used by HTC.

The bottom line is this: there is a border to the left and right of the screen (and, I suspect, the top and bottom) where the finger’s location is not reported. On a piece of test software, when dragging my finger left to right, I found that when my finger was within 20 pixels of the left border, the position was reported as 0–and on the right border, as 319.0. This, despite being able to see my finger’s location reported with one- and two- pixel increments elsewhere on the screen.

I’ve also found that finger-down events within that 20 pixel border are not reported, unless accompanied with a drag outside of that 20 pixel border. Thus, if you design a UI where the user is expected to tap within 20 pixels of the border, the tap event will not be detected by the current hardware. A tap and drag, however, will be detected, which is why when you tap in the notification area at the top of the Android screen it doesn’t necessarily work–but a tap and drag will reliably open the notification area.

Just something to keep in mind when designing for the Android: tap areas near the left or right of the screen that may be 40×40 pixels in size will have the unfortunate problem that half of the potential touch area will be dead to tap events.

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