From Single Touch to Multi Touch

Touch screens have been around for several years now and have been popular in kiosk applications, vertical-market tasks like restaurant ordering systems, and to a more limited extent in tablet PCs. The classic touch screen can intercept the position of your finger on the surface, but returns only one given coordinate. This makes it quite a rough approximation of the position as your finger, unlike a dedicated pen with a more fine tip, will generally touch multiple screen pixels at a time60.

In technical terms, a classic touch screen behaves like a mouse sending mouse down and mouse up Windows messages to the operating system (and hence to the application). This is what is now generally called a single-touch system.

Newer touch screens, in fact, can intercept multiple pressure points at the same time, and send them all in parallel to the system. These multi-touch61 systems bring two different advantages.

First, if you press your big finger over the screen, the driver will reduce the points in proximity to a single one but with a better resolution, so it would be easier to figure out your intended operation. Also, any movement you make with your finger on the screen is interpreted in more precise way, making it easier to perform gestures.

Second, you can select two or more on-screen elements at the same time, by using two or more fingers. There are currently systems being sold which are

60 If you've ever tried using the small buttons of a Windows CE device with your fingers instead of the specific pen you probably know what I mean. I consider it a dreadful experience, but having to use the pen to select a contact and make a phone call wile you are walking is even less natural that trying to hit the name with your finger, or even your fingernail to be more precise!

61 As odd as it might sound, the term "multi-touch" is actually a trademark of Apple Inc. as listed on http: //www.apple.com/legal/trademark/appletmlist.html.

capable of intercepting up to ten touch positions (and although it would be quite odd for a single user to need more, multi-user systems like Microsoft Surface can benefit from more than ten touch points). You can also move two or more fingers on the screen to manipulate the objects you are touching. A classic example is stretching a picture by dragging away the opposite borders with two fingers.

For multi-touch you need support both at the operating system level and at the hardware level. At the operating system level, Windows 7 introduces specific system messages, like wm_touch. This message carries a wealth of information about the input activities of the user, computes the touch to single coordinates plus a couple of key shift states of the traditional mouse messages.

At the hardware level, you need multi-touch hardware with drivers specifically tailored for Windows 7 (as older touch support drivers will simply mimic the traditional mouse messages, as in single touch devices).

Another way to think about touch versus the mouse (and its derivatives) is to consider that in the first case the system works with absolute positions, while in the second it is relative movements that matter. When you touch a screen (or optionally a touch pad with full touch support), you are indicating a specific location. With the mouse (and some movement oriented touch pads) you move it from the current position to a new one, but it is the relative movement that matters. In fact, if you lift the mouse and lower it down in a different place, the effect is like it didn't move!

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