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What the term “multi-touch” means, what it does, and how
2013/12/14 17:35:29

What the term “multi-touch” means, what it does, and how it is used.

Multi-Touch Sensing Technology
Today, there are three technologies in common use that can sense multiple touches. All of these are covered in Touch International’s whitepaper “Choosing the Right Touch Technology.”

The most common multi-touch technology, projective/projected capacitive or pro-cap/ p-cap, is used in all cell phones and e-readers. Projected capacitive is the most widely used because, in addition to sensing multi-touch, the visual appearance of the display is good and the sensor will never wear out.

A second technology is MARS or Multi-Touch Analog Resistive System, also called AMR. This is an older technology which is essentially a 4-wire resistive sensor, cut up into many small 4-wire touch screens. The advantage of this technology is that it works best with pen-input systems because it is pressure sensitive.

The best choice for a very large display is an optical system using cameras. There are two versions, one which uses small cameras in the corners of the flat panel display (SMART/NextWindow), and another which uses an overhead (sometime underneath) camera with a projector (Microsoft Surface).

Multi-Touch Resolution and Speed
Most touch screens, including multi-touch sensors, generate a resolution of 1024 x 1024, or ten bit. The way in which the coordinates are generated is different with each of the three multi-touch technologies but all of them require substantial computing power to do this for up to ten fingers at the same time.

Human perception of “instant” is considered to be one-quarter of a second. To allow for the first point to be generated, processed by the computer and then shown on the display, the first touch point needs to be generated in a tenth of a second. This processing requirement is the same for multi-touch systems, which requires increased computing power.

Some users have noticed a delay when ten fingers are simultaneously drawing on the screen. Typically this is not because there is an inherent delay in the touch system, but because the host computer and its video adapter also need to be more powerful to accommodate this increase in data.

Software Interface
An early problem with multi-touch was how to report up to ten touches at the same time. Microsoft resolved (some would say, benevolently dictated) this problem by standardizing the way multiple input points were interpreted by the computer. To make it easy to integrate single touch displays into the computer environment, the X, Y coordinates (which is all that touch screens do) were treated as an “absolute” mouse. A regular mouse is a “relative” device which just tells the computer which direction to move the cursor. As an absolute mouse, the touch screen tells the computer where to move the cursor (under the finger, of course) by acting as though the mouse was manually moved to the touch point.

Multi-touch screens are treated as though they are multiple absolute mice connected to the computer at the same time. Thus, if the touch screen electronics are capable of generating 10 simultaneous touches, then the computer will see ten mice connected to it. Other operating systems, specifically Linux, have also adopted this method.

ZonesMulti-touch Zones
Projected capacitive and MARS systems generate analog (continuous) coordinates from zones formed by rows and columns; this is analog output from digital inputs. The rows and columns are generally 7-10 mm wide which is the best size to generate 1024 x 1024 coordinates quickly. In addition, the zones must be small enough that only one finger will fit in each zone; if the zones are larger, two fingers can enter one zone, making it impossible for the system to distinguish two separate touches.

Making Multi-Touch Bigger
Most multi-touch applications are for 12.1 inch displays and smaller, excluding camera systems which are typically used for displays 32 inches and larger. Producing multi-touch screens in sizes from 12 to 32 inches presents two problems. First, it takes multiple processors to generate fast touch points. Second, with rows and columns in the 10mm range, there are a large number of connections. These two factors tend to increase the price of multi-touch displays; however, the durability offered to kiosk, point of sale and transaction machines justifies the cost.

Multi-Touch Gestures
A gesture is a description of what happens when two or more touches do “something”. Originally, these actions were coded in the touch controller chip, but now that multi-touch drivers are available in Windows and Linux systems, that function is left to those standard interfaces. Thus, the touch controllers once again just generate coordinates, albeit now there can be simultaneously up to 10 pairs.

There are only three gestures that are unique to multi-touch systems. They are pinch, expand, and rotate. The others -- flick, hold, tap and pan -- can be done with older technologies but are easier with a projected capacitive system.

Multi-DimensionalMulti-Dimensional Multi-Touch
The future of multi-touch is to add the distance (proximity or “Z”) dimension to touch screens. The now familiar video of Jeff Han ( waving his hands over the display to control the images is coming to smaller devices. Similar actions are common with TV news and sports broadcasts. Some game systems are already using cameras to allow multiple users to control the play without the need for wireless remote controls. This proximity sensing is being built into a number of smaller devices to both detect a presence of a person or to control actions without having to touch the display.

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