by Guillaume Largillier, Stantum’s Co-founder and Chief Strategy Officer

Multi-touch interfaces are becoming increasingly popular for handheld devices and portable electronics.  There are a number of technologies that can be used to implement the functionality, but what are the properties that distinguish a good multi-touch technology from a great one, and how can you test for these properties?  Testing a multi-touch interface does not require any special tools or software.  Simple test utilities are usually available to enable you to learn more about the strengths and limitations of any given multi-touch system.  The first tests you’ll want to perform are basic tests that reveal the critical characteristics of any device.

Number of contact points

One of the first tests is to find out how many contact points can be simultaneously detected.  Simply open the program’s drawing application, select a brush color, and try to draw lines with all 10 fingers at the same time.  The number of lines that actually appear will tell you the number of simultaneous touch points supported by your hardware.  Many multi-touch implementations will stop at two or maybe four; if you get all 10 lines, ask a friend to lend a hand and try to find the limit.  Support for multiple touch points enables more interesting interactions with games, and is also important in applications like handwriting capture.  Unless you’re going to be using only basic gestures, look for an implementation that supports more than two simultaneous contact points.

Drawing with 10 fingers on a Stantum Slate Demo

What can be detected

All touch interfaces are designed to detect a fingertip, but what about fingernails, gloved fingers, or a stylus?  Using the drawing application again, try drawing with different pointers and keep track of the results.  Use a glove, your fingernail, a stylus, or a paintbrush.  If supported, fingernails can be very handy for very precise pointing tasks without having to pull out a special stylus; many touch implementations today require a special active stylus, which can be easily misplaced and expensive to replace.  Check to see if an ordinary object like the wrong end of a pen can work as a stylus.  Try a handwriting recognition utility to see if the stylus experience will meet your needs.  Testing with a paintbrush can be very revealing, both about the types of touch that can be detected and the response to the touch area or pressure.

Touch input with a retracted mine pen

Responsiveness

All touch input systems will have some delay between the touch input and the system response.  This can be affected by the scan rate of the touch sensor, as well as the overall speed of the system in processing the input and producing an output.  A slow response will limit the use for games and other applications that require high performance and will generally lead to dissatisfaction in using the device.  You can measure responsiveness quickly without any equipment; just draw a straight line across the screen with a drawing application and note any delay between your finger and the line that appears.  Try it again with multiple touch points to see if there is any increase in the delay.  Another test is to draw a fast scribble between two horizontal lines; if the touch input is not being sampled at a high enough rate, the scribble that appears will not reach all the way to the horizontal lines.

This video capture shows a significant lag when drawing with two contacts on an Asus T91MT

Resolution

The ability of a touch interface to locate a precise point on the screen is the resolution, but don’t expect a touch input system to have the same resolution as a display.  Most graphical objects you’ll manipulate with touch do not require pixel-level accuracy.  The question is whether a given implementation has enough resolution to support the way you will use it.  If you’ll only be zooming maps or selecting icons, high precision may not be needed.  If you need to capture handwriting or use a sensitive touch interface for gaming, precision will be more important.  Usually the best way to evaluate the resolution is with a stylus, which allows the most precise pointing and helps you to discover the touch system’s resolution limits.  Using a drawing program, choose a minimal width line and try to draw the smallest circle you can.  You might also try writing a word, using smaller and smaller letters to see what is the limit where you lose the ability to read it back.

In a subsequent edition of Touch Point, we’ll examine some of the more advanced tests for choosing a multi-touch system.