Transcript MultiTouch - WordPress.com

Daniel A. Taylor
Pitt- Bradford University
Introduction
Touch sensitivity is fairly common in
electronics today…
 …but only for a single point of contact at
any given time.
 Touch-sensitive devices that allow for
multiple points of contact have thus far
been expensive or difficult to produce.

Why multi-touch?

Allows a user to interact with a system
with more than one finger at once
 Touch Keyboards
 Operations using both hands

Also allows multiple users to interact on
the same touch-sensitive platform
 Interactive walls
 Interactive tabletops
Prior approaches to the problem

Matrix of smaller sensors
 Requires many connections, which severely
limits resolution
 Visually translucent - no touch-screen

Video cameras
 Measuring of brightness
 Identifying tracking markers in gel
A new approach…
F rustrated
T otal
I nternal
R eflection
Total Internal Reflection
When light encounters a medium with a
lower index of refraction (e.g., going
from glass to air), its refraction depends
on the angle at which it hits the border.
 Beyond a certain critical angle, light is
not refracted, but instead reflects
entirely within the material.

 This is the basis for fiber optics and other
optical wave guides.
Frustrated Total Internal Reflection
If another material touches that within
which the light is reflecting, the reflection
is frustrated, causing the light to escape.
 This has been used in the past:

 fingerprint imaging
 early touch sensors (1970s!)
 tactile sensors for robotic grips
A Schematic of FTIR
(Kasday, 1984)
Using FTIR for touch sensitivity

A clear acrylic sheet is used as the
touch surface.
 28 inches x 24 inches in prototype
Edges of surface lit by infra-red LEDs to
produce total internal reflection.
 A video camera is mounted under the
surface and facing it.
 When the surface is touched, the light
escapes and registers on the camera.

Using FTIR for touch sensitivity
Basic image-processing techniques are
performed on the camera output to
identify the points of contact.
 Computer-vision techniques are used to
interpret the motion of contact points as
discrete touches or strokes.
 Processing easily handled in real-time
by a 2 GHz Pentium IV processor

Advantages to this approach

High capture rate and resolution
 30 frames per second
 640x480
True zero-force touch sensitivity
Inexpensive to construct
 Scalable to much larger (even wallsized!) surfaces
 Transparent: can be combined with rearprojection display


How is projection achieved?
Disadvantages
Requires significant space behind touch
surface for camera
 Gloves, certain types of styluses, and
even dry skin may not register

 a function of refractive index

Residues on surface (e.g., sweat) also
produce FTIR effect that may build up
Any Questions?