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Modern Operating Systems by Herbert Bos and Andrew S. Tanenb...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf
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Modern Operating Systems by Herbert Bos and Andrew...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf-M ODERN O PERATING S YSTEMS
Modern Operating Systems by Herbert...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf-M ODERN O PERATING S YSTEMS
Page 446
SEC. 5.6
USER INTERFACES: KEYBOARD, MOUSE, MONITOR
415
plastic. However, a thin film of
ITO
(
Indium Tin Oxide
) or some similar con-
ducive material) is printed in thin lines onto the surface’s underside. Beneath it, but
not quite touching it, is a second surface also coated with a layer of ITO. On the
top surface, the charge runs in the vertical direction and there are conductive con-
nections at the top and bottom. In the bottom layer the charge runs horizontally and
there are connections on the left and right. When you touch the screen, you dent
the plastic so that the top layer of ITO touches the bottom layer. To find out the
exact position of the finger or stylus touching it, all you need to do is measure the
resistance in both directions at all the horizontal positions of the bottom and all the
vertical positions of the top layer.
Capacitive Screens
have two hard surfaces, typically glass, each coated with
ITO. A typical configuration is to have ITO added to each surface in parallel lines,
where the lines in the top layer are perpendicular to those in the bottom layer. For
instance, the top layer may be coated in thin lines in a vertical direction, while the
bottom layer has a similarly striped pattern in the horizontal direction. The two
charged surfaces, separated by air, form a grid of really small capacitors. Voltages
are applied alternately to the horizontal and vertical lines, while the voltage values,
which are affected by the capacitance of each intersection, are read out on the other
ones. When you put your finger onto the screen, you change the local capacitance.
By very accurately measuring the miniscule voltage changes everywhere, it is pos-
sible to discover the location of the finger on the screen. This operation is repeated
many times per second with the coordinates touched fed to the device driver as a
stream of (x, y) pairs. Further processing, such as determining whether pointing,
pinching, expanding, or swiping is taking place is done by the operating system.
What is nice about resistive screens is that the pressure determines the outcome
of the measurements. In other words, it will work even if you are wearing gloves in
cold weather. This is not true of capacitive screens, unless you wear special gloves.
For instance, you can sew a conductive thread (like silver-plated nylon) through the
fingertips of the gloves, or if you are not a needling person, buy them ready-made.
Alternatively, you cut off the tips of your gloves and be done in 10 seconds.
What is not so nice about resistive screens is that they typically cannot support
multitouch
, a technique that detects multiple touches at the same time. It allows
you to manipulate objects on the screen with two or more fingers. People (and per-
haps also orangutans) like multitouch because it enables them to use pinch-and-ex-
pand gestures with two fingers to enlarge or shrink a picture or document. Imagine
that the two fingers are at (3, 3) and (8, 8). As a result, the resistive screen may
notice a change in resistance on the
x
= 3 and
x
= 8 vertical lines, and the
y
= 3 and
y
= 8 horizontal lines. Now consider a different scenario with the fingers at (3, 8)
and (8, 3), which are the opposite corners of the rectangle whose corners are (3, 3),
(8, 3), (8, 8), and (3, 8). The resistance in precisely the same lines has changed, so
the software has no way of telling which of the two scenarios holds. This problem
is called
ghosting
.
Because capacitive screens send a stream of (
x
,
y
) coordinates,
they are more adept at supporting multitouch.
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