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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 444
SEC. 5.6
USER INTERFACES: KEYBOARD, MOUSE, MONITOR
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Window 1
Window 2
(a)
(b)
Figure 5-38.
Copying bitmaps using
BitBlt
. (a) Before. (b) After.
have file and information headers and a color table before the pixels. This infor-
mation makes it easier to move bitmaps between dissimilar devices.
Fonts
In versions of Windows before 3.1, characters were represented as bitmaps and
copied onto the screen or printer using
BitBlt
.
The problem with that, as we just
saw, is that a bitmap that makes sense on the screen is too small for the printer.
Also, a different bitmap is needed for each character in each size.
In other words,
given the bitmap for A in 10-point type, there is no way to compute it for 12-point
type. Because every character of every font might be needed for sizes ranging from
4 point to 120 point, a vast number of bitmaps were needed. The whole system was
just too cumbersome for text.
The solution was the introduction of TrueType fonts, which are not bitmaps but
outlines of the characters. Each TrueType character is defined by a sequence of
points around its perimeter. All the points are relative to the (0, 0) origin.
Using
this system, it is easy to scale the characters up or down. All that has to be done is
to multiply each coordinate by the same scale factor.
In this way, a TrueType char-
acter can be scaled up or down to any point size, even fractional point sizes. Once
at the proper size, the points can be connected using the well-known follow-the-
dots algorithm taught in kindergarten (note that modern kindergartens use splines
for smoother results).
After the outline has been completed, the character can be
filled in.
An example of some characters scaled to three different point sizes is
given in Fig. 5-39.
Once the filled character is available in mathematical form, it can be rasterized,
that is, converted to a bitmap at whatever resolution is desired.
By first scaling and
then rasterizing, we can be sure that the characters displayed on the screen or
printed on the printer will be as close as possible, differing only in quantization
Page 445
414
INPUT/OUTPUT
CHAP. 5
20 pt:
53 pt:
81 pt:
Figure 5-39.
Some examples of character outlines at different point sizes.
error.
To improve the quality still more, it is possible to embed hints in each char-
acter telling how to do the rasterization. For example, both serifs on the top of the
letter T should be identical, something that might not otherwise be the case due to
roundoff error.
Hints improve the final appearance.
Touch Screens
More and more the screen is used as an input device also. Especially on smart-
phones, tablets and other ultra-portable devices it is convenient to tap and swipe
away at the screen with your finger (or a stylus). The user experience is different
and more intuitive than with a mouse-like device, since the user interacts directly
with the objects on the screen. Research has shown that even orangutans and other
primates like little children are capable of operating touch-based devices.
A touch device is not necessarily a screen. Touch devices fall into two cate-
gories: opaque and transparent. A typical opaque touch device is the touchpad on a
notebook computer. An example of a transparent device is the touch screen on a
smartphone or tablet. In this section, however, we limit ourselves to touch screens.
Like many things that have come into fashion in the computer industry, touch
screens are not exactly new. As early as 1965, E.A. Johnson of the British Royal
Radar Establishment described a (capacitive) touch display that, while crude,
served as precursor of the displays we find today. Most modern touch screens are
either resistive or capacitive.
Resistive screens
have a flexible plastic surface on top. The plastic in itself is
nothing too special, except that is more scratch resistant than your garden variety
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