Transcript Chapter 1

Chapter 7
Characters and Fonts
Multimedia Systems
Key Points
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Character sets map abstract characters to bit-patterns.
The widely used ASCII character set can only
accommodate 96 printable characters. Eight bit
extensions to ASCII have been developed, culminating in
the ISO 8859 standards.
Unicode, which is identical to the 16-bit subset of ISO
10646 can, with the use of UTF-16 encoding, represent
over a million characters, which is enough for all known
languages.
A font is a collection of glyphs, which are small images
of character shapes.
Fonts may be monospaced or proportionally spaced;
serifed or sans serif; upright, italic or other shapes; they
may vary in width from condensed to extended and in
weight from ultra-light to ultra-bold.
Key Points
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Fonts may be classified as text or display.
The choice of fonts for multimedia productions
must take account of the special requirements of
display on a computer monitor.
Fonts may use bitmaps or outlines of glyphs.
Bitmapped fonts cannot be scaled accurately;
outline fonts need special software to render the
glyphs on screen.
Common outline font formats are Adobe Type1
(PostScript) and TrueType
Multiple Master fonts can be used to generate
arbitrary instances of a typeface.
Character Sets
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Character sets map abstract characters
to bit-patterns.
Abstract character and its graphic
representation
Character repertoire
Code points (code value)
Each abstract character maps to a code
point
ASCII
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ASCII (American Standard Code for Information
Interchange)
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128 code points
0-31, 127: control characters
ISO 646, 1972
8-bit extension
ISO 8859-1: ISO Latin 1
ISO 8859-2: Latin 2, Eastern European
ISO 8859-5 (Cyrillic), 8859-7 (modern Greek)
Shortcomings
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Not achieve universal adoption
256 is not enough
ISO 10646
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Universal Multiple-Octet Coded Character Set
(UCS)
32-bit character set
Hypercube, 4-D cube
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Each character (g, p, r, c)
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256 groups
A group= 256 planes of 256 rows
A row= 256 characters
Group g, plane p, row r, and column c
ISO Latin1= (0, 0, 0, *)
Fig. 7.1
Unicode
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16-bit character set
CJK consolidation (合併)
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字太多
Contemporary major language and classical
forms
Punctuation marks, technical and mathematical
symbols, arrow, dingbats (miscellaneous
symbols), …
Dingbats
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Point hands, stars
Unicode (2)
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39,000 symbols
Reserved for UTF-16 expansion
6400 code points for private use
Not for music notation or other symbolic
writing system
Basic Multilingual Plane (BMP)
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(0, 0, *, *)
Encoding
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Quoted-Printable (QP)
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8-bit ASCII => 7-bit ASCII
128-255= three bytes
First byte: ACSII code for =
 Remaining two: hexadecimal digitals
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MIME content type
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Text/html; charset = iso-8859-1
ISO 10646 Encoding
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ISO 10646
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UCS-4 (4 bytes)
BMP (0, 0, *, *) : top two bytes set to zero
 Universal Character Set
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UCS-2 (2 bytes)
Drop top two bytes
 UCS-2 = Unicode
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Unicode Encoding
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Three UCS Transformation Formats (UTFs)
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UTF-8 (8-bit bytes)
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If their high-order byte is zero, low-order byte < 128=> a single
byte
Otherwise
up to 6 bytes with highest bit = 1
UTF-7
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UTF-7, UTF-8, UTF-16
~ QP, pure ASCII text
UTF-16
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Transforming a subset of the UCS-4 repertoire into pairs of UCS2 values from a reserved range
Access to an extra 15 planes of ISO 10646
UTF-16
UCS-4
x< 0x10000
UTF-16
x
0001 0000..
0010 FFFF
y;z;
y=
((x - 0001 0000) / 400) + D800
z=
((x - 0001 0000) % 400) + DC00
unmapped
x >= 0011 0000
Fonts
• Glyph as a specific representation of a
character
A
A A A
A
A
• A font as a collection of glyphs used for the
visual depiction of characters
• A font is often associated with a set of
parameters (size, posture, weight, …) set to
certain value
Classification and Choice of Fonts
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Monospaced & Proportional
Serif & Sans serif
Upright shape & Italic shape
Condensed & Extended
Weight
Monospaced & Proportional
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Monospaced (or fixed-width)
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Lucida Console
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Each letter occupies the same amount
of horizontal space, so that the text
looks as if it was typed on a
typewriter.
Proportional
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Times New Roman
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Each letter occupies an amount of horizontal space
proportional to the width of the glyph, so that the text
looks as if it was printed in a book.
Serif & Sans serif
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Serifs: little stroke
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MS Reference Serif
C
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Sans Serif Font
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Sans: without
MS Reference Sans Serif
C
Serifs
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Difficult to render accurately at low
resolutions
Hard to read on a computer screen
Sans Serif fonts are widely used for
windows titles and menu entries.
Upright shape & Italic shape
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Upright: vertical strokes
Italic: slanted to the right (Fig. 7.7)
Slanted fonts (Fig. 7.8)
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Share the rightward slope of italic fonts but
lack their calligraphic (書法的) quality
Apply a shear transformation to an upright
font
Some italic fonts: handwriting
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Calligraphic font
Shapes
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Outline fonts
Hollow fonts
Fonts with drop shadows
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Condensed fonts
Extended fonts
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Weight
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Boldface (bold)
Ultra-bold, semi-bold, light, ultra-light
Reserved for Headings
Never use boldface for emphasis, always italics
Italic text renders badly at low resolutions => Bold text
Families
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Atalic version, bold version of an upright
font
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Group in to a family
Lucida Bright family = 20 fonts
When fonts from different families are
combined, their differences can be very
noticeable (Fig. 7.12)
=> Carefully avoided
Text & Display
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Text: for continuous text
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Unobtrusive (不突出)
Problematical: low resolution of monitors
Text for display: 60% larger
Display: heading, signs or advertising slogans on
poster
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Short message
Eye-catching
Fig. 7.13
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Desktop publishing (DTP): printing on paper
No control over fonts that will be used when text is
finally displayed
Software used for display may let users override the
original fonts with those of their own choosing.
Most fonts’ repertoires consist of the letters from some
alphabet.
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Not include lower case letters
Mathematical symbols are usually grouped into their own
fonts, knows as symbol fonts or pi fonts.
Font Measurement
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Absolute length units
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points (pt)
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picas (pc)
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1/72 inch = 0.3528 mm
12 point = 4.2333 mm
Relative length units
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ex units (ex)
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X-height
em units (em)
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The width of a capital letter M
one ex is equal to one-half em
Font Terminology
Horizontal Layout
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Bounding box
Left side bearing
bearingX
Top side bearing
bearingY
Vertical Layout
Kerning
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Kerning is the art of character fitting so that the
space between characters is visually correct
rather proportionally set by the machine.
Most often recommended in headlines, and
larger settings of type, it's the art of carefully
moving characters together so the word looks
and reads better without holes within the word.
Good cases are: Ta, To, Wo, Po or other
situations where a hole is formed by a wide
portion of a letter.
Ligature
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A ligature (連字體) is a set of two or more characters
that have been designed into a harmonious "set".
Kerning, ligatures: High-quality text layout software
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Word processors and web browsers cannot do this.
Bitmap Fonts
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Bitmap Fonts
 Bitmap fonts are by nature pre-rasterized,
they render very quickly, making them a good
choice where speed is important.
 Cannot be scaled gracefully
 Each platform has its own native Bitmapped
font format.
Outline Fonts
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Outline Fonts
 Outline fonts describe the character outlines
with a combination of control points and
curves.
 Cross-plateform
 Adobe type 1 (PostScript fonts), TrueType
 Scaled arbitrarily
 The same font for display and printing.
 Adobe type manager (ATM): Adobe type 1
TrueType & PostScript
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TrueType: quadratic (二次) curves
PostScript: cubic Bezier curves
TrueType font is stored as a series of
points which define the lines and curves
making up its shape.
OpenType unifies Type 1 and TrueType
Hints and Instruction
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Extra information for low resolutions
Type 1: hints
TrueType: instruction
ClearType
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Windows XP
ClearType delivers improved font display
resolution over traditional anti-aliasing.
It improves readability on color LCD monitors
with a digital interface.
Readability on CRT screens can also be
somewhat improved.
ClearType
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This is a picture of ClearType under extreme
magnification, with the sub-pixels of an LCD
explicitly rendered to show the structure of
the ClearType letterforms.
Anti-Aliasing
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Fig. 7.18
Anti-aliasing should be applied to large
fonts.
Multiple Master Fonts
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A new development
Medium weight font might lie half-way between
ultra-bold and ultra-light glyphs
4 Design axes: weight, width, optical size, serif
style
Fig 7.19: 3 design axis
A partial answer to
font substitution problem