◄Up► ◄Next► ◄Previous►
────────────────────────────────────────────────────────────────────────────
 
Screen fonts provide a reasonable approximation of printer output on the
screen. Microsoft Windows applications that compose text for printers need
accurate screen fonts to ensure a WYSIWYG display. This means the screen
fonts must match the printer fonts as closely as possible. The more accurate
the representation, the more accurate the display. This topic presents the
factors font vendors should consider when creating screen fonts.
 
Aspect-Ratio Classes for Display Devices
 
Font vendors should create one set of screen fonts for each of the popular
aspect-ratio classes for displays. The aspect-ratio classes determine the
relative shape of pixels on the device, and therefore affect the design and
implementation of screen fonts. Windows recognizes two major aspect-ratio
classes.
 
Class         Typical device
────────────────────────────────────────────────────────────────────────────
1.33:1 (4:3)  The IBM Enhanced Graphics Adapter
 
1:1           The IBM PS/2 MCGA, VGA, and 8514/a
 
The actual dots-per-inch resolution is not a factor in determining the class
of the device; only the aspect ratio is used.
 
Translating Point Sizes into Line Sizes
 
Because of the variation in screen resolutions, screen font sizes are
discussed in terms of screen line sizes, not typographic point sizes. The
equation for relating point size to equivalent screen line size is as
follows:
 
                    text point size  * lines-per-inch
screen line size =  --------------------------------------
                                    72
 
The lines-per-inch value specifies the logical page size for the device.
When possible, the lines-per-inch closely matches the physical size and
aspect ratio. The common screen lines-per-inch sizes for devices supported
by Windows are as follows:
 
♦  For a 1:1 aspect ratio, 96:96 (VGA), 108:108, 120:120 (8514/a), and
   144:144
 
♦  For a 4:3 aspect ratio, 96:72 (IBM Enhanced Graphics Adapter)
 
Since it is always better to select a smaller size when an exact match is
not possible, the quotient in the equation is truncated if the result is a
fractional line size.
 
Choosing the Correct Range of Line Sizes
 
To accurately represent a printer font on the screen, the font vendor must
make available a reasonable range of screen fonts. However, each screen font
uses memory. Therefore, a font vendor should carefully weigh performance
against a large screen-font variety.
 
The most important factor to consider when creating screen-font files is
memory use. Fewer and smaller screen fonts use less memory, but may degrade
the WYSIWYG quality of the display.
 
Other factors to consider include the following:
 
♦  Legibility Threshold. There is a certain size at which the fonts are
   difficult, if not impossible, to read. For most screens, the value is 6
   lines. For high-resolution screens, the value is around 9-10 lines.
   Therefore, it does not make sense to provide fonts below these sizes.
 
♦  Doubling and Tripling. The screen driver may double or triple a
   small-size font to take the place of an unavailable large font. For
   example, it may double an 18-line font to make a 36-line font, or it may
   triple a 24-line font to make a 72-line font. Doubling is not attractive,
   but acceptable and difficult to avoid. Tripling is unacceptable and can
   be avoided by providing a well-chosen range of screen line sizes.
 
♦  Specific Applications. The application can control how the screen driver
   will select a font for such things as text greeking, doubling and
   tripling, vector font substitution, and downsizing to match widths.
 
Greeking
 
Some applications stop displaying screen fonts below a certain line size and
may display a gray shaded box or other graphic representation. This is
referred to as "greeked" text. Greeking speeds up the screen display.
 
Proof Versus Draft Quality
 
In Windows, proof usually means good, and draft means poor but quick. The
screen driver will never double or triple the size of a font for proof
quality. However, it will double or triple the size of smaller fonts to get
the exact size it wants for draft quality. If the correct size is available,
the screen driver will always choose it.
 
Doubling and tripling occur only when the exact size is not available. For
example, assume the only screen sizes are 10 and 19 lines, and the
application wants a 20-line screen font. In proof quality, the screen driver
would choose the 19-line font. In draft quality, the screen driver would
double the 10-line font.
 
Vector Font Substitution
 
Vector outline fonts are not as attractive as raster screen fonts, but they
look better than doubled or tripled screen fonts. Also, the amount of memory
used by vector fonts is constant while the amount of memory used by raster
fonts increases as the font size increases.
 
Downsizing to Match Widths
 
When the screen fonts do not accurately represent the printer fonts (that
is, the height-to-width ratio of the printer font is different from that of
the screen font), an application can request a smaller font to get the
correct width. The application can display the line more quickly and prevent
the characters from overlapping using this method.
 
This behavior can most often be observed when the font vendor does not
supply matching screen fonts, or the screen fonts do not contain the same
height-to-width ratio as their corresponding printer fonts.
 
Recommended Screen-Font Sizes
 
Nothing produces an accurate display representation than a wide range of
screen font sizes. On the other hand, nothing affects system performance
faster than a large number of screen fonts. Therefore, the selection of
screen fonts must balance desired display quality with system performance.
 
For example, consider two computers with different screen-font
configurations. Computer A contains the screen font sizes 7, 10, and 16
lines. Computer B contains the sizes 7, 8, 9, 10, 11, 12, 13, 14, 15, 16,
17, and 18 lines. If the user creates a page using all the fonts from
Computer B, the differences in sizes will be visible. However, the computer
will load 12 screen fonts to display the page.
 
On Computer A, with doubling and tripling capabilities disabled, the
computer displays the same range of sizes as follows:
 
♦  7, 8, and 9 lines displayed with the 7-line font
 
♦  10, 11, 12, 13, 14, and 15 lines displayed with the 10-line font
 
♦  16, 17, and 18 lines displayed with the 16-line font
 
Much of the WYSIWYG would be lost because several sizes are displayed with
the same font. However, the computer only loaded three fonts to display the
page.
 
Computer B slowly displayed high quality, while Computer A quickly displayed
poor WYSIWYG quality.
 
A font vendor needs to consider the application the user will run with
Windows. For example, Microsoft Excel generally uses body-size fonts (fonts
in the size range of 8 to 12 points). Therefore, a wide range of font sizes
is unnecessary. And Aldus PageMaker, although it uses a large range of
sizes, switches to vector fonts above 24 lines (that is, the default "Vector
text above" setting).
 
Ideally, the user should decide which is more important: display quality or
performance. Many font products contain a font generator that requires the
user to specify which point sizes to build. For such programs, the user
should be given the option to select a predetermined range of sizes.
 
Range        Sizes
────────────────────────────────────────────────────────────────────────────
Publishing   8, 9, 10, 12, 14, 18, and 24 lines.
 
Artwork      7, 8, 9, 10, 11, 12, 14, 16, 18, 21, 24, 28, and 32 lines.
 
General use  8, 10, 12, 18, and 24 lines.
 
If the font-creation program requires the user to indicate exactly which
sizes should be built (that is, there are no predetermined ranges), then the
documentation should provide specific instructions on what numbers to enter,
based upon the user's needs, or intended use for the product.
 
For font packages that give the user no choice in the size range, the vendor
should supply the "Publishing" size range of 8, 9, 10, 12, 14, 18, and 24
lines.
 
 
                                      ♦