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FreeType Glyph Conventions
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version 2.1
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Copyright 1998-2000 David Turner (<a href="mailto:[email protected]">[email protected]</a>)<br>
Copyright 2000 The FreeType Development Team (<a href="[email protected]">[email protected]</a>)
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<table width="100%"><tr valign=center bgcolor="#CCCCFF"><td><h2>
V. Text processing
</h2></td></tr></table>
<p>This section demonstrates how to use the concepts previously
defined to render text, whatever the layout you use.
</p>
<h3><a name="section-1">
1. Writing simple text strings :
</h3><blockquote>
<p>In this first example, we'll generate a simple string of Roman
text, i.e. with a horizontal left-to-right layout. Using exclusively pixel
metrics, the process looks like :
<blockquote><tt>1) convert the character string into a series of glyph
indexes.</tt>
<br><tt>2) place the pen to the cursor position.</tt>
<br><tt>3) get or load the glyph image.</tt>
<br><tt>4) translate the glyph so that its 'origin' matches the pen position</tt>
<br><tt>5) render the glyph to the target device</tt>
<br><tt>6) increment the pen position by the glyph's advance width in pixels</tt>
<br><tt>7) start over at step 3 for each of the remaining glyphs</tt>
<br><tt>8) when all glyphs are done, set the text cursor to the new pen
position</tt></blockquote>
Note that kerning isn't part of this algorithm.</blockquote>
<h3><a name="section-2">
2. Sub-pixel positioning :</h3>
<blockquote>It is somewhat useful to use sub-pixel positioning when rendering
text. This is crucial, for example, to provide semi-WYSIWYG text layouts.
Text rendering is very similar to the algorithm described in sub-section
1, with the following few differences :
<ul>
<li>
The pen position is expressed in fractional pixels.</li>
<li>
Because translating a hinted outline by a non-integer distance will ruin
its grid-fitting, the position of the glyph origin must be rounded before
rendering the character image.</li>
<li>
The advance width is expressed in fractional pixels, and isn't necessarily
an integer.</li>
</ul>
<p><br>Which finally looks like :
<blockquote><tt>1. convert the character string into a series of glyph
indexes.</tt>
<br><tt>2. place the pen to the cursor position. This can be a non-integer
point.</tt>
<br><tt>3. get or load the glyph image.</tt>
<br><tt>4. translate the glyph so that its 'origin' matches the rounded
pen position.</tt>
<br><tt>5. render the glyph to the target device</tt>
<br><tt>6. increment the pen position by the glyph's advance width in fractional
pixels.</tt>
<br><tt>7. start over at step 3 for each of the remaining glyphs</tt>
<br><tt>8. when all glyphs are done, set the text cursor to the new pen
position</tt></blockquote>
Note that with fractional pixel positioning, the space between two given
letters isn't fixed, but determined by the accumulation of previous rounding
errors in glyph positioning.</blockquote>
<h3><a name="section-3">
3. Simple kerning :</h3>
<blockquote>Adding kerning to the basic text rendering algorithm is easy
: when a kerning pair is found, simply add the scaled kerning distance
to the pen position before step 4. Of course, the distance should be rounded
in the case of algorithm 1, though it doesn't need to for algorithm 2.
This gives us :
<p>Algorithm 1 with kerning:
<blockquote><tt>3) get or load the glyph image.</tt>
<br><tt>4) Add the rounded scaled kerning distance, if any, to the pen
position</tt>
<br><tt>5) translate the glyph so that its 'origin' matches the pen position</tt>
<br><tt>6) render the glyph to the target device</tt>
<br><tt>7) increment the pen position by the glyph's advance width in pixels</tt>
<br><tt>8) start over at step 3 for each of the remaining glyphs</tt></blockquote>
<p><br>Algorithm 2 with kerning:
<blockquote><tt>3) get or load the glyph image.</tt>
<br><tt>4) Add the scaled unrounded kerning distance, if any, to the pen
position.</tt>
<br><tt>5) translate the glyph so that its 'origin' matches the rounded
pen position.</tt>
<br><tt>6) render the glyph to the target device</tt>
<br><tt>7) increment the pen position by the glyph's advance width in fractional
pixels.</tt>
<br><tt>8) start over at step 3 for each of the remaining glyphs</tt></blockquote>
Of course, the algorithm described in section IV can also be applied to
prevent the sliding dot problem if one wants to..</blockquote>
<h3><a name="section-4">
4. Right-To-Left Layout :</h3>
<blockquote>The process of laying out arabic or hebrew text is extremely
similar. The only difference is that the pen position must be decremented
before the glyph rendering (remember : the advance width is always positive,
even for arabic glyphs). Thus, algorithm 1 becomes :
<p>Right-to-left Algorithm 1:
<blockquote><tt>3) get or load the glyph image.</tt>
<br><tt>4) Decrement the pen position by the glyph's advance width in pixels</tt>
<br><tt>5) translate the glyph so that its 'origin' matches the pen position</tt>
<br><tt>6) render the glyph to the target device</tt>
<br><tt>7) start over at step 3 for each of the remaining glyphs</tt></blockquote>
<p><br>The changes to Algorithm 2, as well as the inclusion of kerning
are left as an exercise to the reader.
<br>
<br> </blockquote>
<h3><a name="section-5">
5. Vertical layouts :</h3>
<blockquote>Laying out vertical text uses exactly the same processes, with
the following significant differences :
<br>
<blockquote>
<li>
The baseline is vertical, and the vertical metrics must be used instead
of the horizontal one.</li>
<li>
The left bearing is usually negative, but this doesn't change the fact
that the glyph origin must be located on the baseline.</li>
<li>
The advance height is always positive, so the pen position must be decremented
if one wants to write top to bottom (assuming the Y axis is oriented upwards).</li>
</blockquote>
Through the following algorithm :
<blockquote><tt>1) convert the character string into a series of glyph
indexes.</tt>
<br><tt>2) place the pen to the cursor position.</tt>
<br><tt>3) get or load the glyph image.</tt>
<br><tt>4) translate the glyph so that its 'origin' matches the pen position</tt>
<br><tt>5) render the glyph to the target device</tt>
<br><tt>6) decrement the vertical pen position by the glyph's advance height
in pixels</tt>
<br><tt>7) start over at step 3 for each of the remaining glyphs</tt>
<br><tt>8) when all glyphs are done, set the text cursor to the new pen
position</tt></blockquote>
</blockquote>
<h3><a name="section-6">
6. WYSIWYG text layouts :</h3>
<blockquote>As you probably know, the acronym WYSIWYG stands for '<i>What
You See Is What You Get</i>'. Basically, this means that the output of
a document on the screen should match "perfectly" its printed version.
A <b><i>true</i></b> wysiwyg system requires two things :
<p><b>device-independent text layout</b>
<blockquote>Which means that the document's formatting is the same on the
screen than on any printed output, including line breaks, justification,
ligatures, fonts, position of inline images, etc..</blockquote>
<p><br><b>matching display and print character sizes</b>
<blockquote>Which means that the displayed size of a given character should
match its dimensions when printed. For example, a text string which is
exactly 1 inch tall when printed should also appear 1 inch tall on the
screen (when using a scale of 100%).</blockquote>
<p><br>It is clear that matching sizes cannot be possible if the computer
has no knowledge of the physical resolutions of the display device(s) it
is using. And of course, this is the most common case ! That's not too
unfortunate, however because most users really don't care about this
feature. Legibility is much more important.
<p>When the Mac appeared, Apple decided to choose a resolution of 72 dpi
to describe the Macintosh screen to the font sub-system (whatever the monitor
used). This choice was most probably driven by the fact that, at this resolution,
1 point = 1 pixel. However; it neglected one crucial fact : as most users
tend to choose a document character size between 10 and 14 points, the
resultant displayed text was rather small and not too legible without scaling.
Microsoft engineers took notice of this problem and chose a resolution
of 96 dpi on Windows, which resulted in slightly larger, and more legible,
displayed characters (for the same printed text size).
<p>These distinct resolutions explain some differences when displaying
text at the same character size on a Mac and a Windows machine. Moreover,
it is not unusual to find some TrueType fonts with enhanced hinting (tech
note: through delta-hinting) for the sizes of 10, 12, 14 and 16 points
at 96 dpi.
<br>
<p>As for device-independent text, it is a notion that is, unfortunately,
often abused. For example, many word processors, including MS Word, do
not really use device-independent glyph positioning algorithms when laying
out text. Rather, they use the target printer's resolution to compute <i>hinted</i>
glyph metrics for the layout. Though it guarantees that the printed version
is always the "nicest" it can be, especially for very low resolution printers
(like dot-matrix), it has a very sad effect : changing the printer can
have dramatic effects on the <i>whole</i> document layout, especially if
it makes strong use of justification, uses few page breaks, etc..
<p>Because the glyph metrics vary slightly when the resolution changes
(due to hinting), line breaks can change enormously, when these differences
accumulate over long runs of text. Try for example printing a very long
document (with no page breaks) on a 300 dpi ink-jet printer, then the same
one on a 3000 dpi laser printer : you'll be extremely lucky if your final
page count didn't change between the prints ! Of course, we can still call
this WYSIWYG, as long as the printer resolution is fixed !!
<p>Some applications, like Adobe Acrobat, which targeted device-independent
placement from the start, do not suffer from this problem. There are two
ways to achieve this : either use the scaled and unhinted glyph metrics
when laying out text both in the rendering and printing processes, or simply
use wathever metrics you want and store them with the text in order to
get sure they're printed the same on all devices (the latter being probably
the best solution, as it also enables font substitution without breaking
text layouts).
<p>Just like matching sizes, device-independent placement isn't necessarily
a feature that most users want. However, it is pretty clear that for any
kind of professional document processing work, it <b><i>is</i></b> a requirement.</blockquote>
</blockquote>
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