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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
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<body>
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<div id="content">
<h1>Tutorial 3: Working with FontForge auto-instructions</h1>
<p>
The easiest way to invoke the FontForge auto-instructor in the
context of an Xgridfit program is
with <a href="merge-mode.html">merge-mode</a>. It is also possible
to copy and revise instructions generated by the auto-instructor.
Here is a practical example drawn
from <a href="http://junicode.sourceforge.net">Junicode
Regular</a>, in which <b>b</b> is instructed "by hand,"
while <b>uni0180</b> (<b>bbar</b>) has been auto-instructed.
</p>
<p>
Although the outlines of <b>b</b> and <b>uni0180</b> are exactly the
same except for the bar, the instructed versions of these glyphs
look rather different at 20 ppem:
</p>
<table>
<tr>
<td>
<img alt="hand-instructed b" src="b-hinted.png"/>
</td>
<td>
<img alt="bbar auto-instructed" src="bbar-hinted.png"/>
</td>
</tr>
</table>
<p>
There are various subtle differences here, but the obvious ones
are that the top of the bowl is thicker and higher in <b>uni0180</b>
than in <b>b</b> and that <b>uni0180</b> is narrower. If we can
change these things, the two glyphs will match reasonably well.
</p>
<p>
The first step is to get a copy of the program for <b>uni0180</b>.
The easiest way to do so is to run a little Xgridfit utility,
<tt>getinstrs</tt>, against a .ttf or .sfd file containing the
glyph with its instructions (the <tt>--xgridfit</tt> option
ensures that we end up with a complete, valid Xgridfit file):
</p>
<pre>
getinstrs --xgridfit Junicode-Regular.ttf uni0180 > uni0180.xgf</pre>
<p>
Now we're ready to edit the instructions. Here is the program, in
which we have identified the problem areas by running the
FontForge TrueType debugger. We've inserted comments to mark the
places:
</p>
<pre>
<?xml version="1.0"?>
<xgridfit xmlns="http://xgridfit.sourceforge.net/Xgridfit2">
<glyph ps-name="uni0180" xml:id="uni0180" init-graphics="no">
<command name="SVTCA" modifier="y-axis"/>
<push>3 1 0</push>
<command name="CALL"/>
<push>9</push>
<command name="SHP" modifier="rp1"/>
<push>60</push>
<command name="MDRP" modifier="min,rnd,black"/>
<push>27 3 0</push>
<command name="CALL"/>
<push>24</push>
<command name="MDRP" modifier="min,rnd,black"/>
<push>27 24 10</push>
<command name="CALL"/>
<push>64 27 31 9</push>
<command name="CALL"/>
<b><!-- top of bowl --></b>
<push>42 52 3 31 13</push>
<command name="CALL"/>
<push>42</push>
<command name="MDRP" modifier="min,rnd,black"/>
<push>42 52 10</push>
<command name="CALL"/>
<push>64 42 46 9</push>
<command name="CALL"/>
<b><!-- end top of bowl --></b>
<push>20 17 3 31 13</push>
<command name="CALL"/>
<push>38</push>
<command name="SHP" modifier="rp1"/>
<push>20</push>
<command name="MDRP" modifier="min,rnd,black"/>
<push>34</push>
<command name="SHP" modifier="rp2"/>
<command name="SVTCA" modifier="x-axis"/>
<push>63</push>
<command name="MDAP" modifier="rnd"/>
<push>15</push>
<command name="MDRP" modifier="rp0,rnd,white"/>
<push>21</push>
<command name="SHP" modifier="rp2"/>
<push>57</push>
<command name="MDRP" modifier="min,rnd,black"/>
<push>32 39</push>
<command name="SHP" modifier="rp2"/>
<command name="SHP" modifier="rp2"/>
<push>57 15 10</push>
<command name="CALL"/>
<push>64 57 37 9</push>
<command name="CALL"/>
<push>15 57 10</push>
<command name="CALL"/>
<push>64 15 26 9</push>
<command name="CALL"/>
<push>64 15 19 9</push>
<command name="CALL"/>
<b><!-- width of character --></b>
<push>57</push>
<command name="SRP0"/>
<push>49 1</push>
<command name="CALL"/>
<push>0</push>
<command name="MDRP" modifier="min,rnd,black"/>
<b><!-- end width of character --></b>
<push>64 1</push>
<command name="CALL"/>
<push>57 15</push>
<command name="SRP1"/>
<command name="SRP2"/>
<push>7 10 30</push>
<command name="IP"/>
<command name="IP"/>
<command name="IP"/>
<push>49</push>
<command name="SRP1"/>
<push>6 3 46</push>
<command name="IP"/>
<command name="IP"/>
<command name="IP"/>
<command name="SVTCA" modifier="y-axis"/>
<push>60 3</push>
<command name="SRP1"/>
<command name="SRP2"/>
<push>6 13</push>
<command name="IP"/>
<command name="IP"/>
<push>52</push>
<command name="SRP1"/>
<push>0</push>
<command name="IP"/>
<command name="IUP" modifier="y"/>
<command name="IUP" modifier="x"/>
</glyph>
</xgridfit></pre>
<p>
The FontForge code looks a little obscure because it does much of
its work by making calls to a library of functions, which the
auto-instructor inserts into every font. (These well-crafted
functions may also be of interest to Xgridfit programmers. You can
get access to them via the macros defined in
<tt>ff-functions.xgf</tt>, found in the <tt>utils</tt>
subdirectory of the Xgridfit directory tree). However, it is not
really necessary to understand the function calls to see in
general terms what the auto-instructor is doing.
</p>
<p>
We'll start by defining some constants for point numbers: first
the two points that define the top and inside top of the bowl, and
next some key points for the straight stem on the left of the
glyph and the round stem on the right:
</p>
<pre>
<constant name="bowl-top" value="46"/>
<constant name="bowl-top-inside" value="52"/>
<constant name="left-left" value="15"/>
<constant name="left-right" value="57"/>
<constant name="right-right" value="0"/>
<constant name="right-left" value="49"/></pre>
<table>
<tr>
<td>
<img alt="bbar auto-instructed" src="bbar-points.png"/>
</td>
</tr>
</table>
<p>
The next step is to notice how the Junicode <b>b</b> is
instructed. There, <tt>bowl-top</tt> is positioned at the x-height
with a control value (<tt>lc-x-height</tt>), and the distance
between <tt>bowl-top</tt> and <tt>bowl-top-inside</tt> is
regulated with another control value
(<tt>lc-horz-thin-curve</tt>). We'll comment out the offending
instructions and replace them with Xgridfit code that employs the
same control values:
</p>
<pre>
<move distance="lc-x-height">
<point num="bowl-top"/>
<move distance="lc-horz-thin-curve">
<point num="bowl-top-inside"/>
</move>
</move>
<!-- top of bowl
<push>42 52 3 31 13</push>
<command name="CALL"/>
<push>42</push>
<command name="MDRP" modifier="min,rnd,black"/>
<push>42 52 10</push>
<command name="CALL"/>
<push>64 42 46 9</push>
<command name="CALL"/>
--></pre>
<p>
Next, the FontForge auto-instructor has instructed along the
x-axis by first positioning <tt>left-left</tt>, then
<tt>left-right</tt> relative to that, then <tt>right-left</tt>,
then <tt>right-right</tt>. In other words, it has gone strictly
left-to-right. The Junicode <b>b</b> is instructed in a different
order: first the left stem is regulated, then the overall width of
the glyph (<tt>left-left</tt> to <tt>right-right</tt>, with a
control value), and finally the right stem. We can approximate
this behavior in revising the FontForge instructions (we'll ignore
the width of the left stem for now):
</p>
<pre>
<move distance="bpq-char-width">
<reference>
<point num="left-left"/>
</reference>
<point num="right-right"/>
<move distance="lc-vert-curve">
<point num="right-left"/>
</move>
</move>
<!-- width of character
<push>57</push>
<command name="SRP0"/>
<push>49 1</push>
<command name="CALL"/>
<push>0</push>
<command name="MDRP" modifier="min,rnd,black"/>
--></pre>
<p>
We'll address two more details before testing the code. First, the
FontForge auto-instructor regulates the left sidebearing with an
MDRP instruction:
</p>
<pre>
<push>63</push>
<command name="MDAP" modifier="rnd"/>
<push>15</push>
<command name="MDRP" modifier="rp0,rnd,white"/></pre>
<p>
This is the equivalent of the following:
</p>
<pre>
<move>
<point num="left-sidebearing"/>
<move>
<point num="left-left"/>
</move>
</move></pre>
<p>
But Junicode uses a control value to regulate this distance. We
can revise the code very simply, by adding the control value and
changing MDRP to MIRP.
</p>
<pre>
<push>63</push>
<command name="MDAP" modifier="rnd"/>
<push>15 b-left-side</push>
<command name="MIRP" modifier="rp0,rnd,white"/></pre>
<p>
Or we could replace the code (omitting to touch the
left-sidebearing point, since this is unnecessary):
</p>
<pre>
<move distance="b-left-side">
<reference>
<point num="left-sidebearing"/>
<reference>
<point num="left-left"/>
</move></pre>
<p>
Junicode also uses a control value to regulate standard vertical
stems, and so we can make a similar change to the way the
auto-instructor regulates the distance between <tt>left-left</tt>
and <tt>left-right</tt>. We'll just change this:
</p>
<pre>
<push>57</push>
<command name="MDRP" modifier="min,rnd,black"/></pre>
<p>
to this:
</p>
<pre>
<push>57 lc-vert-stem</push>
<command name="MIRP" modifier="min,rnd,black"/></pre>
<p>
We'll just use <a href="xinclude.html">XInclude</a> to include the
revised instructions in the font programming and compile (just
<b>b</b> and <b>uni0180</b>, since we haven't got all day). Since
we're feeling confident, we'll use the <tt>-f</tt> option to pipe
Xgridfit's output to FontForge:
</p>
<pre>
$ xgridfit -g b+bbar -f Junicode-Regular.xgf</pre>
<p>
Here is the result from <tt>Junicode-Regular.ttf</tt> as viewed in
FontForge, with <b>b</b> again for comparison:
</p>
<table>
<tr>
<td>
<img alt="hand-instructed b" src="b-hinted.png"/>
</td>
<td>
<img alt="bbar auto-instructed" src="bbar-revised.png"/>
</td>
</tr>
</table>
<p>
It might have been just as easy to copy the program for <b>b</b>
and edit it; but editing an auto-instructed glyph will usually be
quick and very often the easiest thing to do.
</p>
</div>
</body>
</html>
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