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<div class="sect1">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="the-distinction-between-levels-0-and-1"></a>The distinction between levels 0 and 1</h2></div></div></div>
<p>
So, the above is pretty much what cluster levels 0 and 1 do. The
only difference between the two is this: in level 0, at the very
beginning of the shaping process, we also merge clusters between
base characters and all Unicode marks (combining or not) following
them. E.g.:
</p>
<pre class="programlisting">
A,acute,B
0,1 ,2
</pre>
<p>
will become:
</p>
<pre class="programlisting">
A,acute,B
0,0 ,2
</pre>
<p>
This is the default behavior. We do it because Windows did it and
old HarfBuzz did it, so this remained the default. But this behavior
makes it impossible to color diacritic marks differently from their
base characters. That's why in level 1 we do not perform this
initial merging step.
</p>
<p>
For clients, level 0 is more convenient if they rely on HarfBuzz
clusters for cursor positioning. But that's wrong anyway: cursor
positions should be determined based on Unicode grapheme boundaries,
NOT shaping clusters. As such, level 1 clusters are preferred.
</p>
<p>
One last note about levels 0 and 1. We currently don't allow a
<code class="literal">MultipleSubst</code> lookup to replace a glyph with zero
glyphs (i.e., to delete a glyph). But in some other situations,
glyphs can be deleted. In those cases, if the glyph being deleted is
the last glyph of its cluster, we make sure to merge the cluster
with a neighboring cluster.
</p>
<p>
This is, primarily, to make sure that the starting cluster of the
text always has the cluster index pointing to the start of the text
for the run; more than one client currently relies on this
guarantee.
</p>
<p>
Incidentally, Apple's CoreText does something else to maintain the
same promise: it inserts a glyph with id 65535 at the beginning of
the glyph string if the glyph corresponding to the first character
in the run was deleted. HarfBuzz might do something similar in the
future.
</p>
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