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24 <a name="the-distinction-between-levels-0-and-1"></a>The distinction between levels 0 and 1</h2></div></div></div>
26 So, the above is pretty much what cluster levels 0 and 1 do. The
27 only difference between the two is this: in level 0, at the very
28 beginning of the shaping process, we also merge clusters between
29 base characters and all Unicode marks (combining or not) following
32 <pre class="programlisting">
39 <pre class="programlisting">
44 This is the default behavior. We do it because Windows did it and
45 old HarfBuzz did it, so this remained the default. But this behavior
46 makes it impossible to color diacritic marks differently from their
47 base characters. That's why in level 1 we do not perform this
51 For clients, level 0 is more convenient if they rely on HarfBuzz
52 clusters for cursor positioning. But that's wrong anyway: cursor
53 positions should be determined based on Unicode grapheme boundaries,
54 NOT shaping clusters. As such, level 1 clusters are preferred.
57 One last note about levels 0 and 1. We currently don't allow a
58 <code class="literal">MultipleSubst</code> lookup to replace a glyph with zero
59 glyphs (i.e., to delete a glyph). But in some other situations,
60 glyphs can be deleted. In those cases, if the glyph being deleted is
61 the last glyph of its cluster, we make sure to merge the cluster
62 with a neighboring cluster.
65 This is, primarily, to make sure that the starting cluster of the
66 text always has the cluster index pointing to the start of the text
67 for the run; more than one client currently relies on this
71 Incidentally, Apple's CoreText does something else to maintain the
72 same promise: it inserts a glyph with id 65535 at the beginning of
73 the glyph string if the glyph corresponding to the first character
74 in the run was deleted. HarfBuzz might do something similar in the
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