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