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<title>Level 2: HarfBuzz Manual</title>
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<link rel="up" href="clusters.html" title="">
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<link rel="prev" href="the-distinction-between-levels-0-and-1.html" title="The distinction between levels 0 and 1">
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<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF">
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Level 2
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Level 2 is a different beast from levels 0 and 1. It is simple to
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describe, but hard to make sense of. It simply doesn't do any
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cluster merging whatsoever. When things ligate or otherwise multiple
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glyphs turn into one, the cluster value of the first glyph is
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retained.
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Here are a few examples of why processing cluster values produced at
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this level might be tricky:
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Ligatures with combining marks
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Imagine capital letters are bases and lower case letters are
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combining marks. With an input sequence like this:
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A,a,B,b,C,c
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0,1,2,3,4,5
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if A,B,C ligate, then here are the cluster
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values one would get under the various levels:
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level 0:
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ABC,a,b,c
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0 ,0,0,0
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level 1:
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ABC,a,b,c
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0 ,0,0,5
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level 2:
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ABC,a,b,c
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0 ,1,3,5
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Making sense of the last example is the hardest for a client,
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because there is nothing in the cluster values to suggest that
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B and C ligated with
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A.
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Reordering
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Another tricky case is when things reorder. Under level 2:
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A,B,C,D,E
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0,1,2,3,4
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Now imagine D moves before
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B:
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A,D,B,C,E
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0,3,1,2,4
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Now, if D ligates with B, we
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get:
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A,DB,C,E
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0,3 ,2,4
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In a different scenario, A and
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B could have ligated
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before D reordered; that
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would have resulted in:
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AB,D,C,E
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0 ,3,2,4
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There's no way to differentitate between these two scenarios based
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on the cluster numbers alone.
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Another problem appens with ligatures under level 2 if the
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direction of the text is forced to opposite of its natural
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direction (e.g. left-to-right Arabic). But that's too much of a
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corner case to worry about.
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