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The guarantees and benefits of level 0 and level 1 can be seen with some examples. First, let us examine what happens with cluster values when shaping involves cluster merging with ligatures and decomposition.
Let's say we start with the following character sequence (top row) and initial cluster values (bottom row):
A,B,C,D,E 0,1,2,3,4
During shaping, HarfBuzz maps these characters to glyphs from the font. For simplicity, let us assume that each character maps to the corresponding, identical-looking glyph:
A,B,C,D,E 0,1,2,3,4
Now if, for example, B
and C
form a ligature, then the clusters to which they belong
"merge". This merged cluster takes for its cluster
value the minimum of all the cluster values of the clusters that
went in to the ligature. In this case, we get:
A,BC,D,E 0,1 ,3,4
because 1 is the minimum of the set {1,2}, which were the
cluster values of B
and
C
.
Next, let us say that the BC
ligature glyph
decomposes into three components, and D
also
decomposes into two components. These components each inherit the
cluster value of their parent:
A,BC0,BC1,BC2,D0,D1,E 0,1 ,1 ,1 ,3 ,3 ,4
Next, if BC2
and D0
form a
ligature, then their clusters (cluster values 1 and 3) merge into
min(1,3) = 1
:
A,BC0,BC1,BC2D0,D1,E 0,1 ,1 ,1 ,1 ,4
At this point, cluster 1 means: the character sequence
BCD
is represented by glyphs
BC0,BC1,BC2D0,D1
and cannot be broken down any
further.