[HarfBuzz] difference between harfbuzz and uniscribe?

[Martin Hosken]

Dear Behdad,

Consider a font with the following FEA rule in it:

pos u1014' 79 u1032 u1037;

which adds 79 to the advance of the first glyph in the sequence u1014 u1032 u1037:

[u1014=0+609|u1032=0 at -89,-42+0|u1037=0 at -55,0+0]

vs without the 1037:

[u1014=0+530|u1032=0 at -10,-42+0]

But if we compare with a recent uniscribe we get:

[u1014=0+609|u1032=1 at -10,-42+0|u1037=2 at 24,0+0]
[u1014=0+530|u1032=1 at -10,-42+0]

and if we change the rule to compensate for the advance in the windows case we get:

pos u1014' 79 u1032' <-79 0 0 0> u1037;

harfbuzz gives:

[u1014=0+609|u1032=0 at -168,-42+0|u1037=0 at -55,0+0]
[u1014=0+530|u1032=0 at -10,-42+0]

uniscribe gives:

[u1014=0+609|u1032=1 at -89,-42+0|u1037=2 at 24,0+0]
[u1014=0+530|u1032=1 at -10,-42+0]

Yours,
Martin