CoreText shaping bug?

Simon Cozens

I don't know if there's a better place to talk about this - if so, let me know where would be better and apologies for mentioning it here.

I'm using Gentium Plus and found a bug trying to typeset the word "praȷ̃nā" (wisdom). Here's a test string "aj a/jdotless (U+ 0237) a/jdotless/tildecomb (U+0303)" shaped correctly by HarfBuzz's hb-shape:

and here it is in CoreText:


Adding the combining tilde messes up the spacing.

So, is there a bad assumption in the font that just happens to work with Harfbuzz, or is there a bug in CoreText? (I am not using the Graphite shaper in Harfbuzz, so there's no Graphite magic happening.)

John Hudson

Yes, this seems to be a CoreText issue. I get the same kind of spacing problem testing with Brill and with assorted other fonts with GPOS mark attachment. Interestingly, I get a different, worse problem when testing with Consolas:

I get similar spacing problems to those you illustrate with Apple's own fonts, e.g. Hoefler Text.

I have also confirmed that the problem occurs with other combining marks, not just U+0303.

I have also confirmed that the problem occurs regardless of whether the base letter carrying the mark is encoded as j or dotless j.

However, the problem does not seem to occur when any other base letter is used.

Will activate the Ned signal.

Simon Cozens

Thanks. I was thinking of filing a Radar but that's often like talking to a black hole. I knew someone here would know the right way to get it to the right person...

Kent Lew

Incidentally, isn’t प्रज्ञा usually transliterated as prajñā?  ;-)

Simon Cozens

You're right, and that one I need to take up with my author. :-)