[HarfBuzz] Unsure how to use HB output to combine characters

[Simon Cozens]

On 28/11/2014 18:22, Martin Hosken wrote:
> Is it that the Kokonor font has the diacritics as overstriking, i.e. 
> with a negative x-min (and probably x-max too)? This would account
> for all the advance being on the base character and none on the
> diacritic.

Huh. Well, in *that* case, Harfbuzz (or possibly FT) is not telling me
the truth about how to position my glyphs.

>> What am I doing wrong?
> Allowing the PDF viewer to do shaping?

So I *shouldn't* be getting two glyphs out of Harfbuzz. If I run
hb-shape using the Jomolhari Tibetan font, it gives me one glyph.

Aha! Looking at the Kokonor font repertoire in Font Book, it *does* have
the SA/KA combination sitting right there in glyph position 318.
Harfbuzz should surely be returning that to me. The other Tibetan font
that comes with OS X, Kailasa, has a SA/KA combination but I still get
two separate glyphs out of Harfbuzz.

OS X's own font shaper *can* render Tibetan stacks properly in both
Kokonor and Kailasa, as the text works fine in the browser / text editor
/ etc. That information must be there in the font to render the stacks
correctly, but Harfbuzz is not correctly picking it up.

On the page
http://digitaltibetan.org/index.php/Tibetan_Fonts#Mac_OS-X_Unicode_fonts
, it says "Unfortunately both Mac OS-X 10.5 and 10.6 (which supports
Tibetan) and Adobe's programs support a different set of OpenType
features which are required to correctly render Tibetan glyphs. So far
Chris Fynn's Jomolhari ID font is the only publicly available font that
supports all platforms." Are funny OpenType tables in use?

I'm shocked; it looks like this might not actually be my fault at all.