[HarfBuzz] Some (obsolete) khmer fonts not working (anymore) with harfbuzz

[Jonathan Kew]

On 13/5/13 10:58, Olivier Berten wrote:
> Well... any string with coeng for instance. The string I'm using is:
> ពុម្ពអក្សរខ្មែរ
>
> Here are the results of
> hb-view --shapers=old Kh-Svayrieng.ttf ពុម្ពអក្សរខ្មែរ
> hb-view --shapers=ot Kh-Svayrieng.ttf ពុម្ពអក្សរខ្មែរ

OK, so the problem is triggered by the fact that these fonts (at least 
the one I looked at) lack a 'pref' feature, and as such, harfbuzz-ng 
does not process them with the Indic shaper (which would rely on this 
feature for correct shaping). Instead, it sends them through the 
"generic" shaper.

This works "correctly" for fonts that implemented *all* their behavior 
using "generic" features such as 'liga' and 'clig', which people used to 
do in order to get usable results from applications that did not really 
support Indic or Khmer shaping at all.

However, in this case the font is using features such as 'abvs', 'blwf', 
'pres', etc to do various aspects of the shaping. But these aren't 
applied by the generic shaper.

So I think you'll get the "expected" results if you explicitly tell hb 
to enable these features:

   hb-view --features abvs,blwf,blws,pres,psts Kh-Svayrieng.ttf ពុម្ពអក្សរខ្មែរ

Not sure whether we should try to detect this and handle it 
automatically somehow...

JK