[HarfBuzz] Mark zeroing mess

[Behdad Esfahbod]

I've now confirmed that this is exactly what Uniscribe does for Thai as well.
 I'm going ahead and making the change.

On 16-02-10 03:48 PM, Behdad Esfahbod wrote:
> Hi Jonathan,
> 
> To my surprise, I have produced evince that, contrary to our previous belief,
> Uniscribe does NOT zero mark advances in the default (eg. Latin) shaper based
> on Unicode.  In fact, this is my observation:
> 
> 
>   - With GDEF:
>     - If mark is class=3 in GDEF:
>       -> advance zeroed regardless of whether mark attached in GPOS or not.
>     - If mark is class != 0 in GDEF:
>       -> advance NOT zeroed (and mark attachment obviously doesn't apply.)
> 
>  - No GDEF:
>    - Mark advance is NOT zeroed; and mark attachment doesn't apply.
> 
> So, it looks like:
> 
>   1. Uniscribe is NOT synthesizing any GDEF,
> 
>   2. It's zeroing mark advance purely based on GDEF.
> 
> I think our GDEF synthesis is still a good idea.  But like to change advance
> zeroing to work based on GDEF.  This will fix the following bug:
> 
>   https://github.com/googlei18n/noto-fonts/issues/472#issuecomment-147528411
> 
> as well as a bug Martin and I discovered today, which is: currently GC=Mc
> marks retain their advance, even if they are attached.  That can't be right
> either...
> 
> The Thai and Tibetan shapers also use BY_UNICODE_LATE mark-zeroing.  I'm going
> to test those and change as well, to follow Uniscribe.
> 
> Comments?  Can you possibly get your monster test suite running after this change?
> 
> Thanks,
> 

-- 
behdad
http://behdad.org/