[HarfBuzz] USE and zeroing marks

[Martin Hosken]

Dear Behdad,

I see from the code that the USE shaper doesn't zero marks. But that the USE spec implies that they are:

the width of the base character must be added back using the <dist> feature. This is necessary because OT processing cancels the width associated with a mark. It is necessary to cancel the width of a non-spacing mark because it is not clear where to apply the width of a non-spacing mark during OpenType processing.

I'm hoping that the spacing marks proposal will answer that final question. But it does imply that marks need to have their advances zeroed.

I don't quite follow how not zeroing marks works. If I attach acute with advance of 100 to an a with advance 200, I assume I end up with a total advance of 300? I think the spacing mark proposal helps sort out the overlap problem which is really tricky to resolve otherwise even for those shapers that don't zero their marks.

Yours,
Martin