[HarfBuzz] Documentation-requirements gathering

[Behdad Esfahbod]

On Tue, Jul 17, 2018 at 4:18 AM, Richard Wordingham <
richard.wordingham at ntlworld.com> wrote:

> On Mon, 16 Jul 2018 14:23:10 -0500
> Nathan Willis <nwillis at glyphography.com> wrote:
>
> > - If you are only casually using HarfBuzz but the shaping and layout
> > issues have been a stumbling block in the past, what material would
> > have helped
>
> This is a big hole, and perhaps off-topic


Off-topic indeed.


One problem lies in the change of advancewidth for glyphs
> categorised as marks - there is a subtle difference between
> Uniscribe/DirectWrite and HarfBuzz.  It's also not at all clear to me
> how position changes are supposed to interact with attachment.
>

I don't see a bug open about that.  No bug, no fix.  No bug, no
clarification. :)



> Another nightmare area is the rendering of <base, mark, base, mark>
> when the bases ligate and the marks are required to interact.  The root
> of the problem is that Arabic requires that marks on components of
> ligatures not interact, and that rule seems to apply even if the
> ligature glyph is classified as a base rather than as a ligature.


The first time I heard that this is a problem was a couple weeks ago from
Zachary.  I don't remember anyone filing a bug about this.  Again, no bug,
no fix.


One area where I have just had to assume that 'here be dragons' is how
> mark exclusions affect substitutions invoked by (chained) context
> substitutions.  Context substitutions seem to have odd limitations when
> they invoke multiple ligations - perhaps I'm just seeing bugs in
> Uniscribe.


I know of one bug in HarfBuzz about that. But that's it.  In my experience
they work fine.  Need specific examples.



> There is also the undocumented restriction that subsidiary
> substitutions be applied from start to end.


I don't understand this.


>   Also, it seems that
> invoked substitution have to have a matching context - invoking a
> single substitution that has no effect, not even replacing a glyph by
> itself, has seemed to cause trouble.
>

Need specifics.  What trouble?
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