Re: RTL PUA?

Philippe, I'll need to think about this some more and try to get a
better grasp of what you're suggesting. But some immediate thoughts come
to mind:

If BiDi is to be applied to shaped glyph strings, surely that means
needing to step backwards through the processing that arrived at those
shaped glyph strings in order to correctly identify their relationship
to underlying character codes, since it is the characters, not the
glyphs, that have directional properties. There's nothing in an OT font
that says e.g. GID 456 /lam_alif.fina/ is an RTL glyph, so the
directionality has to be processed at the character level and mapped up
through the GSUB features to the glyphs.

I think you may be right that quite a lot of existing OTL functionality
wouldn't be affected by applying BiDi after glyph shaping: logical order
and resolved order are often identical in terms of GSUB input. But it is
in the cases where they are not identical that there needs to be a
clearly defined and standard way to do things on which font developers
can rely. [A parallel is canonical combining class ordering and GPOS
mark positioning: there are huge numbers of instances, even for quite
complicated combinations of base plus multiple marks, in which it really
doesn't matter what order the marks are in for the typeform to display
correctly; but there are some instances in which you absolutely need to
have a particular mark sequence.]

I've lost track of what the putative benefit of processing BiDi post
glyph shaping is. I think I missed part of your earlier exchange with
Behdad.

JH

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