[MPEG-OTSPEC] Why no GPOS10?

[Greg Hitchcock]

Here was a thread from 2003 for context.

GregH

From: Paul Nelson <paulnel at winse.microsoft.com> 
Sent: Wednesday, September 24, 2003 9:58 PM
To: opentype at topica.com; sarmad.hussain at nu.edu.pk
Subject: RE: [OpenType] Question about Reverse Chaining Contextual SIngle Substitution S

Hi Yannis,

I don't know if you got a satisfactory answer. Here is more information.

Urdu shaping is done based on the last character of the jour. A jour is a group of connected letters. The reason that you have not seen many fonts using this lookup is because it is just recently implemented in Uniscribe. 

Reverse processing is critical for nastaliq shaping to make sure the thin/thick connections are done correctly. Moving from front to end of the jour would require that a long context be set up to get the shaping correct. And then, of course, one more letter would break the shaping and the jour would not have the correct glyphs.

Cursive attachment (GPOS) is a different subject and can easily be done from front to back.

Paul

-----Original Message-----
From: Yannis Haralambous [mailto:yannis.haralambous at enst-bretagne.fr]
Sent: Saturday, September 20, 2003 11:48 AM
To: Opentype; sarmad.hussain at nu.edu.pk
Subject: [OpenType] Question about Reverse Chaining Contextual SIngle Substitution Subs

There is one thing I don't understand about lookup type 8 of GSUB and Urdu :

it is said in the OpenType specs that in Urdu the shape of a glyph depends on the following glyph. Well this is true in Arabic as well, where each glyph shape depends on the preceding and on the following ones. What makes Urdu special is the fact that one is typesetting on an oblique base line, for each word segment.

For obtaining such an oblique typesetting, one could start by using cursive attachment marks in the GPOS table. But the problem is that typesetting is done in the logical order, so that the first glyph would be at normal position, and all other would get lower and lower. 

So it is clear to me that one has to start from the end to get the first glyph at the highest position.

What I don't understand is why the reverse chaining lookup has been implemented for GSUB and not for GPOS. What I have described above is purely positioning.
It could be done with cursive attachment marks. 

What is wrong in my arguments? Why is only GSUB doing reverse lookups?

And a subsidiary question: how come the Nafees Nastaliq font does *not* use a single time such a reverse chaining subtable? Do you know of any fonts using it?

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-----Original Message-----
From: mpeg-otspec <mpeg-otspec-bounces at lists.aau.at> On Behalf Of Simon Cozens
Sent: Friday, July 31, 2020 1:23 PM
To: mpeg-otspec at lists.aau.at
Subject: [MPEG-OTSPEC] Why no GPOS10?

This is more of a historic rationale question than anything else.

GSUB8 was added in OT1.4, giving reverse chaining substitution ("rsub"). 
But at that time there was no parallel addition of reverse chaining positioning. I presume at the time there was a discussion about it and a good reason why it was not eventually added, but I can't get to any opentype mailing list archives.

I'm quite surprised, because knowing the rationale for why it was added (glyph selection in Urdu), any discussion would probably have also touched on nukta repositioning which has to be done in more or less the same way.

Simon