RTL instructions in Feature code?

Hi there,
I’m working on an ancient (no-joining-group) rtl script. It has .init, .medi, .fina features (it works similar to Arabic) and it renders as it should in Glyphs. However, joining is broken in other testing environments - InDesign, TextEdit, Chrome and Safari.
InDesign, is slightly better than the others. Medial forms join, but Finials appear at the beginning of the line, suggesting (I think) InDesign sees the Feature code but is getting it backwards - i.e. isn’t recognising it as an rtl script. So, is it possible to embed/force in rtl instructions in the font file? Or is this an issue for the other software?

Also, any recommendations for instructions on building rtl fonts from scratch?

NB. I am using Glyphs 3.0.4 (3100).
Cheers!

Which script?

Old Uyghur

InDesign will probably not be your first choice for typesetting Old Uyghur. Better try something based on HarfBuzz. They are also quicker to implement it in case it hasn’t been done before.

Okay, sure. I’ll do some research. Thanks!

Old Uyghur is new in Unicode 14, which was released two months ago, so it will take a while for it to become supported in software. HarfBuzz has been updated, but now other software has to pick up the latest version of HarfBuzz, or be updated separately when it doesn’t use HarfBuzz. I wouldn’t expect broad support in operating systems until their major updates in September/October next year. Chrome and Firefox may come earlier.

InDesign now has a “World-Ready Paragraph composer” that’s based on HarfBuzz, but their definition of “world-ready” still seems very limited:
https://helpx.adobe.com/indesign/using/south-east-asian-scripts.html

Are you sure? I think Photoshop has Harfbuzz now, but Indesign still uses its own, limited composer.

Let’s clarify: What I heard from Adobe in 2019 is that the support for Southeast Asian scripts (Khmer, Lao, Myanmar, Sinhala, and Thai) in Illustrator, Photoshop, and InDesign is based on HarfBuzz. I don’t use InDesign myself, so I can’t verify this or whether additional scripts are enabled through HarfBuzz.

Re: OpenType 14 - I think this update includes the Old Uyghur script tag ougr
However this isn’t recognised in Glyphs 3. Will a future update cover these new script tags?

I’ll add it. Thanks for mentioning it.

I added all script tags from the OpenType 1.9 spec.

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Thanks!

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