John Hewson commented on PDFBOX-4189:

For correct text positioning using mixed language information from the 
following tables might be useful:
- GPOS: to adjust the glyph position
- BASE: baseline offsets on a script-by-script basis.
- JSTF: justification information, including whitespace and Kashida adjustments.
- BIDI Mirroring: https://www.unicode.org/Public/10.0.0/ucd/BidiMirroring.txt

bq. here

BASE, JSTF and BiDi are concerned with _paragraph-level_ layout, which happens 
at a higher level than the proposed layout() - which would be concerned with 
only a single script in a single direction (i.e. only OpenType _shaping_). BASE 
and BiDi are related to changes between different scripts, while JSTF is to aid 
in making good line break choices. So all of that functionality will happen 
somewhere else (this fits very closely with the layout code form forms, for 
example). So in layout we're really only going to be concerned with GPOS and 
GSUB features. That way the only options that one might want to pass to layout 
would be this list of which [feature 
flags|https://docs.microsoft.com/en-us/typography/opentype/spec/featurelist] to 

> Enable rendering of Indian languages, by reading and utilizing the GSUB table
> -----------------------------------------------------------------------------
>                 Key: PDFBOX-4189
>                 URL: https://issues.apache.org/jira/browse/PDFBOX-4189
>             Project: PDFBox
>          Issue Type: New Feature
>          Components: FontBox, PDModel
>            Reporter: Palash Ray
>            Priority: Major
>         Attachments: Bengali-text-after.pdf, Bengali-text-before.pdf
>   Original Estimate: 336h
>  Remaining Estimate: 336h
> Implemented proper rendering of Indian languages, which need extensive Glyph 
> substitution. The GSUB table has been read and used effectively to replace 
> some compound words with their respective Glyphs. All tests are passing. I 
> have tested this for the Bengali font. Please review these changes and let me 
> know if it makes sense to incorporate these.

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