[PDFBOX-5808] Add support for GSUB Lookup Type 3

• Type: New Feature
• Status: Closed
• Priority: Major
• Resolution: Fixed
• Affects Version/s: 3.0.2 PDFBox
• Fix Version/s: 3.0.3 PDFBox, 4.0.0
• Component/s: FontBox
• Labels: gsub
• Assignee: Tilman Hausherr
• Reporter: Fabrice Calafat
• Created: 25/Apr/24 19:07
• Updated: 10/Aug/24 07:18
• Resolved: 30/Apr/24 13:03

Description

Add support for the lookup type 3, Alternate Substitution when handling GSUB:

https://learn.microsoft.com/en-us/typography/opentype/spec/gsub#AS

The first available substitution glyph can be used (as done in other libraries)

 

Also, the current implementation of CompoundCharacterTokenizer doesn't account for collision in ligatures

For example, if a font supports ligatures for att and en, the current implementation will not tokenize properly for the word attention. This is because the regex implementation doesn't allow for a proper split

 

I'll open a proposed implementation for the above

Issue Links

• is duplicated by: PDFBOX-5700 Ligature substitution cannot be switched off
• links to: GitHub Pull Request #190