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https://issues.apache.org/jira/browse/PDFBOX-5808?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17841222#comment-17841222
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Tilman Hausherr commented on PDFBOX-5808:
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Re lookup type 3, do you have an example, or a test? I remember looking at that
a long time ago and didn't implement it, because I suspected this was about
"artistic" alternates. I also suspected that these replacements may not be
wanted by all, while only few will bother about classic ligatures. One problem
of the current code is that we can't enable or disable specific GSUB features
(PDFBOX-5700).
> Add support for GSUB Lookup Type 3
> ----------------------------------
>
> Key: PDFBOX-5808
> URL: https://issues.apache.org/jira/browse/PDFBOX-5808
> Project: PDFBox
> Issue Type: New Feature
> Components: FontBox
> Affects Versions: 3.0.2 PDFBox
> Reporter: Fabrice Calafat
> Priority: Major
> Attachments: screenshot-1.png, screenshot-2.png
>
>
> 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
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