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How Lig. carets are used in FF and how do you know which is which?

WAY KYIWAY KYI Posts: 128
edited October 2021 in Font Technology
Please see the attached image, where I separated about 6 lig. carets. I have no idea which one is coming from which glyph. So, I think as the name suggested, are they used for cursor positions and move cursor from one glyph to another? I am at lost that I can't tell which one is for which glyph and how do you handle them. Any technical details and suggestions from Sr. Typographer? Thanks

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    John HudsonJohn Hudson Posts: 2,977
    I’ve always ignored the lig caret aspect of OpenType Layout. It was originally invented with Latin ligatures in mind, where, ironically, it isn’t usually needed because adequate caret positioning can be obtained for most Latin script fonts by dividing the advance width of the ligature glyph by the number of letters it represents, which is what software does if a font does not contain lig caret positions.

    In complex script layout, lig caret positions are problematic in two respects:

    1. A lot of complex script ligatures are vertical, not horizontal arrangements, so having horizontal caret positioning information is not useful (as you have illustrated).

    2. Text editing conventions for many complex scripts work at the cluster level, so it isn’t necessarily possible to position a cursor within a cluster, and stepping through the characters that make up a cluster sometimes only works in one direction and backspace and delete do not function in the same way as in Latin fonts.
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    WAY KYIWAY KYI Posts: 128
    Thank you very much. So, it isn't very useful and I could ignore them. I see the system doesn't allow to put them in negative area or beyond Lig. glyph Lbearing. So the lig. glyph Rbearing is the best place to put them, right? 
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    John HudsonJohn Hudson Posts: 2,977
    If you are using a tool that adds them automatically when you create a ligature glyph, and if you cannot simply delete them, then yes, putting them on the right sidebearing (for a left-to-right script) would be a reasonable thing to do.
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    WAY KYIWAY KYI Posts: 128
    I used FontForge and just noticed them a few days ago. Thank you for your suggestion. 
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