mark on mark for Indic clusters

mitradranirban

I am trying to add Oriya letters to my Monospace font. I am adding Below base form as marks attaching to the base glyph . I have also added the same below-base-forms for base characters for attachment of vowel marks, just like mkmk works for Latin. But shaper is refusing to attach to the vowel mark at the anchor position below the blwf. Is it not allowed to treat a mark as a base for indic  shaping? 

John Hudson

Yes, you can definitely apply marks to marks in the abvm and blwm positioning features. I don’t work with fea code, so I can’t easily spot what might be the problem in your source, but these are the general things to check:

1. mark-to-base and mark-to-mark anchor attachments should be in separate lookups (and generally processed in that order).

2. if using named anchors in your source, avoid using the same anchor names for mark-to-base and mark-to-mark anchors.

3. if filtering marks, ensure that both the blwf marks and the vowel marks are included in the mark sets to be processed in the lookup.

Simon Cozens

Also: mark-to-base and mark-to-mark rules are expressed differently in AFDKO feature syntax. If you managed to compile them into the same lookup, you probably have the syntax wrong. Use "pos mark ..." instead of "pos base ..." for mark-to-mark syntax.

mitradranirban

Thanks @John Hudson , I was putting them in same lookup using same anchor. That's why it was not working. I will try with separate anchors and separate lookups.

mitradranirban

Thanks @Simon Cozens . It worked with Oriya after following your suggestion