I see that Glyphs generates a number of Devanagari OpenType features (like , , , etc.). Can it generate Mark to Base as well, if anchors on the base glyphs and the Marks are present? Or do I need to write this feature into the OpenType feature panel in Font Info by hand?
Glyphs automatically generates mark and mkmk features for any script, as long as it knows them by default. In other words, it only generates the anchors you get by selecting “Set Anchors (Command + U)” or manually placed ones with the same names. It is customisable in the GlyphsData.xml. http://www.glyphsapp.com/tutorials/roll-your-own-glyph-data
(anchors="" is the most important part you need to edit, if you find the default anchors insufficient. accents="" is a grey preview that you can see when you select an anchor.)
In the case of Devanagari, it seems to generate mark feature by default, judging from the xml. I haven’t tried with actual font.
Thank you, Toshe! I just did not have my anchors named properly. Once I fix this, the and marks did generate automatically (you do not see them in the Glyph OpenType feature menu, but they are written into the font file upon font generation).
Now I just have to figure out mark-to-mark spacing
I may not need it at all – at least not with the character set I am working on. But I would like to be able to make it work, theoretically, at some point in the future. Perhaps for the Nukta dot and the u- and uu-Matras, although those could be dealt with through many-to-one substitutions, too.