'mark' overflow?

I made a font which relies heavily on mark positioning. There are several anchors in each glyph and there are a lot of glyphs!
I was always clicking on ‘Compile’ during the development and there was no warning. But in my first attempt to export it to an OTF font, Glyphs hanged for a quite a while and then showed a warning about ‘mark’ feature overflow (I don’t remember the exact wording) and I had to abort. The following attempts didn’t have that warning, but a general ‘unknown’ problem was noted.
Is there a remedy to this? where should I start?

Does adding the Use Extension Kerning parameter in File Info > Font help?

The mark and the kern feature have to fit in 64kB of GPOS table, unless you use extension lookups. That can be done as Rainer suggested.

Thanks Rainer and Georg. Unfortunately it didn’t work. It’s a huge font with unconventional structure. So I really don’t know where to begin…but it’s fun!


Can you send me the .glyphs file?


I had a look at your file.

You set it up correctly but because you don’t have any kerning the Use Extension Kerning flag was not being appleid. I fixed that.

But. Even with the useExtension flag, you have too much mark positioning. But I found a way around it. Please try the latest cutting edge version.

Waw thank you Georg it worked! Now I can go to the next stage which is the bug fix!

No I didn’t have kerning. This will be for later. I have to figure out how contextual kerning (i.e. between alef-ar and kaf-ar.init when there is madda-ar in between) works. So GPOS volume is not going to get any lighter! But a font of 1 MB for this range of language coverage and this sophistication is not too bad!

But for now there a lot of bugs to fix that I can now see, thanks to you.