Interpolating Glyphs files are losing their interpolation

I was adding some OpenType code and renaming some instance weights to an existing typeface when the interpolation status of the glyphs started to shift, first a few got a warning and then more and more. This was with Version 2.5.2 (1172)

Can you send me the .glyphs file please to support (at) (this website without www). I will have a look.

Did you change the positions of the instances?

Will do that this morning.

No, but I did rename them and add to them. Maybe that also causes a change to “position” even if I didn’t drag them around?

With “position” I meant the coordinates. But if you add an instance the master compatibility can change. The font view caches the red corners so it might need a bit to update.

There were no new instances. But that’s a great triage question.

So MAYBE I have found a possible source of the problem. It looks like I had a weight axis listed and I could change the weight values of masters and instances in the UI but there was no weight variable listed under info/Fonts/custom parameter.

Anyway, I will give a file where that’s fixed a go and see what it does.

BTW - I am able to get the same bug in which I loose interpolation compatibility in Version 2.5.2 (1169) so it isn’t something that’s exclusive to Version 2.5.2 (1172)

The test I made failed but now I think that maybe this is a case not so much of the files losing compatibility as much as Glyphs taking a long time to notice and report the interpolation problem with the mark in the corner of the glyph. I’ll keep testing and see if that theory can be shown to be so.

From what I can determine it turns out that it was a combination of the sources not being quite as clean/done as I supposed and also Glyphs seemingly having stricter interpolation standards. Some things I tested that have generated without a hitch in the past can be flagged with the colored corner now.

Did you write anywhere about Glyphs get more strict? If you did decide to do that in a way I think it’s great but it does make the sources more solid/sound/correct I am also a bit concerned about old sources that have been published not generating well now.

I am also interested in having the process that Glyphs uses for pointing these problems out being either

  • more constant (or done when the file opens) at least so that it doesn’t look like things are fine and then breaking for reasons that are unrelated e.g. “I just altered my instance naming” or whatever.

  • manually available so that you ask a question and get an answer.

  • or both because things in a file can change!

As it is it is now it is hard to know why things change and it is easy to guess a reason that’s false.

I’m sorry if this post is a bit rambling. I tried to keep it as short as I could.

It is stricter for variable font export. The MM interpolation to static instances should pretty much be as it was. Changes were only minor.

What about this question of being able to manually ask Glyphs to check this for a whole open file?

You mean this? I don’t really understand what you mean. You can add a filter in the sidebar to give you all glyphs that are not compatible.