FontLab/Glyphs round trip

Hi everybody. I’m trying to figure out a clean way to move a font file from FontLab to Glyphs and back.
I’m using the latest version available of the scripts Glyphs Export and Glyphs Import.
FL 5.1.5 on Mac and Glyphs 2.3 (895).

Testing the workflow I came across some problems. This is what I did:

  • run Glyphs Export from FL;
  • open the .glyphs file in Glyphs and changed the outlines of one glyph, saved and closed;
  • run Glyphs Import from FL and saved the file.

After comparing the original source VFB and the new one created importing from the .glyphs file, this is the result:

  • .notdef glyph from having Index 0, was pushed to the end of the glyph set;
  • some glyphs have changed starting point;
  • some glyphs have been renamed (specifically /mu and a bunch of precomposed fractions, but not all of them).

Is there a way to make the scripts not changing anything?
Is anybody aware of things that could cause this behaviour apart from the scripts themselves?
What I expected is to find a difference only on the glyph that I worked on, and everything else untouched.
Thanks for any advice on this.

In FontLab, are you viewing the font in Index mode or one of the other modes (e.g., Codepages mode)?

In your preferences, check Keep glyph names from imported files before opening the FL export.

Does the starting point change again if you do another round?

And why do you what to do the round trip?

Thanks for the replies.
Mark: I was comparing the glyphs’ index order of the two fonts in Index mode.
Erich: I had the Keep glyph names from imported files checked in the preferences before opening the file.
Georg: I’ve just tried to do another round and the starting point moved again. It seems that it moves to the node that precedes. About the why I want to round-trip: I’m testing possible ways of taking advantage of the different tools I have available; Glyphs can be useful also in situations where you need to target specific tasks, and working on projects developed in FontLab, I need to find an easy and efficient way to go back to VFBs.

I will do further tests in the next couple of days, including the usage of UFO as an intermediate format. From a quick test I did earlier, it seems that exporting/importing UFOs, helps to preserve all what I need. It adds a couple of steps in the process but it could work fine. I will try to summarize everything in a document and I will share my findings.