When I want to make kerning exceptions, as soon as I unlock both locks all other pairs within the group get a value of zero. Why?
There should only be one extra kerning pair: after the first unlocking. In that moment you create a zero kern.
You can get rid of zero-kern pairs with Script > mekkablue > Metrics > Delete Small Kerning Pairs.
OK, thanks! Is there any reason to keep 0 value kern? Or, should I go and delete all zero kern after the kerning is done to keep things tidy?
There can be cases when you have exception pairs with a value of zero. They override the non-zero class kerning value so they do make a difference. If you delete them that would have an impact on your exported font’s observable behaviour.
Yup, I do have those. So ether very carefully remove all other zero kern, or just leave everything as it is? Is there a downside to having a lot of zero value kern pairs?
Good question. I was assuming that makeOTF (the engine used by Glyphs to create the fonts) automatically detects which zero-value kerning pairs have an effect, i.e. only glyph-glyph pairs that override a class-class pair. So, having superfluous zero-pairs in your Glyphs file would not hurt. @GeorgSeifert Do you know?
That does sound like something @GeorgSeifert would solve.
I just made a brief test and it seems that zero class kerning is ignored. But single zero pairs are not.