How to avoid interpolation rounding errors?

When interpolating, it often happens that strokes are inconsistent, for example, in the case of E, one stroke is one unit thicker, or the o is one unit different at the top or bottom. Most of the time you can hardly see this, but with thin weights you do. Is there a way to prevent these inconsistency?

I don’t think you can avoid this. What might help is to use stem width as master positions. You need to pick one that is most important.

Thats what I do. But this only affects the vertical stems. It would be good to have a tool (like to TT hinting tool), which lets you define stroke widths for certain strokes to avoid these inconsistencies. New feature?

If you have stem hints in your Glyphs file, those could be used by Glyphs to keep hinted stems consistent in interpolations. But then something else will have to change, for example a sidebearing, or the two counterspaces in an “m” will become different.

I believe FontLab 5 avoided rounding errors by always rounding down, which comes with other tradeoffs …

Out of curiosity, how thin are your weights that not even hinting can even these out?

30 units. Yes, hinting will probably solve this on screen. Still, it would be nice if the stems were consistent.