Odd Interpolation breaks On-Curve Nodes

Design
1

Hello and happy new year y’all,

I have a setup with two masters (compressed and expanded), which I am trying to interpolate. but some nodes in the middle part of my uppercase “S” don’t behave as expected in the interpolated cuts. instead of being nice and smooth, they get bumpy.

how can i avoid this?

stabil-interpolation-problem_2
stabil-interpolation-problem_21030×1292 297 KB

Above you see the compressed and expanded “S”, below you see the interpolated regular cut with a bumpy path.

Does this topic belong in the “Design” or in the “Glyphs” category?
Thank you!

2

Those are kinks. This happens when there points are on one line but have different angles AND different proportions in the different masters.

3

The kink problem, yes. I recommend:

  1. Put the current outline in the background (select and Cmd-J)
  2. Remove the ‘diagonal’ points on the spine, and
  3. try to recreate the swing of the spine as closely as possible (with the background as reference) with the vertical handles only.

Alternative:
Use Harbortype/Henrique Beier’s Show Smooth Node Angle and Proportion to even out the proportions between all four handles and on-curves across all masters.