Behdad@, you showed me the basic workflow at IUC and I've tried to gather running tools and scripts from your demos at ATypI but I am afraid that's not enough documentation for me to run fontmake and related tools to create a variable font from multiple masters. Could you document the basic steps somewhere, perhaps in the README.md of fontmake on github, or whatever a suitable place might be? Thanks.

Would this guide roughly work? https://github.com/scribbletone/i-can-variable-font

It's also possible to create a variation font with FontForge (which is how I made resources/fonts/Distortable.ttf) but that functionality is a bit under documented and a bit touchy (it doesn't get used very often). The documentation is at https://fontforge.github.io/multiplemaster.html . The way I did it was to create the base 'master', then clone that font (save as new name) and move the points around to create the other 'masters'. Then with all the various 'masters' open you can use the 'MM' menu to create the variation font. Note that I've only done this with newer versions of FontForge, the repo in Ubuntu Trusty is really, really old (they didn't move their source when the upstream changed) so your experience may vary. Anyhow, something you might want to try.

Thanks for the tip - I found a way now based on Glyphs.app and some manual editing of a ttx file plus fonttools. I have a basic reftest and ttx source file now for Blink, works quite nicely and we can extend it for testing hvar etc.

Fixed in https://codereview.chromium.org/2581083003