Suggested check: tnum glyphs have same metrics, across glyphs and styles

Observed behaviour

I noticed that the tabular figures (tnum) of Libre Caslon Text Italic didn’t share the same horizontal width metrics.

This, of course, means that glyphs marked as .tnum and in the OpenType tnum feature are not yet actual tabular figures.

In the case of Libre Caslon Text, the Regular and Bold masters have proper tabular figures and .tnum glyphs:

However, in Libre Caslon Text Italic, many of the glyphs named as .tnum are not yet modified to be tabular:

In the image above, then /yen.tnum is 1222, the dollar.tnum is 1007, and several other glyphs are also completely wrong.

Especially as we process fonts more quickly, this sort of broken OpenType feature will be easy to miss.

Expected behaviour

I think it would be relatively simple to add a check for this problem. My thought is, we could make a list of all width values for glyphs named .tnum, then check if a set of those widths is great than 1. If so, the test fails. I’m not sure whether it’s possible to cross-check other weights in a set of static fonts, but this simple test would go a long way, especially in variable fonts.

Resources and exact process needed to replicate

If you wish, the source files for LC are available at https://github.com/thundernixon/Libre-Caslon. I’m fixing the problem now, however, so you may need to go back to older versions to see this in action.

[thundernixon]

Can you please mention the commit id of the version that has the problem?

@felipesanches Definitely! Specific commits which include the problem:

Libre Caslon Roman; Dec 10, 2018

Libre Caslon Italic; Dec 10, 2018

[hyvyys]

Tabular figures should have the same width across weights, but not widths! When generating fonts from UFO + designspace sources with a Width axis, this check gives false positives.