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

[thundernixon]

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.

[felipesanches]

I'm fixing the problem now, however, so you may need to go back to older versions to see this in action.

Can you please mention the commit id of the version that has the problem? This will help us keep track of it in the future. Otherwise, we'd have to look at the repo looking for a commit that is older than the timestamp of your comment above.

[felipesanches]

OK. This makes a lot of sense. All .tnum in a given font should have the same width. Any .tnum glyph that diverges from that common value should trigger a FAIL. Right?

[felipesanches]

Should we name this com.google.fonts/check/tnum_horizontal_metrics ?

[felipesanches]

The title of the issue states "across glyphs and styles". Will the .tnum glyphs in a Thin always have the same exact width as the ones in a corresponding Extra-Bold?

[felipesanches]

In other words, is the user expected to use different styles of the same typeface and still get consistent tabular alignment? I would guess that this is a reasonable thing to expect but may be hard to achieve when designing a family. How does that work?

[m4rc1e]

I would guess that this is a reasonable thing to expect but may be hard to achieve when designing a family. How does that work?

In many super families, only the RIBBI styles have the same advance widths for tabular glyphs.

[felipesanches]

So is it reasonable to make this a FAILing check across the RIBBI styles?

And maybe with a WARN for inconsistent .tnum widths in the other styles?

[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

[thundernixon]

is the user expected to use different styles of the same typeface and still get consistent tabular alignment

100% yes, between all styles (assuming styles have a constant value for width axis).

By definition, if figures have different widths in different styles, they are not tabular figures. If the width changed between styles, it would be similar to a "monospace" font with different widths (it would fail to meet a central piece of criteria).

Tabular figures need to have the same metrics in all styles in order to allow tables to be set with proper typographic control, but to maintain the placement of decimals and numeric columns between rows. Designing them this way certainly takes some effort, but that's no different from many other aspects of making type properly. I would consider it a FAIL if they don't meet this criteria.

Here's a good explanation of this:

https://www.typography.com/techniques/fonts-for-financials/#tabular-figs

[felipesanches]

So, the plan is to emit a FAIL whenever any .tnum glyph of any style of a whole super-family diverges from a unique glyph-width, right?

[felipesanches]

@thundernixon, while the build instructions on that repo are not updated, could you send me a copy of the relevant font binaries, please?