New check: STAT tables in static fonts

[simoncozens]

What needs to be checked?

Static instances need different stuff in their STAT tables to variable fonts. Fontbakery should tell us if a "variable" STAT table has ended up in a static instance.

Detailed description of the problem

Adobe feature syntax allows for the definition of a STAT table. Fonts built with a hand-coded STAT table in feature syntax may be built either as static or variable, but will end up with the same STAT table. This is a problem, because a STAT table which works on variable fonts will not be appropriate for static instances. The examples in the OT spec of non-variable fonts with a STAT table show that the table entries must be restricted to those entries which refer to the static font's position in the designspace. i.e. a Regular weight static should only have the following entry for the weight axis:

        <AxisIndex value="0"/>
        <Flags value="2"/>  <!-- ElidableAxisValueName -->
        <ValueNameID value="265"/>  <!-- Regular -->
        <Value value="400.0"/>

However, if the STAT table intended for a variable font is compiled into a static, it will have many entries for this axis. In this case, Windows will read the first entry only, causing all instances to report themselves as "Thin Condensed".

Optional fix

Just drop the STAT table from the static.

Resources and exact process needed to replicate

notofonts/latin-greek-cyrillic#429

Expected Profile

• Universal (broadly accepted best practices on the type design community)

This'll mess you up on Windows if you get it wrong.

Expected Result

Fontbakery should:

• WARN if a non-variable font has a STAT table at all, since they're a pretty rare thing and you're almost certainly better off without them (just rely on WWS/style linking).
• FAIL if a non-variable font has more than one AxisValue per axis.

Severity assessment

4

[felipesanches]

I'm starting to work on this one now.

[felipesanches]

The new check was implemented at PR #4151 not including the suggested WARN on the mere presence of a STAT table on static fonts because @Lorp suggested in a conversation that having the STAT table would actually be desirable. Also @twardoch presented some ideas about this topic on a private conversation.

I am closing this issue, and encourage others to open a separate one in case further changes (like the proprosed WARN) are still needed.