lowered copyright symbol and slash with onum

[pmjdebruijn]

During some typesettings experiments, I noticed that the copyright symbol would look a lot better when it's lowered by 1.5pt (in 12pt type), when onum is enabled.

A similar phenomenon happens with the slash.

Please take a look at this sample, which illustrates my point: sample.pdf sample.txt

Presumably the copyright symbol would be just a matter of having an alternate glyph which is added to the onum substitution table?

As illustrated in the PDF, the slash is probably more difficult to handle as it's much more context dependent.

[twardoch]

In theory, it could also be resolved by just specifying positioning, i.e. there could be an "onum" feature defined in the GPOS table as well, with a lookup that moves the specified glyphs down. This would not work everywhere though, because some OpenType Layout engines (most notably Uniscribe) have a hard-coded notion of which features should be implemented via GSUB and which via GPOS (which is stupid).

See: https://sites.google.com/a/twardoch.com/typography/font-tech/gpos-stylistic-sets

This is rather unfortunate (though I think in TeX systems it would work).

The additional complication is that if you use vertical movement in features such as onum, you have no chance to modify kerning, so pairs that have been designed to work with glyphs that sit on the baseline might give undesired effects if applied for glyphs that were moved vertically. Of course an extra lookup in the same onum GPOS feature that corrects the kerning could be done, but this would need to be written numerically or done in VOLT (most GUI editors don't let you design complex accumulative x/y GPOS adjustments).

So practically, duplicating glyphs, moving those down and specifying GSUB substitutions might be easier for the designer to control, especially that then the designer has more explicit control over kerning (whether to use the same kerning as with the baseline-based default variants by including the moved variants in the same classes, or to make separate kerning affecting the moved variants, or to completely forego it).