Reading glyph.height from file

[schriftgestalt]

When reading a .glif, the glyph.height is set to zero if it is not defined in the file. That makes it impossible to distinguish between manually defined zero height glyphs and just default/undefined ones. Would it be better to set glyph.height to None if not defined in the file?

I came across this when round tripping .glyphs > .ufo > .glyphs as the resulting .glyphs files have vertwidth = 0 for all glyphs. That is clearly wrong.

[anthrotype]

It is not "wrong", it's just how the UFO spec defines it currently. I have actually proposed to change the default height value but it will be a breaking change so would probably requires a UFO spec major version bump.

unified-font-object/ufo-spec#95 (comment)

[madig]

How does height work, anyway? Does it always require a vertical origin?

[anthrotype]

Does it always require a vertical origin?

if omitted for a specific glyph, we have it default to the OS/2.sTypoAscender currently:

https://github.com/googlefonts/ufo2ft/blob/0d2688cd847d003b41104534d16973f72ef26c40/Lib/ufo2ft/outlineCompiler.py#L68-L75

height defines the vertical advance in vmtx (like width does for hmtx). The vertical origin is used to compute the top side bearing defined as (verticalOrigin - bounds.yMax)

https://github.com/googlefonts/ufo2ft/blob/0d2688cd847d003b41104534d16973f72ef26c40/Lib/ufo2ft/outlineCompiler.py#L869C30-L872

[madig]

Ok. In my format experiments, I was tempted to treat a UFO glyph's origin and height as a unit, as in, the height is only defined if there's an explicit origin also present. Could this be a band-aid for the g2u2g case? I.e. the origin cannot be implicit, and that's how you can tell if a glyph has a zero height (origin and height diff to 0) or an undefined one (no origin).

@schriftgestalt How does height work in Glyphs.app?

[schriftgestalt]

The height (called vertWidth) defaults to ascender+descender.
The vertOrigin defaults to ascender.
A vmtx table is written if any glyph has a different value set (implicitly or explicitly) for each property.