calculation of top/bottomMargin

[schriftgestalt]

How is fontParts calculating the vertical side bearings? What is the vertical origin?

[anthrotype]

@schriftgestalt I don't know about fontParts, but you may be interested in this related discussion on the ufo-spec repo about verticalOrigin, and the different defaults for glyph.height between defcon (0), on the one hand, and Glyphs.app/FDK (OS2.TypoAscender - OS2.TypoDescender), and my proposal to try align the two: unified-font-object/ufo-spec#95 (comment)

[anthrotype]

ufo2ft sets the default verticalOrigin to OS2.TypoAscender, if a particular glyph doesn't have a public.verticalOrigin in its lib:
https://github.com/googlefonts/ufo2ft/blob/66510c8128f9a447617c8c4d6d5871ef4577f74f/Lib/ufo2ft/outlineCompiler.py#L65

then the top sidebearing is computed as the verticalOrigin - bbox.yMax

https://github.com/googlefonts/ufo2ft/blob/66510c8128f9a447617c8c4d6d5871ef4577f74f/Lib/ufo2ft/outlineCompiler.py#L816-L819

[typesupply]

How is fontParts calculating the vertical side bearings?

Here's the code: https://github.com/robotools/fontParts/blob/master/Lib/fontParts/base/glyph.py#L493

[schriftgestalt]

As far as I see getting the topMargin of BaseGlyph will trigger a self.raiseNotImplementedError() in getHeight.
As far as I can see: bottomMargin = bbox.yMin and topMargin = height - bbox.yMax?

That will produce values not suitable for CJK glyphs. The links that Cosimo posted explain it quite well.