I’m looking at the PUA Vertical Metrics Marker characters U+F132 and U+F133 in some of the SIL Fonts (Gentium Plus Regular v5.000, Charis SIL Regular v5.000) and they are SO useful in diagnosing how applications handle vertical metrics.The Baseline, xHeight, and CapHeight are faithfully represented except …
What do the top-most and bottom-most horizontal lines represent? They’re not the Asc/Descent triplets, and I can’t see a relationship to any of the vertical metrics in the font.
Is there a document that describes the design of these glyphs?
Alternately, is there another place I should be looking for code points that show the Asc/Descent triplets (before I go off and design my own)?
Those glyphs are not designed to reflect the vertical metrics of the font (as in ascender, descender, line gap). The vertical metrics can change, for example, if you use TypeTuner Web and choose a different line spacing.
In order from top to bottom the lines represent:
height of glyphs with ascenders (bdfhkl)
height of capitals
x-height
baseline
depth of glyphs with descenders (fgjpqy)
Generally there are no codepoints or glyphs that provide a visual representation of the line metrics. That would assume that all the line metrics are synchronized, and that can’t be assumed. We recommend that font developers sync them, but there’s not universal agreement on that.
We do not have a document that describes glyphs in detail (our Latin script fonts have over 4000 glyphs!). The closest thing for these glyphs would be the SIL Corporate PUA Assignments docs.
I do synchronize the line metrics, as your guidelines recommend. I am now using my own frame markers for testing that depict the 5 key vertical metrics. I’ve also got a handy overlay PUA glyph that has zero advance width that shows all five vertical metrics.
For non-synchronized vertical metrics, I could imagine little indicators to show the 3 flavors ascender and descender metrics …