Upright glyphs in vertical type advanced one em (or their horizontal
advance, if wider) and turned about a fixed point 0.38 em above the
baseline. Fonts that carry vertical metrics now place them as they ask:
- `FontFace::vertical_glyph`: a glyph's advance down the column (`vmtx`)
and the height of its vertical origin (`VORG`, else the glyph's top
plus its top side bearing, as in TrueType fonts); `None` without
`vhea`/`vmtx` or with values outside a few ems.
`FontFace::ideographic_centre`: the em box centre from an ideograph's
cell, else 0.38.
- The layout advances an upright glyph by its vertical advance and hangs
it from its vertical origin; tate-chu-yoko blocks and point type's
anchor use the font's em box centre. Fonts without vertical metrics
are laid out exactly as before.
- Hiragino's tables give the values the layout assumed (one em, origin
0.88 em), so Japanese vertical type in it renders pixel for pixel the
same; this is the base for proportional vertical metrics (`vpal`) and
for fonts whose em box sits elsewhere.
- `test_fonts::vertical_font`: Source Sans 3 with `vhea`/`vmtx` (and
optionally `VORG`) built in code, one glyph 1.4 em tall hanging from
1.1 em; tested with and without `VORG`.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
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