Font rendering with vertical text

[sugi2000]

I appreciate that resvg supports vertical writing-mode. When testing with Japanese vertical text, I noticed some characters are rotated incorrectly.

The issue appears to affect specific characters like 'yoon', 'sokuon' in vertical writing mode.
https://en.wikipedia.org/wiki/Y%C5%8Don
https://en.wikipedia.org/wiki/Sokuon

As shown in the attached image:

• Left: SVG rendered correctly in browser
• Right: svg2png-wasm output using resvg (incorrect rotation)

[RazrFalcon]

Can you post the input SVG or at least the string itself?

[sugi2000]

Here is the text element.

<text style="writing-mode: vertical-rl;">（すーっ）</text>

[RazrFalcon]

This character is marked as Transformed typographically, with fallback to Upright in unicode-vo. But we handle only Upright in usvg.
Therefore I assume this code should be adjusted.

[sugi2000]

Thank you for pointing this out. I am wondering some characters like the following are rotated correctly:

• U+FF08 FULLWIDTH LEFT PARENTHESIS
• U+30FC KATAKANA-HIRAGANA PROLONGED SOUND MARK

https://www.unicode.org/reports/tr50/tr50-19.html

[sugi2000]

I saw your unicode-vo code. What is the difference between Orientation::TransformedOrUpright and Orientation::Upright?

[RazrFalcon]

What is the difference between Orientation::TransformedOrUpright and Orientation::Upright?

I have absolutely no idea...

[sugi2000]

I understood the way the current code does it. It just rotates upright characters 90 degrees. That's why the parenthesis and prolonged sound mark are positioned slightly offset. But we should change the vertical glyph for these characters. The Japanese fonts have the vertical glyphs, but I don't know how to extract them.

[RazrFalcon]

The Japanese fonts have the vertical glyphs, but I don't know how to extract them.

Are you sure? Fonts can have vertical layout specific metrics, but not separate glyphs.
So the current code is correct, we just have to figure out how to improve it.

[sugi2000]

Yes, I'm sure.

Especially in Japanese and Chinese text, when characters like U+2329 (〈) use different glyphs for horizontal and vertical writing, the substitution is specified in the font's GSUB (Glyph Substitution) table, and the placement information for the substituted glyph is obtained from the vmtx table.

https://www.unicode.org/reports/tr50/tr50-19.html

[RazrFalcon]

GSUB and vmtx are already handled for us by harfbuzz/rustybuzz.
Either way, I would not have time to look into this anytime soon. Maybe someone would send a PR.

[sugi2000]

Thank you. I hope someone would handle if orientation == Orientation::TransformedOrUpright.