Fallback vertical shaping

[khaledhosny]

Firefox tries to use vertical presentation forms when the font lacks ‘vert’ feature. Based on the mailing list discussion HarfBuzz should do it.

[KrasnayaPloshchad]

This is helpful for in some adaptations uses vertical only text layout. For example, on this page several CJK punctuations are used as quote mark for Mongolian text, when they display with Oyun Qagan Tig, they looks unrotated, but when they display with Mongolian Baiti, they looks rotated. BTW Is it possible to let developer disable it if this function is implemented?

[kojiishi]

Out of curiosity, why do you want to disable it?

[KrasnayaPloshchad]

I think this can be configure via an additional API.

[behdad]

No need to make it configurable. If we were to add API for everything, the library would have been unusably complex.

[KrasnayaPloshchad]

This would be helpful for some non-CJK fonts. For example, Oyun Qagan Tig does not have 'vert' feature for U+3008-U300B, Abkai Xanyan does not have the same feature for U+300C-U300F. To get predictable appearence, provide fallback shaping for them would be helpful.

[khaledhosny]

I tried to address the underlying issue differently in LibreOffice:
https://cgit.freedesktop.org/libreoffice/core/commit/?id=0f3861e65d8e652dcc31cf9a2f2b5c1a0a73b86d

Namely, collecting all the output glyphs from vert feature and if the glyph for a character with Tr is not in that set, I rotate it. This might be a little hacky, but it gives better results with fonts not having complete coverage in vert feature, or fonts without a vert feature at all but the vertical presentation forms glyphs are drawn upright like the Oyun Qagan Tig font above.

[khaledhosny]

Now I realize that if HarfBuzz implemented the fallback vertical shaping, my solution will rotate the fallback presentation forms glyphs. Well unless HarfBuzz will fake a vert feature and lookup and hb_ot_layout_lookup_collect_glyphs() would work on it (is this the case for fallback Arabic shaping?).

[behdad]

Would be easier if HB just does the simple thing and LibreOffice doesn't try to be smarter.

[khaledhosny]

But then people will scream REGRESSIONS! to me because that is kinda what pre-HarfBuzz code did (it was parsing the GSUB table in its own).

[behdad]

Ok, not clear what to do anymore. Let's get back to this some other time.

[KrasnayaPloshchad]

When a proportional font as Tangut N4694 used in vertical writing, but does not include such feature for glyph mapped in Tangut codepoints, I think these glyphs should be sightly centered for better rendering.

[KrasnayaPloshchad]

When a proportional font as Tangut N4694 used in vertical writing, but does not include such feature for glyph mapped in Tangut codepoints, I think these glyphs should be sightly centered for better rendering.

According to TDF bugzilla, such glyphs should be allowed to well aligned with strikethrough in vertical layout.

https://bugs.documentfoundation.org/show_bug.cgi?id=105650

[behdad]

@khaledhosny Can you summarize proposed change to HB?

[khaledhosny]

I think this should be closed for now, seems better handled by applications since there are at least two incompatible ways to do it.