[Webkit-unassigned] [Bug 86071] Incorrect behavior of upright text in vertical writing modes

bugzilla-daemon at webkit.org bugzilla-daemon at webkit.org
Tue May 15 08:02:58 PDT 2012


https://bugs.webkit.org/show_bug.cgi?id=86071


Koji Ishii <kojiishi at gmail.com> changed:

           What    |Removed                     |Added
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                 CC|                            |kojiishi at gmail.com




--- Comment #2 from Koji Ishii <kojiishi at gmail.com>  2012-05-15 08:02:03 PST ---
This is a tough one. I can't see the file at URL right now due to 500 error, I can come back to check later, but here're some quick info.

text-rendering: optimizeLegibility lets WebKit use complex code path, and a test[1] indicates that it requires a fix in Core Text:
> The complex text code path mis-renders because of API deficiency.
> There is no way to tell CoreText to use upright horizontal glyphs
> when rendering a vertical line.

One possible fix is not to use complex code path if vertical flow and upright even if text-rendering is set to optimizeLegibility, but I haven't look into its other implications yet.

If this fix has some undesirable side-effect and therefore is not applicable, we have to find a way to fix upright in complex code path, but this is really a tough one. OS X needs Core Text to fix as mentioned above. Complex code path is likely to fail on Windows too, because Uniscribe, the shaping engine we use in complex code path, has the same limitation as Core Text, but I don't think MS is going to fix this because Uniscribe is already deprecated. Probably we have to move on to DirectWrite, the successor of Uniscribe, or other open source shaping engine. If we go this way, I guess it'll be a long way.

For font-feature-settings, I can't see which value the test file is using, but from what I heard from John before, I suspect it's about ligatures. This is also a tough one in regards to figuring out the correct behavior.

1.3.4 of Adobe Technical Notes #5901[2] says:
> Although the conventional ordering of these features is ‘liga’
> followed by ‘vert’ and ‘vrt2’, this font’s vertical-only hiragana
> ligatures necessitates a different ordering, specifically ‘vert’
> and ‘vrt2’ followed by ‘liga’.
so the order of 'liga' and 'vert' are font-dependent, at least in some fonts. I asked this to a guy who created this font before, but didn't get back yet. I'll check with him again.


[1] http://svn.webkit.org/repository/webkit/trunk/LayoutTests/fast/writing-mode/text-orientation-basic.html
[2] http://wwwimages.adobe.com/www.adobe.com/content/dam/Adobe/en/devnet/font/pdfs/5901.Kazuraki_Tutorial.pdf

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