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Bug 391547 - apply rtla OpenType feature for right-to-left runs
apply rtla OpenType feature for right-to-left runs
Status: RESOLVED FIXED
Product: pango
Classification: Platform
Component: general
1.15.x
Other All
: Normal enhancement
: ---
Assigned To: pango-maint
pango-maint
Depends on:
Blocks: 474609
 
 
Reported: 2007-01-01 15:57 UTC by Roozbeh Pournader
Modified: 2012-08-25 19:21 UTC
See Also:
GNOME target: ---
GNOME version: 2.17/2.18


Description Roozbeh Pournader 2007-01-01 15:57:46 UTC 
From how I read Unicode 5.0's bidi, it seems that there is no excuse in not mirroring some characters anymore. If it is mirroring, we must mirror or we are not conforming:

"L4. A character is depicted by a mirrored glyph if and only if (a) the resolved directionality of that character is R, and (b) the Bidi_Mirrored property value of that character is true."

We are not even allowed to claim higher level protocol, as the tailoring rule HL6 only allows mirroring more characters, not less. I assume this is some of the new tightening that was supposed to happen in bidi, for security reasons blah blah.

Apart from mirroring the glyphs graphically, which will not work with things like U+2231 “∱” CLOCKWISE INTEGRAL as the spec says, the only other thing I can find is the OpenType feature 'rtla':

http://www.microsoft.com/typography/OTSPEC/features_pt.htm#rtla

As for usage, none of the default Windows Vista fonts use the feature, but some Adobe fonts may.
Comment 1 Behdad Esfahbod 2007-01-02 07:18:13 UTC 
I don't get it.  What's the bug?  Not mirroring graphically?
Comment 2 Roozbeh Pournader 2007-01-02 10:57:02 UTC 
(In reply to comment #1)
> What's the bug?  Not mirroring graphically?

The bug is "not depicting the characters with a mirrored glyph".

I don't know the proper way to fix the bug, but the only possible way I can see now is supporting 'rtla' and then blaming the fonts that don't have the feature but have both RTL glyphs and glyphs for these characters.

I am letting my mind go loose, but perhaps when displaying such characters, we should search for fonts that have an 'rtla' for the corresponding glyph, similar to what we already do for normal shapes of characters...
Comment 3 Behdad Esfahbod 2007-12-10 23:52:56 UTC 
I asked on opentype list a couple months ago and there was some discussion there.  Don't remember what the conclusion was, it wasn't easy.  More like "apply rtla feature, if that didn't conver the glyph, back out and mirror by char code".  Something awkward.
Comment 4 Behdad Esfahbod 2008-02-01 16:05:51 UTC 
This is being discussed on the opentype list again.  The idea is to fix the definition of rtla for opentype 1.6 spec.  We should just wait for now.  (I have a proposal for splitting it into two features.  I really hope that one wins.)
Comment 5 Behdad Esfahbod 2009-04-06 01:57:48 UTC 
OpenType 1.6 is out.  See end of:
http://www.microsoft.com/typography/otspec/ttochap1.htm
Comment 6 Behdad Esfahbod 2012-08-25 19:21:23 UTC 
Fixed in HarfBuzz.