[dev-context] OpenType Optical Bounds 'opbd' feature

Khaled Hosny khaledhosny at eglug.org
Sun Feb 14 17:06:55 CET 2010

On Sun, Feb 14, 2010 at 02:42:30PM +0100, Hans Hagen wrote:
> On 13-2-2010 18:32, Khaled Hosny wrote:
> >On Fri, Feb 12, 2010 at 07:21:14PM +0100, Hans Hagen wrote:
> >>On 12-2-2010 19:03, Khaled Hosny wrote:
> >>>Is 'opbd' feature supported? According to the spec[1] this feature is
> >>>used to define what I think is equivalent to the protrusion values used
> >>>for optical alignment of the glyphs. I grepped the sources and there
> >>>seem no be special handling of it, I don't have fonts that use this
> >>>feature (I didn't look for one) but I'm planning to use it in my fonts
> >>>and thought I'd check if ConTeXt support it first.
> >>>
> >>>This Typophile thread[2] might be interesting too.
> >>>
> >>>If fonts are needed I can check for free fonts that implement this, or
> >>>prepare a font for testing.
> >>>
> >>>[1] http://www.microsoft.com/typography/OTSPEC/features_ko.htm#opbd
> >>>[2] http://typophile.com/node/48738
> >>
> >>Indeed I need a test font in order to see how it can be implemented.
> >>If there are no fonts we should wonder what the exact spec is. Does
> >>fontforge support it?
> >
> >I found that the font "fxlbi.otf" (of libertine fonts) has both 'lfbd' and
> >'rtbd' (left/right optical bounds) but no 'opbd' feature. Now re-reading
> >the spec, I don't see what the values stored in 'opbd' is used for as
> >the application has to consult 'lfbd' or 'rtbd' feature according to the
> >position of the glyph in the line, I asked on OpenType mailing list but
> >I don't expect getting an answer (nobody implemented this feature, except
> >FontForge AFAIK.) My suggestion is to ignore 'opbd' completely and just
> >use the values from 'lfbd'/'rtbd' directly.
> but are these [lr]fbd meant for protruding or for italic correction

They are meant for protruding, and per the spec should be applied
only at the start/end of line.


 Khaled Hosny
 Arabic localiser and member of Arabeyes.org team
 Free font developer

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