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

Khaled Hosny khaledhosny at eglug.org
Tue Feb 23 19:57:23 CET 2010

On Tue, Feb 23, 2010 at 07:08:10PM +0100, Hans Hagen wrote:
> On 14-2-2010 17:06, Khaled Hosny wrote:
> >>>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.
> >
> >http://www.microsoft.com/typography/OTSPEC/features_ko.htm#lfbd
> >http://www.microsoft.com/typography/OTSPEC/features_pt.htm#rtbd
> well, i decided to take a look at the font but it looks like
> something else ... in that font only digits seem to have such values

I guess because no software is supporting this, it was partially
implemented in the font.

Anyway, existing fonts doesn't really matter; AFAIK, ConTeXt will be the
first typesetting system to support it, so we are free to interpret the
spec the way that makes sense to us.

Having this data in the font is important for Arabic (my main concern)
because it have to be glyph not character based. For example, Heh (ه) in
its isolated form would have different protrusion value than its final
form (ـه), and the only way to do this reliably is to store it in the
font itself.

If a real test suite is needed, I can prepare a special copy of one of
TeX Gyre fonts for this purpose.

> Hans
> -----------------------------------------------------------------
>                                           Hans Hagen | PRAGMA ADE
>               Ridderstraat 27 | 8061 GH Hasselt | The Netherlands
>      tel: 038 477 53 69 | fax: 038 477 53 74 | www.pragma-ade.com
>                                              | www.pragma-pod.nl
> -----------------------------------------------------------------

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

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