From: John H. Jenkins <>
Date: Mon, 22 Aug 2011 11:24:01 -0600

Doug Ewell 於 2011年8月22日 上午10:59 寫道:

> Petr Tomasek <tomasek at etf dot cuni dot cz> wrote:
>>> Some PUA properties, like glyph shapes and maybe directionality, can
>>> be stored in a font. Others, like numeric values and casing, might
>>> not or cannot. An interchangeable format needs to be agreed upon for
>> Why not?
> Where does one store numeric values in a font? Maybe this should be
> taken off-list.

This is actually a relevant point. The major TrueType variants all work primarily with glyphs, not characters. Using them as a place to store information about the *characters* in the text is therefore not a reliable way to provide an override for default system behavior. By the time the rendering engine consults the fonts for layout specifics, large chunks of the text processing will already be completed.

OpenType, for example, expects that the bidi algorithm is largely run in character space, not glyph space, and therefore without regard for the specific font involved. (AAT does almost everything in glyph space, including bidi. I'm not sure about Graphite.)

The net result is that a font is an unreliable way of storing character-specific information useful on multiple platforms. This is one reason why embedding the existing directionality controls within the text itself is currently the most reliable way of getting the behavior one might want in a platform-agnostic way.

Siôn ap-Rhisiart
John H. Jenkins
Received on Mon Aug 22 2011 - 12:26:15 CDT

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