From: John Hudson (tiro@tiro.com)
Date: Tue Aug 23 2005 - 17:33:33 CDT
Adam Twardoch wrote:
>> Yes, precisely. Character level re-ordering is not discussed within
>> the OpenType spec itself because it is not something that happens
>> within the font format.
> I believe this is a narrow-minded view. "OpenType" is not only a font
> format. OpenType Layout is a shaping technology. I think that the
> OpenType specification should give more prominent mention to the fact
> that there is an additional level between the Unicode character code
> stream and the resulted glyph stream.
I don't think it is a 'narrow-minded view', just a description of how the OT *font format*
specification has been written. Note that the registered script, language system and OTL
feature tags are an *appendix* to the specification. There seems to have been a deliberate
editorial decision to separate as far as possible the implementation of OpenType Layout
from the underlying architecture of the font format that makes it possible. You may
criticise this decision, but I'm just describing what I see in the way the specification
has been written.
And I think there are understandable reasons for the separation of the font format spec
and the layout spec, because the latter involves implementation decisions that MS and
Adobe quite sensibly decided should be at the discretion of individual developers.
Remember that one of the reasons GX went nowhere is that major app developers like Adobe
didn't want layout implementation to be dictated to them by the font format.
> This becomes very clear when one
> takes a look at the website
> http://www.microsoft.com/typography/SpecificationsOverview.mspx because
> the script-specific font development specs take some 1/3 of the screen.
> However, if one looks just at the OpenType font specification (e.g. on
> the Adobe website), one can easily overlook the necessity to look at the
> bigger picture.
Yes, which is why we're having this discussion: people are looking at the font format
specification and trying to extrapolate an understanding of the whole. And I agree that
the higher level documentation is lacking (although my 'Windows Glyph Processing' article
is still a pretty good overview of Microsoft's implementation, I think). You and I know
this stuff, Adam, because we've been dealing with it every day for many years now, not
because we read a white paper, and we're still piecing together some bits of the emerging
picture, as are MS and Adobe and everyone else. As Antoine wrote, this is a developing
standard.
By the way, most of the script-specific font development specs at
http://www.microsoft.com/typography/SpecificationsOverview.mspx are in desparate need of
revision and updates, as MS have acknowledged several times. Discussion on the MSN VOLT
community remains the best source of information for complex script font developers.
John Hudson
-- Tiro Typeworks www.tiro.com Vancouver, BC tiro@tiro.com
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