Re: Windows Glyph Handling

From: John Hudson (
Date: Tue Aug 23 2005 - 17:33:33 CDT

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    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
    > 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

    By the way, most of the script-specific font development specs at 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
    Vancouver, BC

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