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Re: OpenType Layout code

From: David Turner
Subject: Re: OpenType Layout code
Date: Wed, 31 May 2000 22:31:40 +0200

Hi Werner,

I'm mailing this to the devel list, given that some people might
be interested by this information:

> >  [ snipped text regarding the rewrite of the OpenType Layout code for FT2 ]
> >
> >   There is however something that I find hard to understand: why is
> >   there a need to "update" the glyph properties each time a lookup
> >   is performed?  I know that the code really assigns properties to
> >   glyphs that have none in the original GDEF table, but I fail to
> >   see in the OpenType specification a paragraph about it.  Could you
> >   tell me where the relevant piece of information is to be found?
> >   I'd like to know the exact details and conditions in order to
> >   experiment with other kinds of implementations/ assignments...
> I fear that you won't find any detailed information on properties.
> There are two kinds of properties.  The first one is what you find in
> a GDEF table, if there is one: Let's call this `glyph properties'.
> For example, if you have the ligature `x' `y'->`a', then the data in
> the GDEF table is used to assign a glyph property to ligature `a'.  If
> there is no GDEF table (as in trado.ttf), my code guesses that after a
> ligature substitution, the resulting glyph is of type `ligature'.
> Note that support for user-defined GDEF data is only implemented to
> support the marvellous trado font, since older Arabic versions of
> Windows (and, I believe, even the new OTL engine) have hard-coded
> support for this font (as Paul Nelson from MS told me).  I hope that
> my solution is generic enough to support other old fonts also --
> nevertheless, I doubt that you'll find another OT font without a GDEF
> table.
> The other kind is what I call `user properties' or `features'.  For
> example, Arabic has four glyph classes, namely isolated, initial,
> medial, and final, so the `fina' lookup should affect only final
> glyphs, the `medi' only medial glyphs, etc.  This isn't documented at
> all in the OT specification (it is just mentioned `that you have to
> keep track' of some data)!  It is a consequence of the implementation,
> and maybe you are able to find a better solution how to handle this.
> After you have applied a lookup you must decide which glyphs are
> affected by the lookup.  My solution is to propagate the user property
> value of the *first glyph* to all the other glyphs.  Example:
> decomposition of the ligature `fl'->`f' `l'.  `fl' has user property
> `foo', so I assign `foo' to `f' and `l' also.
> Does this answer your question?
You bet it does :-) However, I'll rephrase your answer to be sure that
we're speaking about the same thing, this topic seem rather complex:

 - first of all, you're saying that some key important points of
   the OpenType Layout are simply undocumented. That's nasty, to be
   polite, but we'll have to deal with it given that you cleverly
   managed to get it right. Congratulations for your work, by the way :-)

 - apparently, there are two kinds of glyph properties:

     * "static" properties, that are normally defined in the GDEF
       table. An important thing is that not all glyphs have a property
       defined for them in this table.. However, these definitions/properties
       can change during the layout process (according to your code)

     * "dynamic" properties, that are applied to glyphs during
       the layout process, based on the static ones and the lookups,
       substitutions or positioning that occured.

 - moreover, the high-quality Arabic "pardo.ttf" doesn't contain
   a GDEF table, which is why the test program "ftstrtto" in FT 1.x
   does indeed create one "by hand" to describe Arabic properties..

 - currently, when the code performs layout, it modifies the dynamic
   properties, as well as the static ones

If this is correct, I'm wondering if it would not be possible to do
the following:

  - compute as many static properties as possible when loading the OpenType
    tables. For example, it should be possible to parse the substitution
    sub-tables to see that "f" + "l" can give "fl". Then, we're able to
    assign a static property to "fl" it is hasn't one already..

    This would augment the GDEF table a bit, but this work should only be
    done at font open time.. We could also hook the additional Arabic
    properties there..

  - use dynamic properties for the layout process, but without a need to
    modify the static ones. I believe this could greatly speed things

The idea is of course to avoid modifying the GDEF table, or its memory
representation, during the lookup process..

Do you think this is feasible, or am I missing something important ?


- David

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