Re: [ft-devel] [patch] TrueType GX/AAT validator

[George Williams]

On Mon, 2005-08-22 at 18:29, address@hidden wrote:
>     4-5. invalid feature number (117/183)
>     -------------------------------------
>     GX/AAT extension can include 255 different features for layout,  but
>     popular layout features are predefined
Are you sure it's only 255? The 'feat' table stores both feature and
setting ids as uint16. The 'mort'/'morx' chains also use uint16. I
didn't see anything which said they were limited to 255 -- but I haven't
read everything:-)

(And there are certainly fonts which use feature numbers>255,
Osaka.dfont for one)
>     (see http://developer.apple.com/fonts/Registry/index.html).
>     Some  fonts  include  feature  number  which  is  incompatible  with
>     predefined feature registry.
> 
>     In our survey, there are 140 fonts including "feat" table.
>     a)  14 fonts uses too-large feature number (out of defined range).
>     b)  67 fonts uses feature number which should not be used.
>     c) 117 fonts set wrong feature range (nSetting).
>            this infraction is found in mort/morx.
The 'feat' (and 'mort') documentation says:
        Apple has defined a standard set of text features. You may
        include one or more of these or create your own text features.
        Font features that will be supported by your font must be part
        of the Font Feature Registry maintained by Apple Computer, Inc.
To me this seems contradictory. First we are told we can create our own
features, then we are told we can only use standard ones.
(I really dislike Apple's TrueType documentation)

Given that features and settings are in some sense "defined" by the
string(s) attached to them in the 'name' table to me it makes sense to
allow a font to create its own text features rather than restricting
them to the predefined list.

Also my understanding is that there is a difference in philosophy
between Apple & OpenType on features. Apple's feature/settings define
something useful to the user, while OpenType features are expected to be
a sub-part of a greater whole -- an application possibly combining
several features to get a desired effect. If this is true it seems
reasonable to me for a font to be able to define its own features.