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<div class=Section1>
<p class=MsoNormal><span lang=EN-US style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'>To be more precise, Lookup tables operate on glyphs, but they
are triggered by OT features, and features are attributed in the client to
characters. So, I don’t think there’s any break in philosophy. The
only innovation along this line is to introduce a Feature Parameters table that
includes character information indicating what character(s) the feature is
applicable to. But again, this is connected to the features, not lookups, and
so is applied to the character level.<o:p></o:p></span></p>
<p class=MsoNormal><span lang=EN-US style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'><o:p> </o:p></span></p>
<p class=MsoNormal><span lang=EN-US style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'>If two cvXX features are applicable to the same character and
both are applied to the same string, then lookups associated with both features
will be processed over that string, and the interaction of the lookups is
determined by the content and relative order of those lookups as determined by
the font developer. (I think it wouldn’t really make sense to have two
cvXX features applicable to the same character, though.)<o:p></o:p></span></p>
<p class=MsoNormal><span lang=EN-US style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'><o:p> </o:p></span></p>
<p class=MsoNormal><span lang=EN-US style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'>The character information provided in the (optional) feature
parameters table is intended for use by the client in determining UI behavior (should
the client wish to use it).<o:p></o:p></span></p>
<p class=MsoNormal><span lang=EN-US style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'><o:p> </o:p></span></p>
<p class=MsoNormal><span lang=EN-US style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'>As for comments about OT Layout in comparison with GX, let me
offer a different perspective: Dave wrote, “</span><span lang=EN-US
style='font-size:10.5pt;font-family:"Helvetica","sans-serif"'>[O]ne of the
principal advantages of the GX approach is that a developer does not need to
know any of the details described in the previous sections.</span><span
lang=EN-US style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'>” The assertion of developers not needing to know various
details is true of *<b>software</b>* developers, but certainly not true of font
developers. So, there’s just a transfer of burden. <o:p></o:p></span></p>
<p class=MsoNormal><span lang=EN-US style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'><o:p> </o:p></span></p>
<p class=MsoNormal><span lang=EN-US style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'>Also, the comparison at this point is not an apples-to-apples comparison:
he’s comparing the OT font format with the GX line-layout implementation.
If a developer is presented with nothing more than the GX font format, e.g. if
implementing support for TrueType GX fonts on Windows or Linux, then there are
additional details (granted, not exactly all the same ones discussed earlier in
the paper) they must take into consideration than if they were developing an
app in a context in which GX line layout already existed. A software developer
creating an implementation of GDEF, GSUB and GPOS tables has certain additional
work; and a software developer creating an implementation of the feat, mort, and
prop tables has certain additional work; it’s just different work in each
case. I think a more even comparison with GX layout and ATSUI would be GDI text,
WPF, DirectWrite or similar APIs: developers writing at these levels just need
to know how to use the APIs correctly, and most or all of the details are
handled for them.<o:p></o:p></span></p>
<p class=MsoNormal><span lang=EN-US style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'><o:p> </o:p></span></p>
<p class=MsoNormal><span lang=EN-US style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'>I won’t comment on dead ends—this isn’t an
appropriate forum for that.<o:p></o:p></span></p>
<p class=MsoNormal><span lang=EN-US style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'><o:p> </o:p></span></p>
<p class=MsoNormal><span lang=EN-US style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'><o:p> </o:p></span></p>
<p class=MsoNormal><span lang=EN-US style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'>Peter<o:p></o:p></span></p>
<p class=MsoNormal><span lang=EN-US style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'><o:p> </o:p></span></p>
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<p class=MsoNormal><b><span lang=EN-US style='font-size:10.0pt;font-family:
"Tahoma","sans-serif"'>From:</span></b><span lang=EN-US style='font-size:10.0pt;
font-family:"Tahoma","sans-serif"'> mpeg-OTspec@yahoogroups.com
[mailto:mpeg-OTspec@yahoogroups.com] <b>On Behalf Of </b>karstenluecke<br>
<b>Sent:</b> Friday, March 13, 2009 5:13 AM<br>
<b>To:</b> mpeg-OTspec@yahoogroups.com<br>
<b>Subject:</b> [mpeg-OTspec] OFF "cv01"-"cv99", and a more
general question<o:p></o:p></span></p>
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<p class=MsoNormal><o:p> </o:p></p>
<div id=ygrp-mlmsg>
<div id=ygrp-msg>
<div id=ygrp-text>
<p style='margin-bottom:12.0pt'>Character Variants (cv01-cv99)<br>
<br>
So far, OT layout tables operate on glyph level, not on character level. Once
'cmap' has mapped codepoints to GIDs, the character level is completely
ignored. Now character variants smuggle in character level information
(optionally?) which seems to "break" the OTL philosophy.<br>
That you suggest doing it is interesting though because it indicates that
typographic layout behavior may require more than glyph level information and
that OT's inherent neglect of the character level (like paragraph, line, word
boundaries) makes it hard or impossible to do certain things.<br>
<br>
How is this to be implemented? I could imagine that 'cvXX' features that relate
to the same character would be treated as mutually exclusive (a UI could automatically
group 'cvXX' features that relate to the same source character/GID) while those
that relate to different characters could be applied additively. So in effect
one would have different groups of 'cvXX' features, within each groups mutually
exclusive, but groups are additive to each other.<br>
It could work the same way if no codepoints are provided and glyphs are
identified by GID. So I don't really see a need for character level
information. But may miss something.<br>
The behavior I sketched implies that 'cvXX' features should NOT interact with
each other, as InDesign's implementation of the 'ssXX' features allows. (I am
not sure if mutually-exclusive or additive support of 'ssXX' features is
considered as "normal" behavior.)<br>
<br>
Standardizing what?<br>
<br>
You are making OT/OFF a standard. Specifying a font format however is just part
one which is of limited use without part two: defining what a text/layout
engine is to do with the data a font provides, and what to do before applying
behavior defined in layout tables. (So why not add a documentation of what WPF
or Uniscribe do, as models for other text/layout engines?)<br>
For example, I heard that people have problems supporting the 'curs' feature /
GPOS lookup type 3. And some minutes ago I found Mr Opstad's old document
"Comparing GX Line Layout and OpenType layout" (<a
href="http://developer.apple.com/textfonts/WhitePapers/GXvsOTLayout.html">http://developer.apple.com/textfonts/WhitePapers/GXvsOTLayout.html</a>)
which, in the last section "The system support question", pointed out
this conceptual flaw already in 1997, and as far as I see nothing has changed
about it. (Btw, OT layout tables seem to be dead end when it comes to complex
layout behavior, which is nicely illustrated in the Opstad-paper's comparison
of GX's "state machines" vs OT's mere "string matches" in
the sections "Contextual"/"Ligature"/"Insertion".)<br>
<br>
Best wishes,<br>
Karsten Luecke<o:p></o:p></p>
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