From: Peter Constable (petercon@microsoft.com)
Date: Fri Dec 03 2004 - 12:21:44 CST
> From: unicode-bounce@unicode.org [mailto:unicode-bounce@unicode.org]
On Behalf
> Of Gary P. Grosso
> Questions about OpenType vs TrueType come up often in my work, so
perhaps the list
> will suffer a couple of questions in that regard.
>
> First, I see an "O" icon, not an "OT" icon in Windows' "Fonts folder"
for some fonts
> and a "TT" icon for others. Nothing looks like "OT" to me, so are we
talking about the
> same thing?
Indeed, the icon shows "O"; I have no idea where the reference to "OT"
originated.
> Next, if I double-click on one of the "fonts" (files), I get a window
which shows a
> sample of the font, at the top of which is the font name, followed by
either
> "(OpenType)" or "(TrueType)". Can I believe what that says as
indicative of whether
> this is truly OpenType or TrueType?
Yes, but as Antoine's message suggested, you need to understand what it
does or doesn't imply.
To get around terminology hurdles, I will refer to the "sfnt" file
format. This is a file format for fonts that was first introduced when
Apple and Microsoft developed TrueType fonts back around 1990. One of
the features of the sfnt format is that it is extensible: it contains
tables with various types of font-related data, and new tables can be
added without affecting processes that know only about pre-existing
tables.
People have often used the term "TrueType" to refer to this extensible
font file format, but this leads to confusion. "TrueType" fonts are
fonts that use the sfnt format and that use TrueType outlines and
hinting.
OpenType differs from TrueType in two respects:
1) OpenType uses the extensibility of the sfnt format to define
additional tables
2) OpenType allows for either TrueType outlines and hinting, or
Postscript outlines and hinting
Apart from the tables related to Postscript outlines, the OpenType spec
defines six tables not found in the TrueType spec: one for a digital
signature, and five related to advanced typographic capabilities.
Strictly speaking, none of the six new tables are required by the
OpenType spec. Therefore, any TrueType font could be considered an
OpenType font. Of course, that's not particularly useful; generally
people would like to talk about OpenType in contrast to TrueType.
In terms of the icon that is displayed in the Windows font folder, it is
the presence of the DSIG table that determines which fonts get the "O"
icon rather than the "TT" icon.
The distinction that most people are interested in, however, is whether
there is support for advanced typographic capabilities. For instance,
> Mostly how this comes up is we have customers ask if we support
OpenType fonts...
these people are probably interested in advanced typographic
capabilities, not digital signatures. What they are asking, then, is
whether your products support OpenType layout capabilities -- this is
the advanced-typography functionality related to the other fives
OpenType-specific tables.
Three of these tables, in particular, are important:
GSUB - glyph substitution data used to select alternate or
presentation-form glyphs (e.g. ligatures, contextual forms)
GPOS - glyph positioning data used to position combining marks and to
kern glyphs
GDEF - glyph definition data that is used to support substitution and
positioning processes
This is what you need to support in order to support scripts such as
Arabic or Devanagari, or to support fine typography (e.g. true small
caps, swashes, ligatures).
Does that help?
Peter Constable
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