[XeTeX] OpenType fonts in Linux?

[Arthur Reutenauer]

> Strange, Gentium? I was only able to download ttf fonts from SIL website [1]! 
> Looked for OT, but archives contained just *.ttf and *.TTF... How come?

  The naming scheme for OpenType is a bit of mess.  To sum up (the description
below is oversimplified at places; Adam Twardoch, George Williams, Jonathan Kew
and other font gurus, don't hit me on the head):

  OpenType is the name of the format that unifies Apple's TrueType (file
extension .ttf) and Adobe's Type 1 fonts (extension .pfb).  The concrete
packaging of the font file is similar to TrueType, hence all TrueType fonts are
de jure also OpenType fonts.  And for any Type 1 font P, you can build an
OpenType font O which is logically equivalent to P, but the content of the file
is very different.  The most prominent difference (but by far not the only one)
between the two traditional formats is the way glyph shapes are described:
TrueType uses quadratic Bézier curves, PostScript uses cubic ones.  This
difference is reflected in OpenType in that there are effectively two
subformats, thus making the aforementioned unification somewhat moot: Adobe
continues to produce OpenType fonts with Type 1 outlines, while Microsoft ships
OpenType fonts with TrueType outlines.  Apple ships TrueType fonts with
additional AAT tables, that are mostly equivalent, but not quite, to the
OpenType features I mention below (I knew it was impossible to write my e-mail
in a logical order without cross-references...)

  But OpenType is more than the union of TrueType and Type 1: it offers
additional functionalities implemented as *features* in the fonts.  Thus,
traditional fonts in the two legacy formats can be enhanced with features;
TrueType fonts, in particular can be augmented with OpenType features which
older tools, written for TrueType fonts, can mostly ignore harmlessly (since
the file format is similar).  That's really what makes an OpenType font
(something more than what we had before).  There is a widespread convention
(though by no mean systematic) to use .ttf extensions for OpenType fonts with
TrueType outlines (mostly for compatibility reasons on Windows and Mac OS), and
.otf for OpenType fonts with Type 1 outlines.  But as a font user, that's not
really what's interesting to you: you're of course interested in knowing
whether the font has OpenType features or not, which is something only
additional tools can tell you.  This is the case for Gentium, in particular.

	Arthur