[XeTeX] The future of XeTeX

[Philip TAYLOR]

Adam Twardoch (List) wrote:

> * Even with the same core set of commands, if using OpenType fonts, the
> results between LuaTeX and XeTeX will necessarily vary. LuaTeX and XeTeX
> use different mechanisms when it comes to extracting glyph metrics,
> kerning, other positioning commands, and also different mechanisms when
> it comes to processing things like OpenType contextual alternates etc.
> -- and by using different mechanisms, it by necessity arrives at results
> that differ slightly.
>
> No known OpenType Layout engine out there (Microsoft Uniscribe, Monotype
> WorldType, Bitstream Panorama, Adobe World composer, ICU Layout,
> HarfBuzz, Pango, or the LuaTeX engine) is 100% compatible with any
> other, so the same line, or even word, may be typeset slightly
> differently with each of those layout engines. This will, in the end,
> necessarily result in different glyphs being used at times, different
> line-breaking being generated etc. When it comes to Unicode and
> OpenType, it's much more complex than the original 8-bit Western world,
> and cross-platform compatibility is no longer a goal that can be
> achieved at this time.
>
> I'd say the situation is similar to the world of web browsers: HTML, CSS
> and JavaScript are being actively developed, but some snapshots of the
> development are strictly documented by the W3C, yet other factors come
> into play so that a 100% pixel compatibility between Mozilla Firefox,
> WebKit (Chrome or Safari), Microsoft Internet Explorer and Opera is not
> achievable, and probably never will be.

OK, thank you Adam.  I think perhaps I was being unrealistic in
asking whether the two PDFs would be visually identical; for the
very reasons you adduce, it is clear that this can never be the
case.  But differences at the syntactic level are a far greater
concern : I think one should accept that if one passes an extant
XeTeX source through LuaTeX, line and page breaks may well differ,
but if LuaTeX barfs on valid XeTeX source, that is (for me, at
least) a far greater concern (and a reason against adoption, to
be honest).

** Phil.