Are XeTeX and LuaTeX platform dependent?

Question

One of the main goals that Don Knuth had when developing TeX (other than the obvious one of creating high-quality documents) was a 100% platform independence. A document produced on one system should always compile always exactly identical on any machine. It should never happen, that a linebreak or page break would be altered. Therefore the TeX program was very carefully written to ensure that there is absolutely no access to any internally used floating point value and that those parts that actually may use platform dependent code could not affect the typesetting.

Now from my understanding this goal is broken in XeTeX as it hands off "words" to the underlying platform-dependent font machinery and then takes back from there the font glyphs and relative horizontal and vertical positions. Those are then used to calculate line breaks and there it should be possible to generate examples where a document shows different line and/or page breaks if moved from one platform to the next.

I don't know how LuaTeX implements the Opentype support, perhaps the approach here is different. But for LuaTeX I believe that platform dependency may come into play through access to more or less general accessibility of TeX internals (even those that Don deliberately kept inaccessible because of their use of floating point arithmetic).

So my main question here is: is my understanding correct?

And the sub-questions then would be:

• Has somebody experienced a platform dependency in XeTeX?
• Has the LuaTeX font support the same "danger"?

Answer 1

LuaTeX OpenType support is fully implemented using Lua code (as Khaled wrote), which means that it is platform independent as long as identical font files are used.

But note that that is usually only the case for fonts that are part of a portable TeX distribution like TeXLive. LuaTeX also allows one to use system-installed fonts, and these will typically be a little different not only between competing operating systems, but also between different versions (or even updates) of one particular operating system.

LuaTeX uses no system font libraries, not for anything, not even fontconfig.

Anyway, LuaTeX documents should be platform independent if these conditions are met:

• The underlying platform offers IEEE754 double precision floating point as well as 64-bit integer values (but without this, the luatex executable will probably not even compile. Neither will xetex, nor pdftex).
• All used runtime (TeX, Lua, font, pattern, etc.) files are identical.
• No Lua nor TeX code is used that explicitly does platform-dependent things.

The last point is what makes it hard to give a definitive answer. However, that condition exists for pdfTeX and even traditional TeX just the same. It is a bit more obvious in LuaTeX, but it is not at all hard to write platform-dependent documents in any of the other TeX- and TeX-like engines.

Answer 2

At the engine level, LuaTeX is completely platform independent. OpenType font processing is not done by the engine itself, but it provides enough hooks and modules for macro packages to implement it themselves.

• ConTeXt implements its OpenType text layout and font management entirely in Lua, and thus it is platform in dependant as well.
• LuaLaTeX takes its text layout code from ConTeXt with its own font management (in Lua too), so the same applies.
• Other macro packages can use use either of the two or write there own.

In XeTeX all font processing is done by the engine itself:

• Font management (searching and locating system wide fonts) and this is partly system dependant, it uses Apple specific APIs on Mac and FontConfig otherwise to list and locate the fonts.
• OpenType and Graphite text layout are done by external, cross-platform libraries (in TeX Live they are statically linked to the binary). AAT text layout is available only on Mac through Apple APIs as it is an Apple-only technology with no cross-platform implementations.
• XeTeX uses external libraries for other features like line breaking for scripts that does not use space between words, font mappings and input encodings.

Answer 3

Knuth put a lot of thought into preserving line and page breaks. But (at the risk of hijacking this thread) I would suggest that the importance of maintaining precise pagination is now less than it was 30 years ago. It is more important now to have a structured document which can reflow on the fly and be available on different devices. Thankfully, TeX is probably the most structured writing system now as well. :-)

So perhaps we should be concentrating more on how structured and reusable the content is, rather than how consistently it renders on different devices.

Answer 4

Knuth was concerned that any program that specifically called itself "TeX" should pass the trip test. But XeTeX and others don't call themselves "TeX." As close clades of TeX, of course one has expectations about generic behaviours. But XeTeX does many important functions that TeX does not, so of course it isn't TeX-compliant in these areas. Specifically, documents using XeTeX's extensions, such as Unicode input, that would fail in with errors in TeX compile perfectly well with XeTeX.

Because they don't pretend to be TeX, XeTeX and LuaTeX are free from any moral imperative to produce identical output to TeX, or to pass the trip test.