[Dev-luatex] A few newbie questions

Hans Hagen pragma at wxs.nl
Thu Jan 29 09:56:39 CET 2009

Yue Wang wrote:
> Hi,
>> an other issue is that patching whatever bit of tex coude should be done
>> very careful; this has been proven by adding a backend ... rather strict
>>  control over extensions and changes is needed in order to keep tex's
>> reputation of stability up; a one line change could result in for instance a
>> rounding issue (i mention it because we ran into it some time ago) and can
>> have rather drastic consequences; you don't want that to happen with a
>> machinery that has to reproduce a document at the pixel level
>> a patch that might work on ones machine might eventually result in many
>> problems all over the world if only because most users don't update
>> frequently and depend on formal distributions (btw the same is true for
>> fonts and other resources ... small changes can have huge consequences)
> From my experience LuaTeX's result is very different from pdfTeX's
> result. The most notable one is that they give me different line
> breaks even with the same source and tfms given. Last time I was
> working on a Chinese translation of Karl's TeX for the Impatients, It
> occured to me that LuaTeX's linebreak is so different from pdfTeX's,
> and what's worse, LuaTeX gives me more overfull boxes. Of course, I
> use an old version of LuaTeX distributed in TeXLive 2008 (0.25.x, if
> memory serves). I doubt whether it will be possible for LuaTeX to
> produce the similar result as TeX does, (not to say pixel level).

we recenetly tested luatex (a more recent version than the one on tex 
live) with the texbook and there are only a few cases where we see 
differences, especially when ligatures occur at linebreaks; this is 
something that will be sorted out; 100% compatibility will never be 
reached because we use a slightly different route (separation of lig 
building, kerning, hyphenation and par building) but in practice this 
should not be a problem

so, best use a recent version

> But is compactability that important? XeTeX is not compatible with the

it is. of course there will be differences e.g. due to different 
hyphenation patterns (more complete since we have utf and > 256 chars 
can be used in patterns), and also because when using font some of 
traditional tex's limitations are gone (number of distinctive heights 
and depths) as well as some parameters in open type variants might 
differ which also leads to different spacing

but in general, the expected behaviour of tex remains; there is so much 
macro code out there that we will not break anything in that respect 
(unless is can be easily compensated, like some of the removed pdftex 

> good old TeX, but it is still widely used in Asia (Here we don't use
> LuaTeX simply because there is no LaTeX package support). Knuth also

well, it's as with good old tex ... it will take a few years for 
everyone to catch up; when pdftex came around it took a while too before 
all features were supported by all macro packages; i'm not too worried 
about that

a too fast change would also backfire on users; for context that's not 
so much an issue because they is is a beta-update infrastructure and 
most users are willing to use beta's but with latex you need to keep in 
mind that there are many users out there who depend on a real stable 
system (with no experimental stuff that breaks down your document 
processing every now and then)

> suggested that there are several rounding flaws in recent version of
> TeX that he cannot change it, but he recommended that modern
> implementations change that (see TUGBoat, Volume 29 (2008) No.2).

maybe roundingproblems are less an issue in cjk fonts, but in for 
instance arabic typeetting, where many glyphs are pasted together by 
shifting it really matters; one scaled point might result in a .01bp 
shift in a pdf file which is visible to the sensitive eye


                                           Hans Hagen | PRAGMA ADE
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      tel: 038 477 53 69 | fax: 038 477 53 74 | www.pragma-ade.com
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