Re: xml and lua again

[Hans Hagen <pragma@wxs.nl>]

Hi Thomas,

> thanks for your explanations! The point of my question was: can I feed
> the content of tex.dimen["textwidth"] directly back to TeX, and the
> answer to this appears to be "no"; you need to add some unit to it
> (otherwise, you get an error message). Which was a bit confusing to me
> at first, because the name tex.dimen implies that it holds a "real"
> dimension, like \newdim does.

Just switch to philosopher mode for a while and ask yourself what 
implications that would have in the rather fuzzy world of printing.

What is a 'real' dimension? What we call points (pt) is in other 
application also called points but happens to be basepoints in our 
universe (bp). Also, imagine that in good american tradition the 
dimension would have been inches while we all moved on to meters ...

So, Knuth foresaw this (and also wanted predictable calculations and 
wanted to avoid unportable floating points) so he came up with his own 
unit: scaled points. So, a \dimen is just a \count but consider it 
tagged to show you pt for convenience when printed (\the) and the parser 
permits you to enter these numbers as pt/bp/dd/cc/cm/mm etc.

At the lua end all are just integers (with some limited size but that 
might change as Taco and I want to play a bit with adding a couple of 
bytes and see to what extent that will break things).

In metapost the internal unit is bp (because it targets at postscript) 
and there cm, mm etc are just variables that one multiplies with so 
there you can change the universe by just saying "in := cm".

Skips are another story (not to speak of boxes as we do have a dimendef 
but not a boxdef of inserts which are yet another class of animals).

Hans


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