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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