[tex-live] \pdfdecimaldigits=3

Reinhard Kotucha reinhard.kotucha at web.de
Wed Jan 20 05:08:54 CET 2016

On 2016-01-19 at 23:50:13 +0000, Karl Berry wrote:

 > Hi David,
 >     Currently pdftexconfig.tex sets \pdfdecimaldigits=3
 >     In other words was there a particular reason to choose 3....
 > Not that I know of, but then I probably wouldn't know.  (Ross, Thanh?)
 > However, in any case ... changing it at this late date would mean, I
 > think, that most PDF output from pdftex would be gratuitiously different
 > from what was produced before.  That doesn't sound like a good outcome.
 > On the badness scale, I'd say that's rather worse than not being able to
 > share settings ... -k

AFAIR Hartmut Henkel investigated a decade ago and came to the
conclusion that more than 3 decimal digits don't make sense.

As far as the backend is concerned, it sounds reasonable.  PDF's unit
is a "big point", and with \pdfdecimaldigits=3 the resolution in the
PDF file is about 0.3µm.  This should be sufficient in most cases.
If it's not sufficient in a particular case, the configuragtion can be

Hans said that many transformations are involved.  Sure, if too many
"cm" ("concat" in PostScript) operations are written to the PDF file,
rounding errors add up.

Hans distinguishes between LuaTeX internals and the (PDF) backend.

So what does \pdfdecimaldigits actually control?  Since LuaTeX is
written in C I would expect that numbers are either floats, doubles,
or integers internally and numbers are represented with a reasonable

Does \pdfdecimaldigits really control how numbers are processed in the
innards of LuaTeX?  That would be strange.

If \pdfdecimaldigits only controls how numbers are represented in a
PDF file, I don't see any reason to overwrite settings read from the
config file and I still don't understand why a setup for pdftex isn't
appropriate for luatex.


Reinhard Kotucha                            Phone: +49-511-3373112
Marschnerstr. 25
D-30167 Hannover                    mailto:reinhard.kotucha at web.de

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