How to properly change page offsets?

Question

Suppose that we need to change margins. This cannot be done only by changing \hsize and \vsize, because of the default offset of 1in. There is a possibility to change \hoffset and \voffset directly, but this becomes cumbersome. Is there a more universal solution, using output routine, or some other trick? Also, how to change left offset if the page has even number (for two-pages design)?

Answer 1

Is there a more universal solution, using output routine, or some other trick?

For plain TeX, the only "more universal" that I know is the OPmac macros, for example:

\input opmac
\margins/2 a5 (1,1,1,1.4)cm % A5 paper + 1cm margins in two-pages design

In this case, the declared margins are those of the odd pages. The margins of the even pages are just mirrored accordingly. Run texdoc opmac and see section 16, or go the OPmac site for more information.

For LaTeX, you can touch the low level lengths that you can see with the package layout as \hoffset, or as more "universal" (kindly, simple, easy) way, deal with some of the packages of the topic geometry in CTAN, as typearea,anysize, or why not, the ubicuous geometry.

To know that approaches each package, run texdoc <package>. In the case of geometry it is a proper approach as usually it work and there are a nice documentation. With other packages may be you will be less lucky, but you always can run texdoc <package>.sty (now with the extension) and see the source code to understand which methods have been used.

how to change left offset if the page has even number (for two-pages design)?

In LaTeX, use the \layout command (of the package layout) in the document environment!

You will see that in a twoside article \hoffset is always 0pt and it is the same command in odd/even pages. But you but can change \oddsidemargin and \evensidemargin.

In plain TeX you can create these lengths and set \hoffset to odd/even pages conditionally:

% Stolen from a Dan Luecking's answer at comp.text.tex 
\newdimen\oddsidemargin
\newdimen\evensidemargin
\oddsidemargin=.25truein  
\evensidemargin=1truein  
\output={\hoffset=\ifodd\pageno\oddsidemargin\else%
\evensidemargin\fi\plainoutput}

\oddsidemargin=300pt % only to test ...
\evensidemargin=10pt

odd page 
\vfil \break
even page 
\bye

Answer 2

The idea of latex format is based on \moveright by value of margin the box of the page when the page is shiped out.

\vsize=5\baselineskip%%

\newdimen\oddsidemargin
\newdimen\evensidemargin

\oddsidemargin=3cm
\evensidemargin=2cm

\catcode`\@11\relax
\def\plainoutput{\shipout\vbox{%
    \ifodd\pageno
          \let\mymargin\oddsidemargin
    \else
          \let\mymargin\evensidemargin
    \fi
    \moveright\mymargin \vbox{%
              \makeheadline\pagebody\makefootline}%
              }%
  \advancepageno
  \ifnum\outputpenalty>-\@MM \else\dosupereject\fi}
\catcode`\@12\relax

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\bye 

the original output

\def\plainoutput{\shipout\vbox{\makeheadline\pagebody\makefootline}%
  \advancepageno
  \ifnum\outputpenalty>-\@MM \else\dosupereject\fi}

Answer 3

I don't think there is a universal solution, but here's one for horizontal margin that assumes some symmetry (I'm not entirely happy with it, but thought to share anyway):

\catcode`\@=11

% XeTeX doesn't have \pdf[hv]origin (yet?)
\ifdefined\pdfhorigin
\else\csname newdimen\endcsname\pdfhorigin\pdfhorigin=1in \fi
% No \outer allowed inside the skipped conditional (TeX by Topic p. 110)
\ifdefined\pdfvorigin
\else\csname newdimen\endcsname\pdfvorigin\pdfvorigin=1in \fi

\newif\ifspread
\newdimen\bindcorr

% TODO: user could give typeblock size in relation to \pdfpagesize
\def\hmargin{
  \def\next{\hoffset=\dimexpr\dimen@-\pdfhorigin
            \hsize=\dimexpr\pdfpagewidth-2\dimen@
            \ifspread
              \hsize=\dimexpr\hsize+.5\dimen@-\bindcorr
              \dimen@ii=\dimexpr.5\dimen@+\bindcorr-\pdfhorigin
              \def\spreadoutput{\ifodd\pageno\else\hoffset=\dimen@ii
                                \fi\plainoutput}
              \output{\spreadoutput}
            \fi}
  \afterassignment\next\dimen@=}

Usage example:

\pdfpagewidth=6in
\pdfpageheight=9in
\input the_above_codeblock.tex
\spreadtrue
\hmargin 5mm
\input some_text.tex
\bye

Answer 4

Thanks to everyone - your answers helped me to obtain the solution. What I did not understand before is that \hoffset and \voffset are looked at only in the output routine. I would like to share what I came up with. Following is the \margins macro, which I put to the format file and use as, e.g., \margins (2.5,1,2,2)/X, where X stands for 1- or 2- sided layout.

\def\margins (#1,#2,#3,#4)/#5 {%
  \hsize = 210 true mm
  \vsize = 297 true mm

  \advance\hsize by -#1 true cm
  \advance\hsize by -#2 true cm

  \advance\vsize by -#3 true cm
  \advance\vsize by -#4 true cm
  \advance\vsize by -.1 true in % as in plain.tex

  \voffset = #3 true cm
  \advance\voffset by -1 true in % annihilate default gap

  \output = {%
    \if 1#5
      \hoffset = #1 true cm
    \else
      \hoffset = \ifodd\pageno #1 true cm \else #2 true cm \fi
    \fi
    \advance\hoffset by -1 true in % annihilate default gap
    \plainoutput
  }%
}