different type areas on even/odd pages

Question

in some (very rare) cases, it might be desirable to have the type area, particularly the text width, be different on even and odd pages. Let's say we're typesetting a textbook that's supposed to have small illustrations or annotations in the margin of the odd pages -- so we'd need a narrower type area on these. Like this:

Here's an attempt at a solution I came up with. It isn't as smart as I thought it was. I thought we might just combine everyshi and an \ifthispageodd test to change the typearea at every shipout. Obviously, that's not how it works:

\documentclass[paper=a5,pagesize,10pt,DIV=12,twoside=true]{scrreprt}
\usepackage{everyshi,blindtext}
\EveryShipout{%
\ifthispageodd{\areaset{6cm}{12cm}}{\areaset{9cm}{12cm}}}

\begin{document}
\Blinddocument
\end{document}

this results in:

! Bad space factor (0).
<recently read> \@savsf 

l.7 \Blinddocument

Is there a way to improve this? Or something entirely different?

Answer 1

This only works for simple cases, but could be extended and generalised a bit.

\documentclass{article}
\makeatletter

\def\a{Cat dog goat sheep pig rabbit cow kangeroo. }
\def\b{One two three four five six. }
\def\c{\a\a\b\a\b\b\a\a\a\a\b\b\a\b}
\def\d{\c\par\a\b\c\par\b\b\b\b\b\b\par}
\def\e{\par\d\d\b\b\b\b\b\d\a\a\a\a\a\a\a\d}


\def\shp#1{%
\@tempcnta\z@
\loop
\advance\@tempcnta\@ne
\edef\pshape{\pshape 0pt #1 }
\ifnum\@tempcnta<46
\repeat
\advance\@tempcntb46
}
\begin{document}

{
\@tempcntb\z@
\def\pshape{}
\shp{5in}\shp{3in}
\shp{5in}\shp{3in}
\shp{5in}\shp{3in}
\def\par{\ifhmode\\\fi\hspace*{\parindent}\ignorespaces}
\show\pshape
\parshape\@tempcntb\pshape
\e\e
\endgraf
}

\end{document}