Non rectangular page geometry

Question

I am interested in using non-rectangular page geometries for my flipbook idea (in fancyhdr with image and flowing text).

The page geometry that I'm after is that the text has a little rectangle missing from the bottom right (excised rectangle) on every odd page. In my previous question we figured out a way to make change the text height on every page, but not have the right geometry:

\documentclass[twoside]{article}
\usepackage[textheight=15cm]{geometry}
\usepackage{lipsum}
\usepackage[demo]{graphicx}    
\usepackage{fancyhdr}
\usepackage{everyshi}
\usepackage{xifthen}


\newcounter{flipbookcount}
\setcounter{flipbookcount}{1}

\fancyfoot[RO]{\setlength{\unitlength}{1mm}
\begin{picture}(0,10)
\put(-24,-34){\includegraphics[width=5cm]{mypic\theflipbookcount.png}}
\end{picture}
\stepcounter{flipbookcount}
}


\EveryShipout{%
\ifthenelse{\isodd{\value{page}}}%
        {\global\textheight=20cm}% Next page is even
        {\global\textheight=15cm}% Next page is odd
        }

\begin{document}
\pagestyle{fancy}
\lipsum
\lipsum
\lipsum
\lipsum
\lipsum

\end{document}

Is there a way to make that work? I can add a picture as an example if its unclear what I mean.

Answer 1

This is a very partial attempt. It doesn't really work, but could help someone find a better solution.

The idea I have is to set \parshape for every paragraph, depending on the space remaining on the page. Namely, the number of lines that should fit on the page is (\pagegoal-\pagetotal)/\baselineskip. The \parshape begins with lines of full length \hsize, then some shortened lines for the cutout, and further lines are set with full length \hsize. The number of lines with full length is (\pagegoal-\pagetotal-\l_cutout_height_dim)/\baselineskip, the number of shorter lines is the difference.

These calculations unfortunately do not account for the fact that pages can stretch or shrink. Besides, I am not sure whether I should be using \int_div_truncate:nn or \int_div_round:nn (i.e., well, truncating the result or rounding it to the nearest integer) to compute the numbers of lines.

Also, using \everypar directly is not very good idea in general, and I should perhaps use the everyhook package.

\documentclass{article}
\usepackage{lipsum}
\usepackage{expl3,xparse}
\ExplSyntaxOn
  % This code accesses \everypar, \value{page}, \hsize, \pagegoal,
  % \pagetotal, \baselineskip, \parshape, \interlinepenalties from
  % TeX/LaTeX2e directly, as they do not have an expl3 interface.
  %
  \dim_new:N \l__cutout_height_dim
  \dim_new:N \l__cutout_width_dim
  \dim_new:N \l__cutout_hsize_dim
  \int_new:N \l__cutout_long_lines_int
  \int_new:N \l__cutout_total_lines_int
  \NewDocumentCommand { \cutoutbottomodd } { m m }
    { \__cutout_bottom_odd:nn {#1} {#2} }
  \cs_new_protected:Npn \__cutout_bottom_odd:nn #1#2
    {
      \dim_set:Nn \l__cutout_height_dim {#1}
      \dim_set:Nn \l__cutout_width_dim {#2}
      \exp_args:Nx \everypar
        {
          \exp_not:V \everypar
      \exp_not:N \__cutout_everypar_test:
        }
    }
  \cs_new_protected_nopar:Npn \__cutout_everypar_test:
    { \int_if_odd:nT { \value{page} } { \__cutout_everypar: } }
  \cs_new_protected:Npn \__cutout_everypar:
    {
      \dim_set:Nn \l__cutout_hsize_dim
        { \hsize - \l__cutout_width_dim }
      \int_set:Nn \l__cutout_total_lines_int
        {
          \int_max:nn { \c_one }
            {
              \int_div_truncate:nn
                { \pagegoal - \pagetotal }
                { \baselineskip }
            }
        }
      \int_set:Nn \l__cutout_long_lines_int
        {
          \int_div_truncate:nn
            { \pagegoal - \pagetotal - \l__cutout_height_dim }
            { \baselineskip }
        }
      \int_compare:nNnF \l__cutout_long_lines_int > \c_zero
        { \int_zero:N \l__cutout_long_lines_int }
      \parshape
        = \int_eval:n { \l__cutout_total_lines_int + \c_one } \exp_stop_f:
          \prg_replicate:nn \l__cutout_long_lines_int
            { \c_zero_dim \hsize }
          \prg_replicate:nn
            { \l__cutout_total_lines_int - \l__cutout_long_lines_int }
            { \c_zero_dim \l__cutout_hsize_dim }
          \c_zero_dim \hsize
      \interlinepenalties
        = \int_eval:n { \l__cutout_total_lines_int + \c_one } \exp_stop_f:
          \prg_replicate:nn { \l__cutout_total_lines_int - \c_one }
            { 10000 \exp_stop_f: }
          -10000 \exp_stop_f:
          10000 \exp_stop_f:
    }
\ExplSyntaxOff
\begin{document}
\cutoutbottomodd{96pt}{120pt}
\lipsum[1-10]
\large
\lipsum[11-20]
\Large
\lipsum[21-30]
\scriptsize
\lipsum[31-50]
\footnotesize
\lipsum[51-70]
\end{document}

EDIT: code above updated to only put the hole on odd pages. Also, in the previous version, long lines sometimes came from the next page onto the previous page, hence bleeding through the hole. Now I am using eTeX's \interlinepenalties to force a page break.

EDIT2: Use \int_eval:n instead of the long-deprecated \int_eval:w. Also bring the code up to more recent coding conventions.