# How to change \hsize in the middle of a paragraph at pagebreak, especially for two-column?

The original question is: How to get two columns with different width in LaTeX?

I know this older question: How can I create a two-column layout with different widths (and texts) for the two columns? But that does not really need a two-column layout but only pictures in the margin.

In fact, it is not very difficult to set the column widths separately. I can do it manually in LaTeX, and I can use flowfram package for convinence. But it seems weird at columnbreak. \hsize is not changed until a new paragraph. An example:

\documentclass{article}
\usepackage{microtype}
\usepackage{lipsum}
\usepackage{flowfram}
\onecolumntopinarea{static}{0pt}{.3\textwidth}{\textheight}{0pt}{0pt}
\onecolumntopinarea{static}{0pt}{.6\textwidth}{\textheight}{.4\textwidth}{0pt}
\begin{document}
\lipsum
\end{document}


Then I must do a trial run and add a \par manually at the column break point, and add a \noindent at the beginning of next column. But I think it should be automatic, even in this silly approach.

I know it may be very difficult in LaTeX. I'm not familar with the output route of LaTeX. I only want to know is it possible to do this automatically? Is there any new approach for this?

I know little about ConTeXt. It seems also not easy to use variable column width in ConTeXt. But I noticed a figure in manual of MetaFun:

Thus it seems possible with (or without?) help of MetaPost.

Any information is welcome.

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Those last examples will use \parshape, which is set on a whole-paragraph basis. TeX doesn't really allow for mid-paragraph changes in a number of parameters. – Joseph Wright Feb 16 '11 at 6:54
Thanks. But is it possible to add \par and \noindent automatically at pagebreak in the middle of paragraph, or with help of some script like Lua? – Leo Liu Feb 16 '11 at 7:38
possibly a multi-pass approach with some help by the output routine (with or without lua) could be made to do this, but I am not aware of any packages that actually do so. – Taco Hoekwater Mar 9 '11 at 11:14
Could this tutorial article by David Salomon from TUGboat be of help in this issue? – morbusg Mar 9 '11 at 13:28

I don't know about using Lua, but I suspect that what you want is not possible with pdfTeX. As Joseph points out, you cannot change a number of parameters in the middle of the paragraph. The \hsize that is in effect at the end of the paragraph is the one that gets used in TeX paragraph builder.

As for column breaks, TeX accumulates vertical material in its "main vertical list" until it has enough to fill a page and then it runs the output routine. (That's a simplification; see The TeXbook or TeX by Topic for full details.) What this means here is that the paragraph has already been broken up into boxes of width \hsize and added to the vertical list (along with some glue between them to keep the baselines \baselineskip apart). So by the time TeX gets around to deciding on the break point for the first column, material that will appear in the final column has already been typeset.

Edit:
In the comments, Hendrik asks about abusing the output routine and unboxing. Even ignoring the difficulty of finding exactly which material needs to be unboxed, I can't think of a good way to deal with hyphens:

\setbox0\vbox{
\hsize40pt
\rightskip=5pt
\parindent=0pt
hyphen\-ation
}
\setbox0\vbox{
\unvbox0
\global\setbox1\lastbox
\unskip
\unpenalty
\setbox2\lastbox
\global\setbox1\hbox{%
\unhbox2
\unskip
\
\unhbox1
\unskip
\unskip
\unpenalty
}
}
\unhbox1
\bye


Here we set box 0 to have known contents.

\vbox(18.94444+0.0)x40.0
.\hbox(6.94444+1.94444)x40.0
..\hbox(0.0+0.0)x0.0
..\tenrm h
..\kern-0.27779
..\tenrm y
..\tenrm p
..\tenrm h
..\tenrm e
..\tenrm n
..\discretionary
..\tenrm -
..\glue(\rightskip) 5.0
.\penalty 400
.\glue(\baselineskip) 3.37697
.\hbox(6.67859+0.0)x40.0, glue set 12.77771fil
..\tenrm a
..\tenrm t
..\tenrm i
..\tenrm o
..\tenrm n
..\penalty 10000
..\glue(\parfillskip) 0.0 plus 1.0fil
..\glue(\rightskip) 5.0

Since we know that it has two lines, we can directly pull it apart and stuff the contents into box 1.

\hbox(6.94444+1.94444)x60.5557
.\hbox(0.0+0.0)x0.0
.\tenrm h
.\kern-0.27779
.\tenrm y
.\tenrm p
.\tenrm h
.\tenrm e
.\tenrm n
.\discretionary
.\tenrm -
.\glue 3.33333 plus 1.66666 minus 1.11111
.\tenrm a
.\tenrm t
.\tenrm i
.\tenrm o
.\tenrm n

Whereas this would have worked had the word not been hyphenated, here we get
"hyphen- ation"

-
Thank you, I know that. It might be possible to get this feature through multiple compilation, I hope. – Leo Liu Mar 9 '11 at 10:18
Can't one somehow abuse the output routine to unbox the material again and restart accordingly? I'm not opting to try, but merely commenting on the "suspect that what you want is not possible". – Hendrik Vogt Mar 9 '11 at 10:18
@Hendrik: I thought about that. TeX by Topic does say, "it is possible to dissect the page once it is in \box255...inside the output routine." Maybe my statement is incorrect, but since the material in question would be at the top of the page, I'm not sure I can think of a good way to go about this. Maybe invoke the output routine after every paragraph or use \vsplit. – TH. Mar 9 '11 at 11:44
@Hendrik: I've edited my answer in attempt to address your comment. If you can think of a way around that, I'd be pretty interested. – TH. Mar 9 '11 at 12:16
@TH: Finally I had a closer look at your Edit. What I don't quite understand is why you have \  before \unhbox1 in your code. Without it, you get "hypen-" and "ation", and isn't this what we want? Or am I just a bit slow on the uptake? – Hendrik Vogt Mar 27 '11 at 17:07