% macros=mkvi language=us

\definemeasure[onecolumn]   [\columnwidth]
\definemeasure[twocolumns]  [\dimexpr\plustwo  \columnwidth+          \columndistance\relax]
\definemeasure[threecolumns][\dimexpr\plusthree\columnwidth+\plustwo  \columndistance\relax]
\definemeasure[fourcolumns] [\dimexpr\plusfour \columnwidth+\plusthree\columndistance\relax]

\usemodule[abr-04]

\setupbodyfont
  [dejavu,11pt]

\definefont[QuiteLarge][SansBold sa 2.50]
\definefont[NotSoLarge][SansBold sa 1.25]

\setuplayout
  [backspace=.1\paperwidth,
   topspace=.025\paperheight,
   header=0.025\paperheight,
   footer=0.025\paperheight,
   headerdistance=0.025\paperheight,
   footerdistance=0.025\paperheight,
   width=middle,
   height=middle]

\setuppagenumbering
  [alternative=doublesided,
   location=]

\setupwhitespace
  [line]

\setupblank
  [line]

\setuptyping
  [blank=halfline]

\startsetups [pagenumber]
    \hbox to .2\paperwidth {
        \hss
        Page \pagenumber
        \hss
    }
\stopsetups

\setupfooter [before=\vfill,after=\vfill,style=\bf,color=color-4]
\setupheader [before=\vfill,after=\vfill,style=\bf,color=color-4]

\setupfootertexts [] [\setups{pagenumber}]
\setupheadertexts [] [\setups{chapterhead}]

\startsetups [chapterhead]
    \hbox to .2\paperwidth {
        \hss
        \getmarking[chapter]
        \hss
    }
\stopsetups

\setuphead
  [chapter]
  [header=empty]

\definecolor[color-3][.5(red,black)]
\definecolor[color-2][.5(green,black)]
\definecolor[color-1][.5(blue,black)]
\definecolor[color-4][.5(white,black)]
\definecolor[color-5][.5(white,color-4)]
\definecolor[color-6][.2(white,color-4)]

\definecolor[fakerulecolor][white]

\usemodule[typesetting]

\setupcombination
  [style=mono]

\startuseMPgraphic{page}
StartPage ;
    path p ;
    numeric n; n := PageNumber ;
    numeric w; w := bbwidth(Page) ;
    numeric h; h := bbheight(Page) ;
    def DrawBit (expr bit, dx, dy) =
        path p ; p := unitsquare xyscaled(w/5,h/40) shifted (dx*w/10,dy*(h-h/40)) ;
        if n div bit > 0 :
            n := n - (n div bit)*bit ;
            fill p withcolor \MPcolor{color-1} ;
        else :
            fill p withcolor \MPcolor{color-2} ;
        fi ;
    enddef ;
    DrawBit(32,7,1) ;
    DrawBit(16,4,1) ;
    DrawBit( 8,1,1) ;
    DrawBit( 4,7,0) ;
    DrawBit( 2,4,0) ;
    DrawBit( 1,1,0) ;
StopPage ;
\stopuseMPgraphic

\startuniqueMPgraphic{frame}
    fill OverlayBox withcolor \MPcolor{color-5} ;
\stopuniqueMPgraphic

\startuniqueMPgraphic{contrast}
    fill OverlayBox withcolor \MPcolor{color-4} ;
\stopuniqueMPgraphic

\defineoverlay[page]    [\useMPgraphic{page}]
\defineoverlay[frame]   [\useMPgraphic{frame}]
\defineoverlay[contrast][\useMPgraphic{contrast}]

\setuphead
  [chapter]
  [style=\QuiteLarge,
   color=color-3,
   number=no]

\setuplist
  [chapter]
  [command=\MyListCommand,
   alternative=command]

\starttexdefinition unexpanded MyListCommand #1#2#3
    \snaptogrid
        [line,-line]\vbox {
            \tfb
            \setstrut
            \strut#2
            \quad#3
        }
\stoptexdefinition

\setupmargindata
  [style=\tttf,
   color=color-3]

\starttexdefinition unexpanded ShowFile #1
    \margintext
        {\cldcontext{string.match("#1","\letterpercent d+")}}
    \typefile
        [range={environment}]
        {#1}
\stoptexdefinition

% \typefile[range=4]  {...}
% \typefile[range={=}]{...} % start after comment lines

\starttexdefinition unexpanded ShowEnvironmentFile #1
    \margintext
        {\cldcontext{string.match("#1","\letterpercent d+")}}
    \typefile
        [range={=startenvironment}]
        {#1}
\stoptexdefinition

\startdocument

\startMPpage
StartPage ;
    path p ; numeric w, h, d ; picture q ;
    if true :
        q := image(draw textext.urt("\ssbf PAGES") withcolor \MPcolor{color-6}) ;
    else :
        q := image(draw textext.urt("\ssbf PAGECOLUMNS") withcolor \MPcolor{color-6}) ;
    fi ;
    w := bbwidth(Page) ;
    h := bbheight(Page) ;
    d := h/100 ;
    fill Page withcolor \MPcolor{color-1} ;
    p := Page xysized (w,d) ;
    for i := 5d step 2d until h-5d :
        fill p shifted (0,i) withcolor \MPcolor{color-2} ;
    endfor
    p := Page xysized(w/10,h) ;
    for i = 0, .3, .6, .9 :
        fill p shifted (i*w,0) withcolor \MPcolor{color-1} withtransparency(1,.75) ;
    endfor ;
    def do_it(expr sz, sh) =
        p := Page xysized sz ;
        fill p shifted sh withcolor \MPcolor{color-3} withtransparency(1,.5) ;
        draw q xysized (bbwidth(p),bbheight(p)) shifted sh ;
    enddef ;
    do_it ((8w/10,13d),(.1w,81d)) ;
    do_it ((5w/10,13d),(.4w,67d)) ;
    do_it ((2w/10,17d),(.1w,20d)) ;
    do_it ((2w/10,15d),(.1w,56d)) ;
    do_it ((1w/10,10d),(.4w,40d)) ;
    do_it ((2w/10,13d),(.7w,27d)) ;
    draw textext.urt("\ssbf Hans Hagen")
        xysized (5w/10,5d)
        shifted (.4w,6d)
        withcolor \MPcolor{color-5} ;
StopPage ;
\stopMPpage

\page[empty] \setuppagenumber[number=1]

\setupbackgrounds
  [page]
  [background=page]

\starttitle[title=Contents]

    \placelist[chapter]

\stoptitle

\startchapter[title=Introduction]

{\em This manual as well as the mechanism it describes is under construction. Don't
use page columns in production. Experimenting is of course fine.}

The \TEX\ engine is a progressive typesetter: it takes input and does something
with it and can do that for quite a while. It can process massive input and
produce massive output. In that process it never looks back. Now, it is of course
possible to collect content and delay some aspects of the rendering but to what
extent that content can be manipulated is limited. In \LUATEX\ one can do a bit
more but even then the basic typesetting process is mostly linear.

When a text gets typeset there are several properties that play a role:

\startitemize
\startitem
    the width of the lines in a paragraph which is the main unit of processing
\stopitem
\startitem
    the target height of what gets output, normally this is the height of the
    text area, excluding headers and footers
\stopitem
\startitem
    the parameters that determine when the so called output routine gets called,
    for instance successive broken lines, widow and club penalties
\stopitem
\startitem
    the presence of so called inserts, for instance footnotes or floating bodies
\stopitem
\startitem
    the space between lines and paragraphs
\stopitem
\startitem
    explicitly entered elements or spacing or injected by macros hooked into for
    instance \type {\everypar}
\stopitem
\stopitemize

It is important to realize that \TEX\ has no concept of what a page is. It takes
input and occasionally hand over the intermediate result to the output routine.
There decisions are made to flush out a page, either of not with ornaments like
headers, footers, content left over from previous encounters, footnotes as well
as maybe specific trickery liked by the backend that targets at some viewer.

That said, so called multi|-|column layouts are {\em not} part of the concepts
present in \TEX. As far as the engine is concerned there is only one column.
Among the reasons for not having a native mechanism for columns is that it is
very hard to come up with a mechanism for splitting them in ways that suits
everybody. In fact, the amount of control needed would make such a mechanism very
complex and still limited. You can compare this to math typesetting. That
subspace has some fixed characteristics if only because otherwise mathematicians
could not communicate. However, there is some control over spacing and one can to
some extend intervene. In \LUATEX\ some of the hard coded properties are made
configureable but still one gets pretty much what the engine considers right.
Such an approach would not work with columns. In fact, it doesn't even work well
with tables, for which \TEX\ provides the alignment mechanism: different kind of
tables demand different solutions, which is why in \CONTEXT\ we have old
fashioned tables, tabulated tables (that span pages and can contain paragraphs),
natural as well as extreme tables that resemble \HTML\ tables, line tables and
framed tables.

So, when we want to support columns we need to write something in \TEX\ lingua
and for that the \type {\vsplit} operation can be of help. So far we have in
\CONTEXT\ four mechanisms:

\startitemize[n]
\startitem
    the original \MKII\ mechanism that permits mixed single and multi|-|column
    text flows, with limited support for floating bodies
\stopitem
\startitem
    a reimplementation of mixed columns in a mix of \TEX\ and \LUA, a bit better
    with balancing and penalties for keeping section heads with the text, but
    with very limited support for floating bodies; it works ok with grids
\stopitem
\startitem
    a reimplementation of \MKII\ columnsets, a mechanism that is meant for
    magazine like layouts where floats can span columns and positioned at will:
    this one assumes relative simple text flows but as a bonus provides spreads;
    this one is strongly grid based
\stopitem
\startitem
    page based columns, where we use the normal (single column) page builder to
    collect columns that then get combined into a page: this one is quite robust
    but one has to keep an eye on floats as we don't span across columns
    otherwise than in top or bottom floats but, on the other hand, one can use so
    called side floats; this one is still experimental
\stopitem
\stopitemize

Given specific demands one can come up with alternatives but it's very hard to
combine all demands into one mechanismn simply because anything can be part of
the text flow. Because it is not much fun to write such mechanisms and because
no project ever pays for fundamental development, one just has to accept what
\CONTEXT\ provides, be it with limitations.

The first mechanism is more or less obsolete and replaced by the second. For
instance, we now use the second one when we have an itemize that demands columns.
I must admit that I don't like columns and never use them. I was involved in
projects where the designer wanted columns but in the end the complexity of the
content didn't work well with that that decision: inconsistent whitespace (due to
many small sections, lots of images, many tables, math formulas, etc) as well as
the result having more pages that the single column variant with side and margin
floats. Occasionally I use columned itemize or columned tables of contents and of
course registers are often in columns.

One can wonder why columns are supported anyway. It's mostly useful for
newspapers that have wide pages or journals that have small print. For magazines
and special layout it's fine too although it can look bad when columns are narrow
and fancy layout tricks are used. I guess that at some point their use will
disappear. A document on a phone or tablet is one narrow columns. An internet
page is one long column. A printed book, given that there will be such items in
the future is normally typeset in one column. And journals as well as newspapers
more and more get replaced by media that better serve their purpose. So, apart
from an intellectual challenge, implementing extensive multi|-|column support is
a waste of energy. Which is why we can accept limitations.

\stopchapter

\startchapter[title=Just text]

As with the other mechanism you can define a column instance and use that later.
You can also directly pass the settings with the \type {\startpagecolumns} command
but best is to define an instance.

\FourPages{pagecolumns-001}

\ShowFile{pagecolumns-001}

We always start at a new page and end on a new one. Footnotes and such don't span
columns and we don't balance. When the number of columns equals one you get the
normal routine but with the requested settings.

\stopchapter

\startchapter[title=Floats]

You can have floating bodies in columns. When a float doesn't fit it will be
flushed at the top of the next page.

\FourPages{pagecolumns-002}

\ShowFile{pagecolumns-002}

\SixPages{pagecolumns-007}

\ShowFile{pagecolumns-007}

Side floats are also supported but we don't treat them special so you get the
same behaviour as with single column layouts.

\FourPages{pagecolumns-005}

\ShowFile{pagecolumns-005}

\FourPages{pagecolumns-003}

\ShowFile{pagecolumns-003}


\stopchapter

\startchapter[title=Footnotes]

Footnotes are supported but we assume sane usage. You can always revert to
endnotes if needed.

\FourPages{pagecolumns-004}

\ShowFile{pagecolumns-004}

\stopchapter

\startchapter[title=Pages]

Going to a new page happens with \type {\page} and progressing to the next column
with \type {\column}.

\FourSpread{pagecolumns-006}

\ShowFile{pagecolumns-006}

\stopchapter

\stopdocument