Re: metafun

[Aditya Mahajan <adityam@umich.edu>]

On Sat, 23 Oct 2010, Herbert Voss wrote:

> It is a hard job to understand how all works ...

I agree. That chapter is confusing if you do not know already how the mechanism
works.

> I tried to create the first example of the
> documentation "Page 257 Anchors and layers",
> but failed with mkii and iv. The page is created,
> but without the graphical elements. Not using
> \startbuffer--\stopbuffer makes no difference.

You are simply setting the positions. You also need to tell ConTeXt to do 
something with the positions.

1. First define a position graphic that does something with the anchors 
that it is passed.

\startMPpositiongraphic{mypos:arrow}
   pair c[] ;
   initialize_box(\MPpos{\MPvar{self}});
   c[1] := cxy ;
   initialize_box(\MPpos{\MPvar{to}});
   c[2] := cxy ;
   drawarrow c[1] -- c[2] withpen pencircle scaled 2bp ;
\stopMPpositiongraphic

cxy is the center of the box. See the defn of initialize_box in mp-core.mp 
for other values. I wish these were better documented.

2. Then define a position overlay that specifies the relation between the 
anchors.

\startpositionoverlay{arrow}
\setMPpositiongraphic{X-1}{mypos:arrow}{to=X-2}
\setMPpositiongraphic{X-2}{mypos:arrow}{to=X-3}
\stoppositionoverlay


3. Define an overlay that is mapped to MP position overlay.

\defineoverlay[arrow][\positionoverlay{arrow}]

4. Hook the overlay as a page background

\setupbackgrounds[page][background=arrow]

5. Then everything will work.

\starttext

\section {Anchors and layers}

In a previous section we saw that some \hpos{X-1} {words} were
\hpos{X-2}{circled} and connected by an \hpos{X-3} {arrow}.
As with most things in \CONTEXT, marking these words is separated
from declaring what to do with those words. This paragraph is keyed
in as:

\stoptext

Aditya
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