Re: [NTG-context] 3. Re: \MPpos, pxy, initialize_box(), oh, my! (Hongwen Qiu)

6 Jun 2010

      hmm, that's interesting.  the source of the metafun manual is a logical place to look... hadn't thought of that, but where can i find it?  i searched the source browser for metafun and metafun.tex to no avail.  the challenge of learning a new system like context is like cooking out of someone else's kitchen: everything you need is probably there somewhere, it's just a matter of knowing where to look.

yes, i'd like to know how this family of macros---\fpos, \tpos, mpos:box and mpos:par and friends, etc.---work, and what information you can get from them.  the only mention of these on this mailing list seems to be that bunch of messages you forwarded, which was back in 2008.  if i can get a bit of a kickstart toward figuring them out, i'd be compelled to put the basics in the wiki.

assuming mpos:box and mpos:par were some sort of built-in MPgraphics, i compiled the following short context document, where mpos:box and mpos:par had no effect :(

\setupMPvariables[mpos:box][linecolor=darkred]
\setupMPvariables[mpos:par][linecolor=darkred]

\startpositionoverlay{testing_paragraph_outline}
	\setMPpositiongraphic{pos1}{mpos:box}{self=pos1}
	\setMPpositiongraphic{pos1}{mpos:par}{self=pos1}
\stoppositionoverlay

\defineoverlay[mylayer][\positionoverlay{testing_paragraph_outline}]
\setupbackgrounds[page][background=mylayer]

\starttext
\fpos{pos1}Something special.\tpos{pos1}
\stoptext

On 6/06/2010 12:34 a.m., ntg-context-request@ntg.nl wrote:
...
Date: Sat, 05 Jun 2010 19:12:28 +0800
From: Hongwen Qiu
To:ntg-context@ntg.nl
Subject: Re: [NTG-context] 3. Re: \MPpos, pxy, initialize_box(), oh
  my! (Hongwen Qiu)
Message-ID:<4C0A311C.6010808@gmail.com>
Content-Type: text/plain; charset="iso-8859-1"; Format="flowed"
? 2010?06?05? 10:56, Adam Fuller ??:
...
...
my initial post had a mistake:  i meant to refer to the/Metafun/
 manual, not the Metapost manual.  that probably didn't help.
Actually, I know that you mean the/Metafun/  manual.
 i guess what i should have said is, i know that pxy can give me the
 bounding/box/  (the rectangle with the maximum horizontal and vertical
 extents of its contents, be they set in horizontal or vertical mode),
 but can it give me the subset of that, the bounding/path/, if you
 will, when the contents are text which flows over several lines.  the
 bounding/path/  may not be rectangular, although it will be something
 like the union of the bounding boxes of the text's individual lines.
So now, I understand what mean.
 i think if you look at the correct reference, the difference of the
 effects is clear.  i think the challenge is that you don't know where
 the line breaks will occur until you typeset it.   --adam
And the source code from the manual may help you:
\setupMPvariables[mpos:box][linecolor=darkred]
\setupMPvariables[mpos:par][linecolor=darkred]
\startpositionoverlay{backgraphics}
    \setMPpositiongraphic{A-3}{mpos:box}{self=A-3}
    \setMPpositiongraphic{A-4}{mpos:box}{self=A-4}
    \setMPpositiongraphic{b:A-5}{mpos:par}{self=A-5}
    \setMPpositiongraphic{b:A-6}{mpos:par}{self=A-6}
\stoppositionoverlay
\fpos {A-5} Because the text must be laid on top of
the graphic, the graphic must precede the first word in the
typeset stream or it must be positioned on a separate layer.
In the latter case it can be calculated directly after the
paragraph is typeset, but in the former case a second pass
is needed. \tpos {A-5}
Given the above code, I think you can get some information about the
horizontal and vertical pos of texts from \fpos and \tpos. And the
mpos:par is defined in anch-pgr.mkii and anch-pgr.mkiv. But I haven't
dig into the code which implement mpos:par. Hope you can find useful
information there.
And hope the following message I just searched out will help you too:
http://archive.contextgarden.net/message/20080107.024554.314d9fee.ca.html