? 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