some secret mp commands

[Hans Hagen <pragma@wxs.nl>]

Hi mp friends, 

While I was playing a bit with the graph macros that come with mpost
[actually, extending the format package] i uncovered some commands that are
not defined in the mp manual, but nevertheless quite interesting. I will
cover them in the metafun manual, but here is already an impression:

vardef inverted expr p =
  save pp ; picture pp ; pp := nullpicture ;
  for i within p :
    addto pp if stroked i : doublepath else : contour fi pathpart i
    dashed dashpart i withpen penpart i
    withcolor white-(redpart i, greenpart i, bluepart i) ;
  endfor ;
  pp
enddef ;

beginfig(1)
  colorcircle(10cm,red,green,blue) ;
  currentpicture := inverted currentpicture ;
endfig ;

So, one can disassemble a picture using a within loop and *part primitives!
Another one is truecorners but I still have to find out what it does. [i
found a few more by browsing the mp.pool file, like fontsize that reports
the real font size [so something infont "string" at size is now possible.] 

Hans  
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