Hans et al,
Let me try to be more specific. Here is a file which is very typical of
the way I draw graphs for my mathematics classes. When you compile, you
will see the graph of the absolute value of x-1. That is, it draws the
graph of y=|x-1|, but only after clipping it to the cpath. What I
would like to do is draw the graph with the drawdblarrow command so
that the graph of the absolute value ends in arrowheads. So, I want to
clip the graph to the bounding box, save the picture, null the picture,
then somehow extract the path into the variable F in its clipped form
(not the original path F that extends outside the clip path) so that I
can replace the command draw pic with drawdblarrow F.
beginfig(0);
numeric a, c;
a=1;
% define function f(x)=|x-a|
vardef f(expr x)=
abs(x-a)
enddef;
% draw line with given point and slope
path F;
F:=(-5,f(-5));
for x=-5 step 1 until 5:
F:=F--(x,f(x));
endfor;
% initialize scale
numeric u; 10u=3in;
% the line
F:=F scaled 1u;
draw F withcolor blue;
% clipping path
path cpath;
cpath:=(-5,-5)--(5,-5)--(5,5)--(-5,5)--cycle;
cpath:=cpath scaled 1u;
% clip and save current picture
picture pic;
clip currentpicture to cpath;
pic:=currentpicture;
currentpicture:=nullpicture;
% erase currentpicture
currentpicture:=nullpicture;
% draw grid
for k=-5u step 1u until 5u:
draw (-5u,k)--(5u,k) withcolor 0.85white;
draw (k,-5u)--(k,5u) withcolor 0.85white;
endfor;
% draw axes
drawarrow (-5u,0)--(5u,0);
drawarrow (0,-5u)--(0,5u);
% label axes
label.rt(btex $x$ etex, (5.2u,0));
label.top(btex $y$ etex, (0,5.2.u));
label.bot(btex $5$ etex, (5u,0));
label.lft(btex $5$ etex, (0,5u));
% redraw line
draw pic;
endfig;
end.