MetaFun - Pragma ADE

These definitions of anglebetween are far from perfect. If we don't start in the origin, we get the
curve in the wrong place and when we swap both points, we get the wrong curve.
The solution for the displacement is given in the METAPOST manual and looks like this (we package
the macro a bit different):
def anglebetween (expr endofa, endofb, common, length) =
(unitvector (endofa-common){(endofa-common) rotated 90} ..
unitvector (endofb-common)) scaled length shifted common
enddef ;
As you can see, we compensate for the origin of both vectors. This macro is called with a few more
parameters. We need to pass the length, since we want to add the shift to the macro and the shift
takes place after the scaling.
pair a, b, c ; a := (2cm,-1cm) ; b := (3cm,1cm) ; c := (-1cm,.5cm) ;
drawarrow c--a ; drawarrow c--b ;
drawarrow anglebetween(a,b,c,1cm) withcolor .625red ;
That the results are indeed correct, is demonstrated by the output of following example:
However, when we swap the points, we get:
This means that instead of rotating over 90 degrees, we have to rotate over −90 or 270 degrees.
That way the arrow will also point in the other direction. There are undoubtedly more ways to
determine the direction, but the following method also demonstrates the use of turningnumber,
which reports the direction of a path. For this purpose we compose a dummy cyclic path.
vardef anglebetween (expr endofa, endofb, common, length) =
save tn ; tn := turningnumber(common--endofa--endofb--cycle) ;
show tn ;
Marking angles A few applications
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