MetaFun - Pragma ADE

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pair uv ; pickup pencircle scaled 1mm ; autoarrows := true ;
draw fullcircle scaled 2cm withcolor .625red ;
for i=(10,35), (-40,-20), (85,-15) :
draw origin--i dashed evenly withcolor .625white ;
drawarrow origin--unitvector(i) scaled 1cm withcolor .625yellow ;
endfor ;
draw origin withcolor .625red ;
The circle has a radius of 1cm, and the three line segments are drawn from the origin in the direction
of the points that are passed as arguments. Because the vector has length of 1, we scale it to
the radius to let it touch the circle. By setting autoarrows we make sure that the arrowheads are
scaled proportionally to the linewidth of 1 mm.
An application of this macro is drawing the angle between two lines. In the METAPOST manual you
can find two macros for drawing angles: mark_angle and mark_rt_angle. You may want to take
a look at their definitions before we start developing our own alternatives.
The previous graphic demonstrates what we want to accomplish: a circular curve indicating the
angle between two straight lines. The lines and curve are drawn with the code:
pair a, b ; a := (2cm,-1cm) ; b := (3cm,1cm) ;
drawarrow origin--a ; drawarrow origin--b ;
drawarrow anglebetween(a,b) scaled 1cm withcolor .625red ;
where anglebetween is defined as:
def anglebetween (expr a, b) =
(unitvector(a){a rotated 90} .. unitvector(b))
enddef ;
Both unitvectors return just a point on the line positioned 1 unit (later scaled to 1cm) from the
origin. We connect these points by a curve that starts in the direction at the first point. If we omit
the a rotated 90 direction specifier, we get:
A few applications Marking angles