MetaFun - Pragma ADE

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draw b withpen pencircle scaled 1mm withcolor .625yellow ;
draw point 0 of a withpen pencircle scaled 2mm ;
draw point 0 of b withpen pencircle scaled 2mm ;
The rotation 120 can be calculated from the relative starting points and time the journey will take.
Alternatively we can use the time along the path, but this would be a bit more fuzzy later on. 16
After a bit of playing with drawing paths between the two points, you decide to make a macro. We
want to feed the angle between the paths but also the connecting path. So, we have to pass a path,
but unfortunately we don't have direct access to the points. By splitting the argument definition
we can pass an expression first, and a wildcard argument next.
\startuseMPgraphic{gamble}
def Gamble (expr rot) (text track) =
path a ; a := fullcircle scaled 3cm ;
path b ; b := fullcircle scaled 2cm rotated rot ;
pair aa ; aa := point 0 of a ;
pair bb ; bb := point 0 of b ;
path ab ; ab := track ;
draw a withpen pencircle scaled 1mm withcolor .625red ;
draw b withpen pencircle scaled 1mm withcolor .625yellow ;
draw aa withpen pencircle scaled 2mm ;
draw bb withpen pencircle scaled 2mm ;
drawarrow ab withpen pencircle scaled 1mm withcolor .625white ;
setbounds currentpicture to boundingbox a enlarged 2mm ;
draw boundingbox currentpicture withpen pencircle scaled .25mm ;
enddef ;
\stopuseMPgraphic
Because at this distance nobody will bother us with the thickness of the pen and colors, we code
them the hard way. We create our own universe by setting a fixed boundingbox.
We leave the Earth in the most popular way, straight upwards and after a few cycles, we leave
it parallel to the surface. The path drawn reminds much of the trajectories shown in popular
magazines.
In case you wonder why METAPOST talks about the time on a path, you now have a cue.
Killing time A few applications
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