MetaFun - Pragma ADE

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fill (0,0)--(ww,0)--(ww,hh)--(w,hh)--(w,h)--(0,h)--cycle ;
fill (ww,0)--(w,0)--(w,hh)--cycle ;
The points that are used to construct the paths are defined using the constants w, h, ww and hh.
These are defined as follows:
w := 4cm ; h := 2cm ; ww := 1cm ; hh := 1.5cm ;
In this case we draw two shapes that leave part of the rectangle uncovered. If you have a background,
this technique allows the background to ‘show through' the graphic.
A not uncommon practice when making complicated graphics is to use unfill operations. Since
METAPOST provides one, let us see what happens if we apply this command.
fill (0,0)--(w,0)--(w,h)--(0,h)--cycle ;
unfill (ww,0)--(w,hh)--(ww,hh)--cycle ;
This does not always give the desired effect, because METAPOST's unfill is not really an unfill, but
a fill with color background. Since this color is white by default, we get what we just showed.
So, if we set background to black, using background := black, we get:
Of course, you can set the variable background to a different color, but this does not hide the fact
that METAPOST lacks a real unfill operation.
Welcome to MetaPost Drawing pictures