MetaFun - Pragma ADE

pab := pab cutafter (pab intersectionpoint pb) ;
pickup pencircle scaled 1pt ;
draw pab withcolor .625yellow ;
anchor_box(\MPanchor{\MPvar{from}}) ;
fi ;
\stopMPpositiongraphic
The command \startMPpositiongraphic defines a graphic, in this example we have called it
mypos:circle.
The METAPOST macro initialize_box returns the characteristics of the box as identified by \MPpos.
After this call, the corners are available in llxy, lrxy, urxy and ulxy. The center is defined by cxy
and the path stored in pxy. When we are finished drawing the graphic, we can anchor the result
with anchor_box. This macro automatically handles positioning on specific layers.
The position macro \MPpos returns the current characteristics of a position. The previously defined
G positions return:
position page x y width height depth
G-1 149 182.84163pt 287.9047pt 31.97pt 7.26pt 2.83pt
G-2 149 110.1036pt 275.33267pt 21.15999pt 4.69pt 2.83pt
G-3 149 228.85555pt 275.33267pt 27.68001pt 7.26pt 0.2pt
G-4 149 368.61012pt 262.76065pt 16.46999pt 6.21pt 0.2pt
The numbers represent the real pagenumber p, the current position (x,y), and the dimensions of
the box (w,h,d) if known. These values are fed directly into METAPOST graphics but the individual
components can be asked for by \MPp, \MPx, \MPy, \MPw, \MPh and \MPd.
In the previous definition of the graphic, we saw another macro, \MPvar. When we invoke a
graphic or attach a graphic to a layer, we can pass variables. We can also set specific variables
in other ways, as we will see later.
\setMPpositiongraphic{G-1}{mypos:circle}
\setMPpositiongraphic{G-1}{mypos:line}{to=G-2}
In the second definition, we let the variable to point to another position. When needed, we can
ask for the value of to by \MPvar{to}. For reasons of convenience, the current position is assigned
automatically to from and self. This means that in the line we saw in the graphic:
initialize_box(\MPpos{\MPvar{self}}) ;
\MPvar{self} will return the current position, which, fed to \MPpos will return the list of positional
numbers. We already warned the reader: this is not an easy chapter.
5.3 More layers
Overlays are one of the nicer features of CONTEXT and even more nice things can be build on top of
them. Overlays are defined first and then assigned to framed boxes using the background variable.
You can stack overlays, which is why they are called as such. You can use the special overlay called
foreground to move the topmost (often text) layer down in the stack.
More layers Positional graphics
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