mfpic-doc.pdf.

4.12 FOR ADVANCED USERS. 66
The 〈value〉 must be a constant of the appropriate type or a METAFONT expression returning the
appropriate type. It can also be (or include) other variables previously defined. The \setmfcolor
command has been enhanced so that in recent METAPOST the 〈value〉 can be any of the three types
of colors METAPOST allows: numeric (for grayscale color), rgbcolor or cmykcolor. The data type of
〈value〉 will be examined, and the variable 〈name〉 will be declared to be a variable of the appropriate
type. The same is true of \setmfvariable{color}.
As an example of their use, since dimensions are numeric data types in METAFONT, the command
\setmfnumeric{my_spc}{5pt}
\setmfnumeric{my_dia}{.8pt}
would set the METAFONT variables my_spc and my_dia to the values 5pt and .8pt, respectively.
After that, these variables can be used in any drawing command where a dimension is required:
\plot[my_dia,my_spc]{Triangle}\rect{(0,0),(1,1)}
will plot the rectangle with small triangles of diameter .8pt, spaced 5pt apart.
The knowledgeable user may realize that path and picture are METAFONT data types, and
may want use them in \setmfvariable. It is also true that at some level, MFPIC figure macros
produce a path and \mfpimage produces a picture. However, MFPIC commands cannot be used in
the value portion of \setmfvariable. The TEX code that most MFPIC commands produce would
be meaningless to METAFONT. You can store the path produced by figure macros with \store, and
store pictures in variables with \mfpimage or even \tile.
With the obvious exception of the \globalsetmfvariable command, these commands define
the variable locally. That is, the variable will revert to any previous definition (or become undefined)
at the end of the mfpic environment it is defined in. It is in fact local to any METAFONT
group. In MFPIC, only \connect . . . \endconnect, \mfpimage . . . \endmfpimage, and \mfpic
. . . \endmfpic create METAFONT groups in the graph file.
A warning about variable names. METAFONT and METAPOST allow multi-part variable names
like ‘arrowhead length’ or ‘X.r’ The part after the first space or ‘.’ is called a suffix. In META-
FONT, variable settings are global unless explicitly made local. The code of the \set... commands
does make the variable setting local. However, METAFONT syntax forbids this localization when a
variable name has a suffix. Moreover, if you localize a variable, METAFONT will localize all variables
with that name plus any suffix. Even more, localizing a variable renders all variables with the
same name plus suffix locally undefined. The command \globalsetmfvariable simply omits the
localization part, so suffixes are permitted, but it cannot ‘globalize’ something that has previously
been localized within the same group.
For example, suppose you use the example code in subsection 4.4.3 and define a custom arrowhead
path myAH and the corresponding clearing path myAH.clear. Suppose now you try to make
this head the default for the \arrow command by doing the following.
\setmfvariable{path}{Arrowhead}{myAH}
Then this assignments is local and makes Arrowhead.clear undefined (locally). You cannot use
\setmfvariable to define Arrowhead.clear; that will produce an error from METAFONT. You
need to do
\setmfvariable{path}{Arrowhead}{myAH}
\globalsetmfvariable{path}{Arrowhead.clear}{myAH.clear}
and both assignments will be local. To make both assignments global, use the global version in both.