mfpic-doc.pdf.

4.4 MODIFYING THE FIGURES. 32
\partpath{〈frac1〉,〈frac2〉}. . .
\subpath{〈num1〉,〈num2〉}. . .
\trimpath{〈dim 1 〉,〈dim 2 〉}. . .
\trimpath{〈dim〉}. . .
These macros modify the following path by producing only a part of it. In \partpath the parameters
〈frac1〉 and 〈frac2〉 should be numbers between 0 and 1. The path produced travels the
same course as the path that follows, but starts at the point that is the fraction 〈frac1〉 of the original
length along it, and ends at the point 〈frac2〉 of its original length. If 〈frac1〉 is greater than 〈frac2〉,
the sense of the path is reversed. In \subpath, the two numbers should be between 0 and the number
of Bézier segments in the path. This is mainly for experienced METAFONTers and provides an
MFPIC interface to METAFONT’s ‘subpath’ operation.
The \trimpath macro takes two dimensions separated by commas and trims those lengths off
the initial and terminal ends of the following path. Alternatively, it takes one dimension and and
trims that length off of both ends. If any of 〈dim 1 〉, 〈dim 2 〉 or 〈dim〉 is missing, it is taken to be 0pt.
This works by finding the points of intersection between the path and circles around the endpoints
with the given dimensions as radii. If the path is shorter than either dimension, it will not intersect
either circle and nothing will be trimmed. Similar problems can occur, at one end or the other, if the
path is shorter than the sum of the dimensions.
\parallelpath{〈dist〉}...
This modifying macro takes the following path and returns a path that follows beside it, keeping
a fixed distance 〈dist〉 to the left. If 〈dist〉 is negative, it keeps to the right. Left or right is from the
point of view of a traveller following the given path from start to finish. The distance is a pure number
in graph coordinates. Note: this should be compared to the first optional argument of \doubledraw
(see subsection 4.5.1), which requires an absolute dimension like 2pt, even though it is implemented
using the internal code of \parallelpath.
The calculation of the parallel path is approximate and rather inefficient. It is likely to produce
inexplicable small loops where it tries to follow the inside of tight turns (radius less than 〈dist〉).
Actual corners, (which might be thought of as turns of radius 0) are usually detected and dealt with
in a reasonable manner. However, if the path is made up of segments of length 〈dist〉 or less, this is
unlikely to work correctly at all.
\arccomplement. . .
This macro, to work properly, must be followed by an arc of a circle. It produces the complementary
arc. That is, it produces the circular arc, which would, if appended to the following arc,
complete the circle. The complementary arc will have the same direction, clockwise or anticlockwise,
as the original. The arc that follows doesn’t have to be produced by \arc, as in the following
example:
\draw[blue]\arccomplement
\draw[red]\partpath{0,.333}
\circle{(0,0),1}
This will draw 1/3 of this circle in red and the rest of it in blue.
METAFONT cannot check if a path is really a circular arc. The METAFONT code, like that of
\makesector (see subsection 4.4.1), selects three key points on the arc, then it produces the rest
of the circle much the same way as the internal code of \arc[t] (the three point option for \arc).
Thus, it will produce some arc from the end of any following path to its beginning (or a straight
line if the three chosen points happen to lie in a straight line). However, the result needn’t bear any
significant relation to the original path.