mfpic-doc.pdf.

4.2 COMMON OBJECTS. 21
\arc[〈format〉]{〈specification〉}
\arc*[〈format〉]{〈specification〉}
This figure macro produces a circular arc specified as determined by the 〈format〉 optional parameter.
As with \circle, the optional 〈format〉 parameter determines the format of the other parameter,
as indicated below. The user is responsible for ensuring that the parameter values make
geometric sense. The starting point of each arc is at the first specified angle or point and the ending
point is at the last one.
The star-form produces the complementary arc. That is, instead of the arc described below, it
produces the rest of the circle from the ending point to the starting point of the arc described.
\arc[s]{〈p 0 〉,〈p 1 〉,〈θ〉}
\arc[p]{〈c〉,〈θ 1 〉,〈θ 2 〉,〈r〉}
\arc[a]{〈c〉,〈r〉,〈θ 1 〉,〈θ 2 〉}
\arc[c]{〈c〉,〈p 1 〉,〈θ〉}
\arc[t]{〈p 0 〉,〈p 1 〉,〈p 2 〉}
\arc[r]{〈p 0 〉,〈p 1 〉,〈r〉}
\arc[q]{〈p 0 〉,〈p 1 〉,〈r〉}
The optional arguments produce arcs according to the following descriptions.
[s] The point-sweep form is the default format. It draws the circular arc starting from the point 〈p 0 〉,
ending at the point 〈p 1 〉, and covering an arc angle of 〈θ〉 degrees, measured anticlockwise
around the center of the circle. If, for example, the points 〈p 0 〉 and 〈p 1 〉 lie on a horizontal line
with 〈p 0 〉 to the left, and 〈θ〉 is between 0 and 360 (degrees), then the arc will sweep below
the horizontal line (in order for the arc to be anticlockwise). A negative value of 〈θ〉 gives a
clockwise arc from 〈p 0 〉 to 〈p 1 〉.
[p] The polar form draws the arc of a circle with center 〈c〉 starting at the angle 〈θ 1 〉 and ending
at the angle 〈θ 2 〉, with radius 〈r〉. Both angles are measured anticlockwise from the positive
x axis. If the first angle is less than the second, the arc has an anticlockwise sense, otherwise
clockwise. A negative radius is a mathematical error, but the result is the arc on the opposite
side of the circle, as if both angles were increased by 180 degrees
[a] The alternative polar form differs from the polar form above only in the order of the arguments.
This seems (to me) a more reasonable order, and matches the order \sector requires (see
below). The [p] option is retained for backward compatibility.
[c] The center-point-angle form draws the circular arc with center 〈c〉, starting at the point 〈p 1 〉,
and sweeping an angle of 〈θ〉 around the center from that point. This is the fundamental method
for drawing arcs. All other methods are converted to this or the point-sweep method. Even the
point sweep form is converted to this one for angles greater than 90 degrees.
[t] The three-point form draws the circular arc which passes through all three points given, in the
order given. Internally, this is converted to two applications of the point-sweep form.
[r] The point-radius form draws an arc on the circle that \circle[r] would produce. The arc
starts at the point 〈p 0 〉 and ends at 〈p 1 〉. Of the two possible arcs on that circle, it produces the
shorter one: the one with an angle θ less than 180 degrees measured anticlockwise around the
center of the circle. A negative radius is a mathematical error, but the result is the short arc on
the other circle with a clockwise sense.
[q] The alternative point-radius form is the same as [r] except it produces the longer arc: the one
with angle θ larger than 180 degrees measured anticlockwise around the center of the circle. A
negative radius is a mathematical error, but the result is the longer arc on the other circle with
a clockwise sense.