Arcs require the following parameters, typically formatted as follows:
Radius<spc>StartAngle<spc>EndAngle<nl>
where Radius is a double defining the radius of the arc, StartAngle is a double containing the angle, in degrees, between the local X-axis and the start of the arc, EndAngle is a double containing the angle, in degrees, between the local X-axis and the end of the arc, <spc> is the space character and <nl> is the newline sequence.
Arcs are drawn, counter-clockwise from the start angle to the end angle, on the local X-Y plane. In wireframe mode, an arc is drawn as a fragment of a circular circumference; in solid mode, an arc is drawn as a wedge (acute angles) or like a cheese round with a wedge cut out (obtuse angles). In both cases, the arc's local origin is at centre of the circle of which it is a fragment.
Arcs exist as either coarse arcs or fine arcs; the former are drawn with fewer triangles (or arc-sections, if drawn as a wireframe) than the latter. With the introduction of VR Graphics (using the OpenInventor scene graph rendering engine), ACE creates all new arcs as fine arcs and converts coarse arcs to fine arcs during the read process.
There is one special case: an arc that has a start angle of 0 and an end angle of 360 is a Circle.