arc.cornerRadius()

arc.cornerRadius([radius])

If radius is specified, sets the corner radius to the specified function or number and returns this arc generator. If radius is not specified, returns the current corner radius accessor, which defaults to:

function cornerRadius() {
  return 0;
}

If the corner radius is greater than zero, the corners of the arc are rounded using circles of the given radius. For a circular sector, the two outer corners are rounded; for an annular sector, all four corners are rounded. The corner circles are shown in this diagram:

The corner radius may not be larger than (outerRadius - innerRadius) / 2. In addition, for arcs whose angular span is less than π, the corner radius may be reduced as two adjacent rounded corners intersect. This is occurs more often with the inner corners. See the arc corners animation for illustration.