class Magick::RVG::Rect

def round(rx, ry = nil)

are the x- and y-axis radii. If y is omitted it defaults to x.
Specify optional rounded corners for a rectangle. The arguments 
def round(rx, ry = nil)
  rx, ry = Magick::RVG.convert_to_float(rx, ry || rx)
  raise ArgumentError, "rx, ry must be >= 0 (#{rx}, #{ry} given)" if rx < 0 || ry < 0
  @args << rx << ry
  @primitive = :roundrectangle
  self
end