This command computes, for each possible distance \(r\),
the integral of the pixel values lying inside the disc of radius
\(r\) centred at the origin.
If result="fv" (the default) the result is a function
object f of class "fv". For each value of radius \(r\),
the function value f(r) is the integral of X
over the disc of radius \(r\).
If result="im" the result is a pixel image, with the same
dimensions as X. At a given pixel, the result is
equal to f(r) where r is the distance from the given
pixel to the origin. That is, at any given pixel, the resulting value
is the integral of X over the disc
centred at the origin whose boundary passes through the given pixel.