(a) a vector giving the frequency components (number of cycles per unit length, kx and ky ) of a general 2D sine wave in x- and y-direction; or (b) the length of a spatial frequency vector. In the 2D Fourier transform plane, the spatial frequency vector gives the location of any point. Points that have the same [absolute] spatial frequency lie on a circle with radius sqrt(kx**2 + ky**2). For representation of 3D objects or images, the spatial frequency vector is three-dimensional, with components (kx , ky , kz), corresponding to 3D sine waves, and points with the same [absolute] spatial frequency lie on the surface of a sphere with radius sqrt(kx**2 + ky**2 + kz**2).