1. A method for rendering inhomogeneous scattering medium, the method comprising: (a) receiving a RBF model density field which is expressed by a weighted sum of a set of radial basis functions (RBFs) each having a RBF center, the RBF model density field representing at least approximately a density field of an inhomogeneous scattering medium defined in a volume space; (b) computing a value of source radiance at each RBF center; (c) approximating values of the source radiance at other points in the volume space by interpolating from the values of the source radiance at the RBF centers; (d) computing an effective exitant radiance at point x in the volume space at least partially based on the source radiance; and (e) rendering an image of the inhomogeneous scattering medium based on the effective exitant radiance.