1. A computer-implemented method for creating artistic effects for volumetric illumination in a scene comprising participating media, wherein the method is performed by a computer system programmed to perform the method, comprising: receiving data representing the scene comprising the participating media; obtaining a plurality of photon beams for the scene; receiving a user specification of a target appearance of the scene, wherein the user specification describes illumination effects for the scene; receiving from a user an instantiation of a function to be used in calculating a radiance due to the participating media; generating, using one or more processors associated with the computer system, a plurality of modified photon beams representing volumetric illumination in the scene, wherein the generating incorporates one or more modifications of at least one of the plurality of photon beams based on the user specification; calculating the radiance due to the participating media using the plurality of modified photon beams and the instantiation of the function received from the user; computing, using one or more processors associated with the computer system, a shading of the scene using the calculated radiance; and generating a rendering, using the one or more processors associated with the computer system, of the scene.