This paper describes a software-based method for interactive transfer function modification. Our approach exploits the fact that, in general, a user will rarely want to modify the viewpoint and the transfer functions at the same time. In that spirit, we optimize the latter by first fixing the viewpoint and then storing a list of pre-shaded, but uncolored, samples along each ray. Then, each time the RGBA transfer function is modified, the algorithm traverses the sample lists, colors the samples, and composites them along each ray until full opacity is reached. Since neither the expensive sample interpolation nor the shading are no longer necessary, we can obtain near-interactive framerates for a variety of datasets.