1. A method executed by a processor for adjusting a rendering sampling rate of an image, comprising: providing an initial sampling rate; determining a regional variation value representing regional data, color, and alpha variation; determining an accumulated opacity value representing opacity accumulated over a ray in the image; determining a regional opacity value representing regional opacity; accessing a quality setting representing a default sampling rate obtained from a memory; and dynamically adjusting the rendering sampling rate of the image, by adjusting a rate of ray sampling for each voxel, as a function of the regional variation value, the accumulated opacity value, the regional opacity value, and the quality setting, thereby maintaining a predetermined image quality level.