A system and method of determining a sampling rate for rendering a volumetric image, the method includes determining a regional variation value representing regional data, color, and/or alpha variation. An accumulated opacity value is determined that represents opacity accumulated over a ray in the image, and a regional opacity value is determined that represents regional opacity. A quality setting is accessed that represents a default image quality, and the sampling rate is adjusted as a function of the regional variation value, the accumulated opacity value, the regional opacity value, and the quality setting. The regional variation value may be determined by iteratively generating a maximum regional variation table during pre-processing, quantizing the maximum regional variation table where the transfer function is defined based on a histogram of the volumetric image, and computing the regional variation value using the maximum regional variation table. Also disclosed are methods to calculate the accumulated opacity value, and the regional opacity value, as well as systems and methods of rendering a volumetric image by using the sampling rate.