A method for rendering a volumetric image includes providing a digitized volumetric image comprising a plurality of intensities corresponding to a domain of points in a 3-dimensional space, casting a ray through said volumetric image for a first pass to determine a threshold intensity for said ray, casting said ray through said volumetric image for a second pass, and sampling only those points whose intensity value is selected by said threshold intensity to determine a rendering value for said ray, wherein said rendering value is associated with a pixel from which said ray was cast.