There is described a method for generating a 2-D image of a 3-D object represented by a volume data set comprising a multiplicity of data points each having an opacity value. A plurality of notional rays are cast through the 3-D object and for each ray, a ray path is divided into a plurality of base sampling intervals defined by data points on the path. If it is determined that a difference in opacity values across a base sampling interval can become greater than a pre-determined value, successively smaller sampling regions are generated within the base sampling interval until it is determined that a difference in opacity values across each generated smaller sampling interval in the base interval is less than the pre-determined threshold. Values indicative of an interaction between the ray and the 3-D object in the sampling intervals along the path are accumulated using a direct volume rendering procedure to determine a pixel value in the 2-D image.