A technique for generating DRR (digitally reconstructed radiography) images includes dividing sample points along a ray trajectory through a 3-dimensional dataset of attenuation values into stages. Each of the stages includes a plurality of the sample points. The attenuation values are summed along the ray trajectory for each of the sample points. Intermediate summation values are computed for each of the stages along the ray trajectory and for each of the stages the attenuation values summed within a given stage are summed with a previous stage intermediate summation value calculated for a stage previous to the given stage to generate a given stage intermediate summation value. A final accumulation value being representative of a total attenuation summed along the ray trajectory is calculated.