1. A computer implemented method for generating a DRR (digitally reconstructed radiography) image, the method comprising: summing attenuation values along a ray trajectory through a 3-dimensional dataset of attenuation values to generate a final accumulation value representative of a total attenuation summed along the ray trajectory; and computing intermediate summation values in stages along the ray trajectory, wherein each of the stages includes a subset of the attenuation values and wherein 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.