1. An image processing apparatus comprising: a volume data unit for obtaining a volumetric image data set; and a rendering process unit configured to: set a position of a light source; determine a plurality of sample points with respect to the volumetric image data set; for each sample point, determine at least one offset point, wherein the at least one offset point is positioned at a respective offset distance from the sample point; for each offset point, determine an irradiance arising from light virtually emitted from the light source; determine or refine an irradiance at each sample point based on the irradiance at its respective at least one offset point; and render, based on the determined or refined irradiances at the sample points, an image.