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 or each 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.