The invention relates to a method to and a computer program product for lighting a computer graphics image to be rendered. It is assumed that a plurality of light sources illuminate one or more objects of the scene to be rendered thus causing a complex lighting situation. For reducing the complexity of the lighting situation and saving computing time the lighting condition is pre-calculated by approximating at least two and preferably all light sources by a single fictitious light source in a pre-calculating step. In the step of pre-calculating, there is computed an approximated light direction for at least two selected ones of the light sources and an approximated light color for the selected light sources by weighting color contributions thereof. The step of pre-calculating is performed for selected areas of a surface of objects present in the scene and results in coefficients that are stored, for these selected areas, in a lightmap or in vertices representing the object surface. The coefficients are retrieved by subsequent method steps for rendering the computer graphics image using the approximated light condition. The invention applies not only to diffuse lighting but is, with a minor modification, also of use for calculating specular lighting.