1. A method for a computer system, the method comprising: displaying a first image rendered using a three-dimensional (“3-D”) model of an object and an illumination texture map associated with the 3-D model, the illumination texture map having initial texel values that are based at least in part on an effect of a first light source on a surface of the 3-D model; receiving user input selecting illumination parameters for a second light source; rendering an updated image of the object using the illumination texture map, wherein rendering includes calculating an effect of the second light source on the surface of the 3-D model such that the updated image shows a cumulative effect of the second light source and the first light source; accumulating the effect of the second light source into the illumination texture map by updating one or more of the texel values based on the effect of the second light source; displaying the updated image; and storing the illumination texture map including the one or more updated texel values.