1. A method for the reduction of artifacts based on an unequal representation of the same material classes in various locations in a three-dimensional image data set, reconstructed from two-dimensional x-ray projection images in which an image datum describing an attenuation value is allocated respectively to a voxel, the method comprising: locating at least two material class regions in a post-processing step, which receive image data which is homogeneously distributed and lies in an expected material class interval of the attenuation values; considering a characteristic of the material class regions and determining a smooth homogenization function, which is to be applied to the image data of the entire image data set; and applying the smooth homogenization function to the image data of the image data set.