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 performed by a computer processor and 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, wherein the image data set is divided into image slices along a division direction, and wherein an image slice homogenization function is determined for each image slice and the homogenization function for the entire image data set is derived from the image slice homogenization function.