A method of constructing an object data set of data elements representing a physical property is described, in which the construction of the object data set includes assigning attributes to respective data elements. Such attributes are used, for example, in volume rendering. The construction of the object data set is achieved in such a way that the relative contributions of the physical property to the values of the respective data elements are derived on the basis of the relative contributions of the physical property to the data values of the data elements. These contributions are calculated by modeling transition changes in the image using a plot of grey value and gradient magnitude. Sampling of data points along the maximum gradient allows identification of the ranges of the physical property contributing to the selected voxel and calculation of the relative amounts.