A method of volume rendering comprising constructing an object data set, the object data set containing data elements being voxels, the data elements assigning data values being grey values to respective positions in a multi-dimensional geometrical space, the data values representing a physical property of an object being an attenuation of X-ray radiation caused by the relative densities of tissue within the object, wherein the construction of the object data set includes assigning attributes to respective data elements, the method being characterized in that it comprises: for each tissue transition of a plurality of tissue transitions, determining a respective model of how the grey values and gradient magnitudes vary when crossing an edge in the direction of the gradient, wherein the edge is a Gaussian smoothed step edge between two types of tissue each with a constant grey value, wherein this is done by sampling both the grey value and the gradient magnitude along the direction of the maximum gradient and plotting these to produce a graph of grey value - gradient magnitude relation, for each data element, deriving a tissue mixture represented by relative contributions of different types of tissue to the data values of the respective data elements, wherein by using the determined models any voxel of the object data set is classified as belonging to one of the tissue transitions, wherein for each voxel in the object data set a direction of the gradient is determined and grey values and gradient magnitudes are sampled along that direction, wherein it is determined which model fits best to the sampled grey values and gradient magnitudes, wherein this best fitting model defines on which type of transition the respective voxel is located, wherein the exact position of the respective voxel on the graph of the best fitting model identifies the respective relative contributions of the two tissues of the tissue transition and hence the respective tissue mixture; mapping the respective tissue mixture to color and opacity; assigning said color and opacity as attributes to said each data element. for each tissue transition of a plurality of tissue transitions, determining a respective model of how the grey values and gradient magnitudes vary when crossing an edge in the direction of the gradient, wherein the edge is a Gaussian smoothed step edge between two types of tissue each with a constant grey value, wherein this is done by sampling both the grey value and the gradient magnitude along the direction of the maximum gradient and plotting these to produce a graph of grey value - gradient magnitude relation, for each data element, deriving a tissue mixture represented by relative contributions of different types of tissue to the data values of the respective data elements, wherein by using the determined models any voxel of the object data set is classified as belonging to one of the tissue transitions, wherein for each voxel in the object data set a direction of the gradient is determined and grey values and gradient magnitudes are sampled along that direction, wherein it is determined which model fits best to the sampled grey values and gradient magnitudes, wherein this best fitting model defines on which type of transition the respective voxel is located, wherein the exact position of the respective voxel on the graph of the best fitting model identifies the respective relative contributions of the two tissues of the tissue transition and hence the respective tissue mixture; mapping the respective tissue mixture to color and opacity; assigning said color and opacity as attributes to said each data element. deriving a tissue mixture represented by relative contributions of different types of tissue to the data values of the respective data elements, wherein by using the determined models any voxel of the object data set is classified as belonging to one of the tissue transitions, wherein for each voxel in the object data set a direction of the gradient is determined and grey values and gradient magnitudes are sampled along that direction, wherein it is determined which model fits best to the sampled grey values and gradient magnitudes, wherein this best fitting model defines on which type of transition the respective voxel is located, wherein the exact position of the respective voxel on the graph of the best fitting model identifies the respective relative contributions of the two tissues of the tissue transition and hence the respective tissue mixture; mapping the respective tissue mixture to color and opacity; assigning said color and opacity as attributes to said each data element.