1. A method of generating data for use in constructing a two-dimensional image rendered from a time sequence of volumetric data sets each representing a subject in three spatial dimensions at one of a succession of times within the time sequence, the method including: classifying samples of the volumetric data into one of a plurality of sample classes according to changes in the value of a respective sample within the time sequence; determining pixel values from samples in different respective samples classes according to a rendering of volumetric data within a respective samples class; selecting from amongst said pixel values one pixel value for use in constructing said two-dimensional image.