Image data processing method and apparatus

A medical image processing apparatus comprises a buffer; and processing circuitry configured to: obtain a volumetric image data set; determine, from the volumetric image data set, a plurality of intervals along a path through the volumetric image data set, each interval having a respective depth from a reference position of the path; for each of the plurality of intervals, determine respective parameter values of a respective continuous function representative of a transparency of the interval; store the parameter values for the continuous functions to the buffer; and generate a rendered image using the stored parameter values for the continuous functions.