1. An image rendering apparatus for rendering an image from a set of image data representing a volume, the apparatus comprising a memory and processing circuitry, the memory configured to store a one-dimensional look-up table that includes predetermined values of colour and opacity, the processing circuit configured to perform a sampling process that comprises: determining values of opacity for a plurality of sample positions along a specified path through the volume based on values of the image data at the sample positions, wherein the sample positions are separated by sample intervals; determining values of colour/grayscale for the plurality of sample positions along the specified path, based on values of the image data at the sample positions; determining whether there is a transition to or from an opacity state at a transition point within a sample interval between the sample points, wherein the opacity state is higher than a predetermined value and wherein the determining of whether there is the transition to or from the opacity state comprises determining, for each of the sample positions, whether the sample interval includes an image data value which corresponds to the transition to said opacity state according to a transfer function to emphasize a characteristic of the rendered image, and if there is the transition to or from the opacity state modifying at least part of the sampling process for at least one of the sample positions such that at least one of the values of colour/grayscale or opacity is determined based on a value determined from the one-dimensional look-up table of the image data at the transition point rather than on a value of the image data at said at least one of the sample positions, wherein the determined values of opacity and colour/grayscale are processed to determine a pixel value fora pixel of the image, the determining of whether there is the transition to or from the opacity state comprises determining, for each of the sample positions, whether there is a transition to or from a complete opacity state within the sample interval of the transfer function pertaining at that sample position, and the memory is further configured to store at least one transition point marker that represents a value of image data value at which the opacity value according to the transfer function exceeds or falls below a threshold, the determining of whether there is the transition to or from the complete opacity state comprises determining, for each sample position, whether there is a transition point marker within the sample interval for the image data value pertaining at that sample position, and points of transition are identified in the one-dimensional look-up table, and presence of the transition point is determined from a comparison of stored transition point data and a range of intensity values that make up the sample interval.