1. An image rendering apparatus comprising:— an image data unit for obtaining volumetric image data representative of a three-dimensional region; a rendering unit configured to perform a rendering process on the volumetric image data that includes a sampling process that comprises, for each of a plurality of sampling paths, determining a respective color or grayscale value for a corresponding pixel based on a plurality of sampled points along the sampling path, wherein:— for each sampling path, the sampling process performed by the rendering unit comprises for each of at least some of the sampled points:—calculating a significance factor for the sampled point based on at least accumulated opacity along the sampling path for the sampled point; selecting for the sampled point one of a plurality of rendering calculation processes in dependence on the calculated significance factor; and performing the selected rendering calculation process to obtain at least one image data value for the sampled point; and wherein for each sampling path the rendering unit is configured to determine the color or grayscale value for the corresponding pixel based on the determined image data values for the plurality of sampled points for the path.