1. An image rendering apparatus, comprising: processing circuitry configured to obtain volumetric image data representative of a three-dimensional region; and 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 comprises, for each of at least some of the sampled points on the sampling path, calculating a significance factor for the sampled point, wherein the significance factor for the sampled point is a sample contribution that is a function of both an opacity at the sampled point and of an accumulated opacity calculated from opacity at sampling points along the sampling path up to the sampling point, such that sampled points that make a higher contribution to the color or grayscale value have a higher significance factor than sampled points that make a lower contribution to the color or grayscale value; selecting, for the sampled point, one of a plurality of rendering calculation processes in dependence on the calculated significance factor, wherein the plurality of rendering calculation processes comprise a first rendering calculation process and a second rendering calculation process that each calculate a diffuse contribution differently; and performing the selected rendering calculation process to obtain at least one image data value for the sampled point, and for each sampling path, the sampling process comprises determining the color or grayscale value for the corresponding pixel by combining the determined image data values for each of the plurality of sampled points for the path.