1. Image rendering apparatus for rendering an image from a set of image data representing a volume, the apparatus comprising: a rendering unit 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; at least one of a) and b): a) determining values of colour/grayscale for the plurality of sample positions along the specified path; b) determining gradient values for a plurality of gradient positions along the specified path; and determining whether there is a transition to or from a high opacity state, and if there is a transition to or from a high opacity state modifying at least part of the sampling process, wherein the rendering unit is further configured to process the determined values of opacity, colour/grayscale and/or gradient to determine a pixel value for a pixel of the image.