Apparatus for, and method of, rendering image data

Image rendering apparatus for rendering an image from a set of image data representing a volume, the apparatus comprises 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) determining values of color/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 pad of the sampling process, wherein the rendering unit is further configured to process the determined values of opacity, color/grayscale and/or gradient to determine a pixel value for a pixel of the image.