1. An apparatus for mapping a gaze point on a scene image to a corresponding point in a reference image, wherein said scene image and said reference image have been captured from different positions and orientations by a head-mounted camera which can change position and orientation due to movement by a subject, said apparatus comprising: a head-mounted camera for taking said scene image; an eye tracker that determines a direction of the subject's gaze and maps the gaze direction onto said scene image; processing means for: identifying a plurality of characteristic features and their locations in said reference image; re-identifying said plurality of characteristic features and their locations in said scene image; determining a point transfer mapping that transforms point positions between said scene image and said reference image based on the locations of said plurality of characteristic features detected in said reference image and said scene image; and using said point transfer mapping to map a gaze point which has been determined in said scene image to its corresponding point in said reference image.