An entertainment device comprises an input, a marker detector and a failure boundary calculation processor. The input is operable to receive a captured image from a video camera. The marker detector is operable to detect a fiduciary marker within the captured image, and is also operable to estimate a distance and angle of the fiduciary marker. The failure boundary calculation processor is operable to calculate at least one of an additional distance and an additional angle from the currently estimated distance and angle of the fiduciary marker at which recognition of the fiduciary marker is assumed to fail.