Data processing apparatus comprising: a camera configured to capture successive images according to an image capture period, wherein each image among the successive images is captured during a respective image capture period such that respective different portions of the captured image are captured at respective different capture times within the image capture period; an optically detectable indicator located within the successive images, the optically detectable indicator having multiple optically detectable states and configured to transition between the multiple optically detectable states according to a transition pattern having interspersed periods in the multiple optically detectable states; the transition pattern including a reference period and two or more shorter second periods, wherein the two or more shorter second periods are shorter than the image capture period, the two or more second periods being separated by a separation period different to the image capture period, and wherein the reference period encompasses at least a whole image capture period; a processor configured to detect timing information from the successive images captured by the camera of the optically detectable indicator, wherein the processor is configured to detect the timing information in response to a position, within a given captured image among the successive images, of an image of the optically detectable indicator; and a detector configured to detect, from images captured by the camera, information indicating a relative location of a remote marker with respect to the camera, and to associate a timestamp with a detected relative location indicating a time at which the relative location was detected. a camera configured to capture successive images according to an image capture period, wherein each image among the successive images is captured during a respective image capture period such that respective different portions of the captured image are captured at respective different capture times within the image capture period; an optically detectable indicator located within the successive images, the optically detectable indicator having multiple optically detectable states and configured to transition between the multiple optically detectable states according to a transition pattern having interspersed periods in the multiple optically detectable states; the transition pattern including a reference period and two or more shorter second periods, wherein the two or more shorter second periods are shorter than the image capture period, the two or more second periods being separated by a separation period different to the image capture period, and wherein the reference period encompasses at least a whole image capture period; a processor configured to detect timing information from the successive images captured by the camera of the optically detectable indicator, wherein the processor is configured to detect the timing information in response to a position, within a given captured image among the successive images, of an image of the optically detectable indicator; and a detector configured to detect, from images captured by the camera, information indicating a relative location of a remote marker with respect to the camera, and to associate a timestamp with a detected relative location indicating a time at which the relative location was detected.