Detecting an object in an image

A system for detecting an object comprises a transmitter/emitter module adapted to remotely alter the appearance of an object, an imaging device arranged to capture image data while the appearance of the object is altered, and a processing module arranged to select a region within the captured image data and compare a variation in pixel values in the region to an expected variation based on information about the altered appearance of the object, to determine whether the object is located in the region. In some embodiments, the object may include a liquid crystal LC shutter switchable between states having different optical properties, such as a transparent state and an opaque state, and the transmitter/emitter may be adapted to send a signal to the object to switch the LC shutter. In other embodiments, the object may include a polarising filter, and the transmitter/emitter may be adapted to illuminate the object with polarised light to alter the luminance of the polarising filter in a captured image. The object may be a pair of glasses wearable by a user, for example active or passive glasses for viewing visual data reproduced in a 3 D reproduction mode. The system may execute a facial-recognition algorithm to determine whether the user is wearing the glasses, and switch a display into the 3 D mode if the user is wearing the glasses.