An apparatus for extracting image data of objects in an input image data, comprising: a display device having a display screen; a display controller arranged to control the display device; an imaging device operable to take an image of objects (63, 64, 65) placed before the display screen in order to generate input image data having a plurality of pixel values; a locating and tracking unit arranged to detect a respective position of each of the objects in the image, and to track the respective positions of the objects; a generating unit arranged to generate, on the basis of the tracked position and colour of each of the objects (63; 64; 65), data of a respective display image which has a colour and a pattern, and which is to be displayed in only a portion (63c; 64c; 65c) of the display screen that surrounds the object (63; 64; 65), wherein the colour of the display image remains set at a colour that is predetermined for the respective object and different from the colour of the respective object, the display controller being arranged to control the display device to display each of the display images generated by the generating unit; a controller arranged to control the imaging device to take an image of the objects (63, 64, 65) and the display screen when the display device displays the display images in cooperation with the display controller; and an extractor arranged to extract image data of the objects (63, 64, 65) by comparing the input image data generated by imaging device with data of the display images generated by the generating unit, the colour of each display image being predetermined for easy extraction of the respective object. a display device having a display screen; a display controller arranged to control the display device; an imaging device operable to take an image of objects (63, 64, 65) placed before the display screen in order to generate input image data having a plurality of pixel values; a locating and tracking unit arranged to detect a respective position of each of the objects in the image, and to track the respective positions of the objects; a generating unit arranged to generate, on the basis of the tracked position and colour of each of the objects (63; 64; 65), data of a respective display image which has a colour and a pattern, and which is to be displayed in only a portion (63c; 64c; 65c) of the display screen that surrounds the object (63; 64; 65), wherein the colour of the display image remains set at a colour that is predetermined for the respective object and different from the colour of the respective object, the display controller being arranged to control the display device to display each of the display images generated by the generating unit; a controller arranged to control the imaging device to take an image of the objects (63, 64, 65) and the display screen when the display device displays the display images in cooperation with the display controller; and an extractor arranged to extract image data of the objects (63, 64, 65) by comparing the input image data generated by imaging device with data of the display images generated by the generating unit, the colour of each display image being predetermined for easy extraction of the respective object.