1. An apparatus for altering images, the apparatus comprising: an input unit operable to receive a left image and a right image; and an image altering unit which is operable to alter the received left image and the received right image and to output an output image signal including the altered left image and the altered right image, wherein the left image is altered by changing a first portion, which is near a boundary area in the left image, and wherein the right image is altered by changing a second portion, which is near a boundary area in the right image.