3D video is spreading not only as movies but also as television content or video for games along with the popularization of home-use televisions and computer displays which allow 3D display. 3D video provides viewers with illusion of depth by providing parallax between images presented to the right eyes and the left eyes. Such 3D video allows the viewers to perceive depth stereoscopically and to enjoy more realistic and powerful video. In particular, 3D video games allow players to enjoy a deeper experience. However, it is said that 3D video causes heavier visual fatigue (eye strain) than conventional two-dimensional video. One cause that has been discussed is that the visual line moves to a position in the depth direction virtually displayed in the 3D video. Although the distance from the angle formed by both eyes to the virtual position is perceived, focus of the eyes is adjusted to the screen position at which the video is presented, because the focus is adjusted to the video. As a result, the angle of the eyes contradicts the focus adjustment. Furthermore, it is believed that extreme movement in the depth direction or sudden depth changes due to scene changes in video causes excessive eye movements, resulting in causing fatigue that is different from fatigue caused by 2D video.
Hence, in order to avoid causing extreme fatigue after viewing 3D video, there is a method, as in Patent Literature 1, for measuring fatigue by presenting test video in the middle of viewing and measuring eye movements performed when the visual line follows the point of gaze that moves on the screen.
Furthermore, there is also a method, as in Patent Literature 2, for measuring fatigue of viewers using the information from the video that is being viewed.