Besides Hitachi, Sony is also working on similar technologies. Single-view displays project only one stereo pair at a time. Multi-view displays either use head tracking to change the view depending on the viewing angle, or simultaneous projection of multiple independent views of a scene for multiple viewers (automultiscopic). Such multiple views can be created on the fly using the 2D-plus-depth format. Various other display techniques have been described, such as holography, volumetric display, and the Pulfrich effect, which was used in "Doctor Who" Dimensions in Time, in 1993, by "3rd Rock From The Sun" in 1997, and by the Discovery Channel's "Shark Week" in 2000. Stereoscopy is the most widely accepted method for capturing and delivering 3D video. It involves capturing stereo pairs in a two-view setup, with cameras mounted side by side and separated by the same distance as is between a person's pupils. If we imagine projecting an object point in a scene along the line-of-sight for each eye, in turn; to a flat background screen, we may describe the location of this point mathematically using simple algebra. In rectangular coordinates with the screen lying in the Y–Z plane, with the Z axis upward and the Y axis to the right, with the viewer centered along the X axis; we find that the screen coordinates are simply the sum of two terms. One accounting for perspective and the other for binocular shift. Perspective modifies the Z and Y coordinates of the object point, by a factor of D/(D–x), while binocular shift contributes an additional term (to the Y coordinate only) of s·x/(2·(D–x)), where D is the distance from the selected system origin to the viewer (right between the eyes), s is the eye separation (about 7 centimeters), and x is the true x coordinate of the object point. The binocular shift is positive for the left-eye-view and negative for the right-eye-view. For very distant object points, the eyes will be looking along essentially the same line of sight. For very near objects, the eyes may become excessively "cross-eyed". However, for scenes in the greater portion of the field of view, a realistic image is readily achieved by superposition of the left and right images (using the polarization method or synchronized shutter-lens method) provided the viewer is not too near the screen and the left and right images are correctly positioned on the screen.