Stereoscopy - Stereo Window

Stereo Window

For any branch of stereoscopy the concept of the stereo window is important. If a scene is viewed through a window the entire scene would normally be behind the window, if the scene is distant, it would be some distance behind the window, if it is nearby, it would appear to be just beyond the window. An object smaller than the window itself could even go through the window and appear partially or completely in front of it. The same applies to a part of a larger object that is smaller than the window.

The goal of setting the stereo window is to duplicate this effect.

To truly understand the concept of window adjustment it is necessary to understand where the stereo window itself is. In the case of projected stereo, including "3D" movies, the window would be the surface of the screen. With printed material the window is at the surface of the paper. When stereo images are seen by looking into a viewer the window is at the position of the frame. In the case of Virtual Reality the window seems to disappear as the scene becomes truly immersive.

In the case of paired images, moving the images further apart will move the entire scene back, moving the images closer together will move the scene forward. Note that this does not affect the relative positions of objects within the scene, just their position relative to the window. Similar principles apply to anaglyph images and other stereoscopy techniques.

There are several considerations in deciding where to place the scene relative to the window.

First, in the case of an actual physical window, the left eye will see less of the left side of the scene and the right eye will see less of the right side of the scene, because the view is partly blocked by the window frame. This principle is known as "less to the left on the left" or 3L, and is often used as a guide when adjusting the stereo window where all objects are to appear behind the window. When the images are moved further apart, the outer edges are cropped by the same amount, thus duplicating the effect of a window frame.

Another consideration involves deciding where individual objects are placed relative to the window. It would be normal for the frame of an actual window to partly overlap or "cut off" an object that is behind the window. Thus an object behind the stereo window might be partly cut off by the frame or side of the stereo window. So the stereo window is often adjusted to place objects cut off by window behind the window. If an object, or part of an object, is not cut off by the window then it could be placed in front of it and the stereo window may be adjusted with this in mind. This effect is how swords, bugs, flashlights, etc. often seem to "come off the screen" in 3D movies.

If an object which is cut off by the window is placed in front of it, an effect results that is somewhat unnatural and is usually considered undesirable, this is often called a "window violation". This can best be understood by returning to the analogy of an actual physical window. An object in front of the window would not be cut off by the window frame but would, rather, continue to the right and/or left of it. This can't be duplicated in stereography techniques other than Virtual Reality so the stereo window will normally be adjusted to avoid window violations. There are, however, circumstances where they could be considered permissible.

A third consideration is viewing comfort. If the window is adjusted too far back the right and left images of distant parts of the scene may be more than 2.5" apart, requiring that the viewers eyes diverge in order to fuse them. This results in image doubling and/or viewer discomfort. In such cases a compromise is necessary between viewing comfort and the avoidance of window violations.

In stereo photography window adjustments is accomplished by shifting/cropping the images, in other form of stereoscopy such as drawings and computer generated images the window is built into the design of the images as they are generated. It is by design that in CGI movies certain images are behind the screen whereas others are in front of it.