This way it is independent of the application's coordinate system, which makes it easier to position objects appropriately in the scene. To model viewer attention realistically, the fusion distance should be adjusted to match the object in the scene that the viewer is looking at. This requires knowing where the viewer is looking. If head and eye tracking equipment is being used in the application finding the center of interest is straightforward. A more indirect approach is to have the user consciously designate the object being viewed.
Clever psychology can sometimes substitute for eye tracking hardware.
If the animated scene is designed in such a way as to draw the viewer's attention in a predictable way, or if the scene is very sparse, intelligent guesses can be made as to the viewers center of interest. The view direction vector and the vector separating the left and right eye position are perpendicular to each other. The fusion distance is measured along the view direction. The position of the viewer can be defined to be at one of the eye points, or halfway between them. In either case, the left and right eye locations are positioned relative to it.