Fusion is a method to enhance stereoscopic effects. For the walking, driving and flight simulation this distance is typically kept constant, but for the trackball and terrain manipulators in which the user is zooming in at out and needing god's eye and ant's eye views, you can use the distance between the eye point and point of rotation as the fusion distance. This keeps the point of interest glued to the display when you have an external display surface. But when the goal is to produce an accurate stereoscopic rendering this enhancement is not normally desired as it breaks accuracy.

In geometrical terms, fusion distance is the distance from the eyes where the lines of sight for each eye converge. At this distance objects in the scene will appear to be on the surface of the display ("in the glass''). Objects farther than the fusion distance from the viewer will appear to be "behind the glass'' while objects in front will appear to float in front of the display. The latter illusion is harder to maintain, since real objects visible to the viewer beyond the edge of the display tend to destroy the illusion.

In Vizard, you can either specify fusion in distance units (e.g., distance from viewer to the display surface in units corresponding to the simulated model), or a dimensionless proportion. If you select to use fusion in distance mode, the ratio of fusion distance to screen distance is used as a scale factor to multiply the inter-pupillary distance. If fusion distance is used in proportional mode, you specify the ratio directly without worrying about what the screen distance is.

The default fusion distance value,mode is (1.0,viz.PROPORTIONAL).