This refers to the separation of the left and right images on the projection device or display screen. Positive Parallax puts an object behind the screen (on screen objects in the left eye image are to the left of the same objects in the right eye image). Negative parallax puts an object in front of the screen (on screen objects in the left eye image are to the right of the same objects in the right eye image).
Zero or neutral parallax puts an object on the screen (on screen objects in the left eye image are overlaid on the same objects in the right eye image).
The only difference between stereo cameras should be parallax or angle between the axes of the lenses, as in Camera Convergence – anything else can disturb the stereo viewing experience. This requires close attention, so that the cameras are set-up the same and with the same filters. Color differences, skewing, vertical misalignment, differential weave and hop, lens flares, poor VFX fixes, scratches and dirt can all cause problems.
Fast cuts between shots with strong positive and strong negative parallax can be unsettling in some circumstances. This is because the eyes and brain are being asked to jump uncomfortably quickly between positions and then make sense of the result. This can be mitigated by the use of ‘handing off’ – dynamically changing the convergence of an outgoing shot in relation to an incoming shot. Another method of dealing with this is trying wherever possible to cut between shots that are somewhat close in parallax.
Vertical parallax is a vertical offset between stereo images and is very uncomfortable to watch – so it is necessary to remove it during post production if there has been camera misalignment during shooting.
Note: The term ‘Parallax’ is sometimes used interchangeably with ‘Congruence’ or ‘Disparity’.