Zero parallax plane

The plane at the convergence distance falls on the screen itself: the zero parallax plane (screen plane). The screen acts like a window opening onto the scene; this is called the stereo window.

Main subject and the screen plane

The eye always focuses on the screen, but vergence goes to the perceived point. If the main subject is near the screen plane, vergence and focus meet at the same distance, and the vergence–accommodation conflict is smallest. This is why the main subject is usually placed near zero parallax.

Window violation

If an object that appears in front of the screen touches the left or right edge of the frame, the frame cuts the object. Yet the object is in front of the frame. This contradiction is called a window violation, and it causes discomfort for the viewer.

The solution is a floating window: the edges of the image are masked with thin black strips, and the window appears to have moved in front of the object. At the left edge the left-eye image is masked, and at the right edge the right-eye image. The width of the strip equals the parallax of the violating object.

Changing it in post-production

The zero parallax plane can also be changed after the shoot. When the images are shifted horizontally (HIT), all parallax changes by the same amount. The scene moves forward or back, and the depth range stays the same. Whether different screens need separate masters is calculated from this shift.