Viewer’s Left and Right Eye Orientation to a Stereoscopic Image Pair

The position of an object in front of or behind the screen is determined by how the viewer’s eyes see the image pair.

Your eyes receive an object’s image pair differently depending on where the object should appear in depth.

Objects appearing on the screen plane (green actor) don’t seem to have an image pair but they do. The pair of images is superimposed over each other. Objects that appear on the screen plane have zero parallax separation (zero parallax setting orZPS).

When an object appears behind the screen plane (blue actor) the image pair is correctly oriented to your eyes. Your left eye looks to the left image and your right eye looks to the right image. Objects that appear behind the screen plane have positive parallax.

When an object appears in front of the screen plane (red actor) the image pair is flipped. The left eye image is offset to the right and the right eye image is offset to the left. Objects that appear in front of the screen plane have negative parallax.

1. Objects appearing in front of the screen require the viewer’s eyes to converge and cross. We converge and cross our eyes when we look at close objects in real life, too. You are converging when you read this book. The difference is that in a 3D movie you are converging in front of the screen even though the image of the object is on the screen surface. Notice that the image pair is flipped. The viewer’s left eye sees the screen right image and the viewer’s right eye sees the screen left image.
2. Objects that appear on the screen plane require the viewer to converge or cross at a point on the screen surface.
3. Objects that appear behind the screen require the viewer to converge at a point beyond the screen. The image pair is no longer flipped. Now, the viewer’s right eye sees the right image and the viewer’s left eye sees the left image.
4. Objects appearing at infinity require the viewer’s gaze to remain parallel. This is similar to what the eyes do in real life when we look at distant objects.
5. Objects with a parallax separation greater than 2.5 inches of measured screen distance will appear farther away than infinity. This causes the eyes to diverge.