As described previously, a major issue with screen edges is when an object placed in front of the screen plane (has negative parallax) exits screen in one eye's image before the other.

In this instance the illusion of 3-D is broken as without both the left and right eye containing the same portion of the image there can be no 3-D effect.

If the object exits the screen quickly there probably isn't too much of a problem as the brain won't have time to register the issue, but if the object lingers or moves slowly out of frame it will cause a problem.

The only real way to overcome this issue is to either change the convergence point, such that all objects are behind the screen window, or manage it in post by 'zooming' into the image to remove the object from both the left and right eyes.

However, a more dynamic way to deal with the problem is to dynamically crop just the eye that has more of the object such that it matches the other eye.

The following image shows a person breaking frame to the right sides of the image; The chap in the black t-shirt. Close each eye in turn while looking through a set of Anaglyph glasses to see the differences.

Images curtesy of Pietro Carlomagno - www.carlomagno3d.it - and taken from the set of 'Inferno'.

This following image has a floating window used to crop the extra information within the right eye images such that the information contained within each is identical... You have to work with us on this as it's very hard to see the benefit of this in static images!

Images curtesy of Pietro Carlomagno - www.carlomagno3d.it - and taken from the set of 'Inferno'.

This approach has become known as 'floating windows', or more accurately 'floating crops', as the edges of the left and right eyes are cropped dynamically depending on the objects at the edge of screen.

Note that although the chap in centre frame is well in from of the screen plane there is no issue as the top and bottom crop points are identical. If the camera were panning vertically there would be an issue as this would no longer be the case...

This floating window concept can be a very interesting way to manage issues with objects leaving the screen unevenly and causing stereoscopic failure.

Interestingly, a likely reason objects leaving screen unevenly causes problems has a lot to do with the evolution of human vision, and how historically predators were often glimpsed in the periphery of vision, and so a disturbance to the left or right brings back natural survival instincts.

Probably explains why the same isn't true with upper or lower periphery vision - not so many predators there...

Next Page - Setting Convergence

Contact steve@lightillusion.com for more info.