Primer: The Principles Of 3D Video And Blu-ray 3D

Other Considerations

Brightness

Because they block alternate pixels, rows, or frames of video from each eye (depending on the type of 3D display you have chosen), less than half the light from a 3D display system reaches your eyes. To minimize crosstalk on frame sequential display systems, active shutter glasses block both eyes during the transition period between the display of each video frame. For all of these reasons, it is helpful to choose a 3D display with high brightness levels.

It is also important to avoid any reflections on the screen of your TV or display, as these reflections will be seen at a fixed depth (the distance from your eye to the display), making it a bit harder for your eyes to naturally focus on whatever you are interested in.

Due to both concerns, (brightness and reflections) you will find that 3D video is best viewed in a dark room.

Accommodation Disparity

Although objects may appear to be in front of or behind the display, they are not really there. Because the image is really coming from a flat screen, to see the 3D video clearly, the muscles in your eyes must keep your eye lens focused to the distance of the screen. The fact that the 3D video is really only in focus on a flat plane creates a disparity between one visual cue (accommodation) and the other visual cues. 

When your eyes try to focus on 3D objects that appear to be close to you, your eyes will naturally converge inward while trying to accommodate for viewing a nearby object. Unlike the real world, all objects in a 3D video will only be in focus on the display. If you try to focus on objects that appear to be right in front of your nose, you will be disappointed, as you instead lose focus.

Fortunately, it seems that most people are able to adjust to this disparity without much difficulty, letting them relax and enjoy a 3D video without losing focus.

Blur Disparity

In the real world, our eyes focus on the objects at which we are looking. Objects that are nearer or farther appear out of focus. Because a 3D video is presented on a flat screen, the blur gradient that we experience in the real world will not be seen in a 3D video. If the 3D video is shot with a wide depth of field, the majority of the scene will be in focus, allowing the audience to see any part of the scene clearly when they focus on the screen.

If the director or cinematographer chooses to use a narrow depth of field, scenes may be shot with the subject in focus and other areas out of focus. While this technique can approximate the blur gradient we experience in the real world, it has the drawback of causing objects that we would normally be able to focus on to be out of focus, and impossible to focus on.

Blur disparity is an unavoidable issue, regardless of how a 3D video is shot or rendered. Studies have suggested that blur disparity and accommodation disparity tend to provide a cue to the brain that although it is seeing a stereoscopic view of a three-dimensional scene (real or computer-generated), the actual 3D video presentation is on a flat screen.

Eye Strain

We naturally view our world in 3D, and so a good 3D production makes it easy to suspend the disbelief that we are not actually “on scene,” live and in-person. However, the viewer will naturally try to focus to different distances, depending on the apparent distance of subjects and scenery in the 3D video. When you not only scan your eyes from side to side, but focus in and out, your eye muscles get a bigger workout than you would get from watching a video in 2D. Once you are able to adjust to the brave new world of 3D video, you will find yourself relaxing and enjoying, instead of trying to actively focus on objects near and far. 

Motion Sickness

Motion sickness is normally caused by a disagreement in your brain between what you see and the motion that you feel (by your inner ear, which gives you your sense of balance). Motion sickness can also be caused by a disagreement within the visual system of your brain. If a 3D video is shot, displayed, or viewed poorly, the 3D depth perception of the objects in the scene may conflict with the 2D depth information that we perceive. These conflicts can cause the viewer to suffer similar symptoms to common motion sickness (fatigue, headache, dizziness, or in the worst case, nausea). 

3D producers know how to minimize the potential for problems by:

• keeping subjects in the 3D comfort zone, at roughly the distance of the convergence point of the camera (at least most of the time)
• avoiding focusing on objects that are extremely close to the camera (your eye will try to focus on the object as it if is close to you, when your eye needs to focus to the distance of the display)
• avoiding zooming in and out (which changes the scale of the 3D space)
• avoiding excessive camera motion (for example, flying through a jungle; the audience has suspended the disbelief that they are watching a movie, and now the subconscious part of their brains are more prone to be concerned when their eyes are telling them that they are flying through the jungle but the sense of balance from their ears is telling them that they are sitting still)
• keeping near subjects away from the edge of the frame (where the picture for one eye could leave the frame)
• being sure that all content is 3D (producers cannot use flat 2D backgrounds or effects in a 3D production)
• minimizing the use of a narrow depth of field (causing parts of the scene to be out of focus – causing problems for viewers who attempt to focus on these objects)

Fortunately, experienced 3D producers know how to avoid these problems. 

Consumers can minimize the potential for problems by:

• choosing a high-quality 3D display and 3D glasses solution (minimizing ghost images caused by crosstalk)
• minimizing reflections on their TV or display (reflections are 2D)
• viewing 3D content from the center of the direction that the display is facing (or from the center of a 3D theater – keeping the relative distance of all parts of the scene centered and in proportion)