What is Binocular Overlap and why should you care?

Binocular Overlap

Binocular overlap refers to the visible overlapping portion between the two eyes of a stereoscopic vision system. In other words, it describes how much of the viewed scene can be seen by both eyes as opposed to by just one of the eyes.

Binocular overlap.
Source: David Johnson, University of Utah

The visual field varies from person to person, but typically extends 60 inward (toward the nose) and 100 degrees outwards, and typically 60 degrees above and 75 below the horizontal meridian. As such, the binocular overlap region is 120 degrees horizontally (-60 to +60 degrees for each eye). Since each eye can see about 160 degrees, the binocular overlap is 120/160 = 75%.

Binocular overlap is particularly important for depth perception. When the brain sees an object with both eyes, the relative angles in which this object is visible gives an estimate of how far this object is located. If the object is far, far away, the angle in which it is seen by both eyes is practically the same. If the object is very close, the angles are much different.

Binocular overlap in goggles

Manufacturers of goggles and HMDs have a decision to make with regards to how much binocular overlap to incorporate in their products. Let’s examine a standard professional HMD with eyepieces that each have 60 degree diagonal field of view and a 4:3 aspect ratio. Using the “diagonal field of view and aspect ratio conversion table“, 60 degrees diagonal translate into 48 degrees horizontal and 36 degrees vertical. As you can see in the drawing below, there is a left eye and a right eye:
Independent images of the left and right eyepieces, each at 60 degree diagonal field of view
Note: the fill colors in the left and right eyes are just to separate them graphically and are not intended to convey anything else. If the eyepieces were installed to have 100% overlap between them, the binocular (combined) field of view would also be 48 degrees horizontal and 36 degrees vertical, resulting in a binocular field of view of 60 degrees diagonal. This is shown below. If you are looking at an object that is far away (and thus the distance between the eyes is not relevant), everything that can be seen in the left can also be seen in the right.
100% overlap, 60 degree binocular  field of view

Now let’s assume that the manufacturer decided to install the eyepieces with 75% horizontal overlap, meaning not with full overlap between the eyepieces but with partial overlap. The result is shown below. 75% overlap means that the overlapping region between the eyepieces is 75% of 48 degrees, so 36 degrees. Thus, the binocular horizontal field of view is 60 degrees: 12 degrees that are just shown in the left eyepiece, 36 overlapping degrees and 12 degrees shown in the right eyepiece. Thus, the diagonal field of view of the combined image is sqrt(60*60 + 36*36) = 70 degrees, larger than what we get with the 100% overlap:

Partial (75%) overlap, 70 degree total diagonal field of view made of individual 60 degree eyepieces

Advantages of partial overlap

  • Wider field of view, and thus greater immersion. In the example above, we created an HMD with 70-degree field of view using 60-degree eyepieces.
  • Improved aspect ratio. The aspect ratio of the eyepieces in our example was 4:3 = 1.333. The aspect ratio over using 75% overlap became 60:36, or 5:3 = 1.666. This aspect ratio is between the 16:9 HD1080 (1920×1080) standard and the 16:10 WUXGA (1920×1200) standard, making the result more suitable for viewing wide screen content, assuming the wide screen content is correctly divided between the two screens (more on that below).

Disadvantages of partial overlap

  • Binocular rivalry. Consider the a green circle that is shown in the eyepieces. Because of the location of the circle, it will be fully shown in the left eyepiece but cut off in the right eyepiece. In fact, when a person looks through both eyepieces at the same time, that person might notice the leftmost border of the right eye and this might look unusual or distracting. The image in the binocular view continues more to the left, but the right eyepiece no longer shows the object. Some people may find this distracting because of binocular rivalry. Instead of seeing a summation of the two images, our perception switches from one image to the other. If the field of view is larger than in our example, say 100 degrees in each eye, this is less of a problem because the discontinuity of the image is outside the central vision area.
Binocular rivalry caused by a partial-overlapping visual system
  • Compatibility challenges with non-3D content. One of the nice things in a fully-overlapped system is that you can view standard content – a computer desktop, Microsoft Word, a YouTube video or live video from a Web cam – without much effort. The same content is presented on both eyes. The application does not need to know that is is viewed in a goggle as opposed to a computer monitor In the case of partial-overlap, that is not the case. If the same exact image is presented in both eyes, eye strain will result because the eyes will try to merge the two images even though they are shown in different angles. In most cases, applications need to be aware that they are being viewed in a partial-overlap system. The exception is hardware that automatically splits a wide screen image into two left and right images (see the description of the zSight electronics below), but that is often not available.

What’s the exact math?

Check out this post for the exact math and useful reference tables.

Can you have it both ways (a bit promotional)?

What if you don’t want to make the trade-off between between immersion/field of view and compatibility/binocular rivalry? Can you have it both ways?
The zSight HMD has an innovative mechanism (which is patent pending) that allows users to switch between full and partial overlap. A lever that is located between the eyepieces can be moved from left to right. In the left position, the eyepieces are arranged for full overlap. In the right position, the eyepieces are tilted so that they are partially-overlapped. Moreover, the zSight electronics identifies when it is presented with a wide screen video signal and automatically creates two different images, one for each eye.
Left: zSight in full overlap mode; Right: zSight in partial-overlap mode

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