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Video Glasses & Mobile Digital Theater spec background

Video Glasses and/or Mobile Digital Theater, also called in literature Head Mounted Devices (HMD), will have the same performance parameters to take into account and which determines the quality and application of the device.
Some spec points will be explained here.
  • Field Of View (FOV) – Humans have around 180° FOV, but most HMDs offer considerably less than this. Typically, greater field of view results in greater sense of immersion and better situational awareness. Most people do not have a good feel for what a particular quoted FOV would look like (e.g. 25°) so often manufacturers will quote an "apparent screen size". For example, "Our new HMD presents an image that looks like a 72 inch screen at 10 feet." However, this tends to be quite misleading since people focus on the screen size rather than the distance and manufacturers will often increase the distance until the screen size sounds good. Most people sit about 60 cm away from their monitors and have quite a good feel about screen sizes at that distance. To convert the manufacturer's "apparent screen size" to a desktop monitor position, just divide the screen size by the distance in feet, then multiply by 2. So 72 inches at 10 feet becomes 14.4 inches. A 14 inch monitor on your desk sounds a lot less impressive than "72 inches at 10 feet" which is why they do not explain it that way.
  • Resolution – HMDs usually mention either the total number of pixels or the number of pixels per degree. Listing the total number of pixels (e.g. 1600×1200 pixels per eye) is borrowed from how the specifications of computer monitors are presented. However, the pixel density, usually specified in pixels per degree or in arcminutes per pixel, is also used to determine visual acuity. 60 pixels/° (1 arcmin/pixel) is usually referred to as "eye limiting resolution", above which increased resolution is not noticed by people with normal vision. HMDs typically offer 10 to 20 pixels/°, though advances in micro-displays help increase this number.

  • A slightly different view:

    The resolution is measured in terms of minutes of arc per pixel, which includes these two factors. We can now compare these HMDs with a normalized measure, which will give us a better idea of the overall visual quality perceived. Practically speaking, this means that with this measure unit, we can compare two HMD that use different FOV or raw displays resolution just by comparing their normalized resolution results.

    Let us first define what an arc minute per pixel is. One arc minute represent 1/60 of a degree. So the angle defined by the left and right extent of a picture element (pixel) define a value in arc minute per pixel. The figure 1a illustrate this definition.

    Figure 1

    For the human, the maximum angular resolution of the eye is around 1 arc minute for the smallest point that can be seen (it can be as little as 1/3 minutes of arc per pixel in some specific circumstances). This high visual acuity region is localized in the fovea on the retina, where there is the highest density of cones. These cones are mostly responsible for the central color vision.

  • Binocular overlap - measures the area that is common to both eyes. Binocular overlap is the basis for the sense of depth and stereo, allowing humans to sense which objects are near and which objects are far. Humans have a binocular overlap of about 100° (50° to the left of the nose and 50° to the right). The larger the binocular overlap offered by an HMD, the greater the sense of stereo. Overlap is sometimes specified in degrees (e.g. 74°) or in percent indicating how much of the visual field of each eye is common to the other eye. Well , "what is actually stereo overlapping?"

    Stereo overlapping is a process by which the individual field of views of each eye are voluntarily shifted horizontally so that the global HMD FOV is larger. What does this mean? The following figure illustrates the general principle:

    In classical HMD designs, particularly low-cost implementations, the stereo overlapis set at 100%. This means that both eyes see the entire field of view of both LCD displays. This configuration has the advantage of providing a stereoscopic scene over the entire FOV of the HMD device. The inconvenient of using such scheme is that the maximum usable FOV is the FOV of each single LCD display making the HMD. In other terms, if the HMD optics is such that each eye sees a 50 degrees wide scene, the total FOV seen by both eyes will also be 50 degrees wide.

    High-end HMD makes use of overlapping techniques exactly to overcome this. That is, what if we could get more FOV and still use the same displays? The answer to that question is to shift each display FOV horizontally so that only a partial region of each falls in the central region of the total HMD's FOV like seen on figure 5. This technique will effectively enable a wider total FOV. The inconvenient here is that we lose the ability to display stereoscopic information on some areas of that extended FOV. For the two side regions, as shown on figure 5, only the image of one of both displays reaches that area. For stereoscopy to be achieved, two images are required for each pixels element displayed in the total FOV region of the HMD. As previously mentioned, the human visual system only requires stereoscopic information in a limited central region of its total field of view. This is to say that the humans visual system is an overlapping stereoscopic visual system.

    It has been reported that a central stereopsis region of about 20 degrees is sufficient to provide a good sense of depth to the user, while providing other types of cues in the peripheral regions such as speed and movements. The high-end HMD models make uses of that principle to maximize the immersion effect in term of maximizing horizontal FOV while maintaining the perception of depths in the scene. This is not an ideal solution unfortunately. In most implementations using overlapping, the overlapregion is fixed at the center of the global HMD FOV. The problem arises when the user looks on the sides by turning his eyes inside the HMD. Since the stereoscopic central region does not move with eye movements, the users will perceive disturbing effects while watching in these situations. Fusion of images may be momentarily lost, thus causing a break in the immersion sensation. To limit this problem, a possible partial solution is to voluntarily set the overlapregion to a value greater than what the human vision system requires. This is basically providing a safety margin in case the user's gaze changes. Some very costly HMD devices uses active eye tracking systems to move the stereoscopic region along with the user's eye movements. The moving overlapping region is often called a "high-resolution inset".

  • Distant focus ('Collimation'). Optical techniques may be used to present the images at a distant focus, which seems to improve the realism of images that in the real world would be at a distance.
Inter-Pupillary Distance (IPD). This is the distance between the two eyes, measured at the pupils, and is important in designing Head-Mounted Displays. For the Video Glasses and Mobile Digital Theater products, Orangedwarf is offering, these are optimized for the design of their products.

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