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VR Beginner's Guide - Part I - The VR Glossary


It is the year of VR

In addition to being remembered as the year of the great pandemic, universal quarantine and social distancing, 2020 will probably also be remembered as the year of VR. In fact, this is probably the best time to enter the world of virtual reality thanks to the arrival on the market of exceptional quality viewers at affordable prices and decidedly interesting titles such as Asgard Wrath. The release of Half Life Alyx it then represents, after 16 years of waiting, the year 0 of virtual reality. The moment in which the VR experience surpasses that of traditional productions in general quality, immersiveness, depth of gameplay, giving us a title that will mark the history of this new way of approaching the videogame medium and beyond.



However, entering this world still fragmented among many platforms can be unsettling for a neophyte, and that is why we will try in this series of guides to offer you a general overview of the VR world, accompanying you in this experience that we are sure will upset you.

Step number 1: the Glossary of VR

First of all, you need to learn a series of terms that will come in handy later in choosing your viewer. Not all viewers, in fact, are born equal and in order to choose the one that best suits you, you must first read up on some rather new terms of this new technology, on the features and types of viewers available. In this Glossary of VR we will try to give you the basics to then be able to evaluate with greater awareness, the strengths and weaknesses of the individuals of the individual viewers.


Stand-alone and Tethered

A stand alone viewer is a device that does not need a connection to a PC or other device to function. Inside, in fact, there are processing components such as a CPU and a GPU, a memory and a battery and therefore the same can be used on the move. An example of viewers of this type are Oculus Go and Oculus Quest, both equipped with a Snapdragon cpu used in smartphones (Snapdragon 821 for Oculus Go and Snapdragon 835 for Oculus Quest)


A tethered viewer, on the other hand, is a viewer that must necessarily be connected to a PC or another device, such as a smartphone or console, to function. Basically, the viewer is equipped with a screen, lenses and other display technologies, but not a processing system. The connection can be made via cable, or via wireless devices. Examples of this type of viewers are the Valve Index, the HTC Vive Cosmos and the Oculus Rift S. Generally the quality of the experiences obtainable with tethered viewers is superior, given the greater processing power. On the other hand, however, tethered viewers requiring a cable connection (with exceptions such as Htc Vive which has a wireless module as an accessory) are more complicated to manage at home.

3DOF and 6DOF

The term 3DOF  stands for 3 degrees of freedom while on the contrary 6DOF stands for six degrees of freedom.

Controllers with 3 degrees of freedom (3DoF) are limited to rotational tracking. 3DoF controllers have no positional tracking, which means we can't reach or move our hand back and forth or up and down. Having a 3DoF-only controller is like having a hand and wrist without an arm. Examples of controllers of this type are controllers for Google Daydream, Samsung GearVR and Oculus GO


Controllers with 6 degrees of freedom (6DoF) have both rotational and positional tracking. Unlike controllers with 3DoF which are bound to orientation, controllers with 6DoF are able to move freely in 3D space. A 6DoF controller allows us to move with the controller forward, behind our shoulders, move our hands on our body or close to our face. To take the example above, with a 6DoF controller it is like having arms as well as hands. The 6DoF system also applies to additional headphones and trackers (eg feet, props). A 6DoF controller should be considered the minimum wage for an immersive VR experience. HTC Vive, the different Oculus Rift, Rift S, Oculus Quest, as well as of course Valve Index and Pimax, are all equipped with 6DoF controllers.


The same speech made for the controllers also applies to the viewer. A 6DoF viewer is able to recognize the subject even if it moves back and forth in space, therefore you can move in the virtual world as you move in real life, without prejudice to the limits of the traced area; with a 3DoF viewer it will only be possible to look around from a fixed point and nothing else.

Tracking inside out - inside in

By tracking we mean the tracking of a viewer in space. There are two tracking systems: Inside in, or external tracking, e I, or internal tracking. By external tracking, or inside in, we mean a system that works thanks to sensors external to the viewer (from one to 4 in the most common configurations to be fixed on the walls at the corners of the movement area) that measure the movements of the individual . These sensors are called differently, base stations, lighthouse, beamer. The standard is the one defined by SteamVR, so much so that today all the main VR headsets are part of the SteamVR 1.0 or 2.0 category of headsets.
The first consumer VR headsets, such as the original Oculus Rift, the original HTC VIVE and even the most recent Pimax and Valve Index, all use exclusively external (or inside in) tracking systems.


The biggest pro of external tracking is that it is the more accurate of the two systems, but it forces you to fix devices in the room and therefore the viewer is bound to the initially chosen area.

By tracking inside out instead we mean a tracking system present on the device and which generally works with a system of cameras placed directly on the viewer. The main advantage is the greater portability and versatility of this solution, even if this means less precision than inside systems. Most second-generation VR headsets now use inside-out tracking, such as the Oculus Quest, Oculus Rift S, and HTC VIVE Cosmos.


IPD or interpupillary distance

Another essential term for venturing into the world of Virtual reality is IPD, a value that indicates the distance between the center of the pupils of the two eyes. Obviously this value is not the same for everyone so it is essential to know your IPD because the quality of the VR experience depends on it. The more this corresponds to the distance between the center of the two lenses of the viewer, the higher the perceived visual quality will be. On the contrary, if the distance is not set correctly, the image can be blurry if not even able to generate what is called Motion Sickness.

IPD is measured in millimeters; you can measure it yourself using a metric ruler or you can download a phone application to do it.

Screen Door Effect

The "screen door effect" is so called because you have the impression of looking at an image from a mosquito net (screen door in English). This effect is caused by a not particularly resolution of some displays, which seen closely allow you to distinguish the pixels and the space between them that draws a real mesh, like a mosquito net). This is an effect that can be perceived to a greater extent by some individuals, and particularly when viewing light monochromatic backgrounds, which emphasize the inter-pixel space. Increasing the resolution of the panel is not the only possible solution to avoid the screen door effect, the management of the subpixels is also fundamental (see the next item).

sub-pixels

Each pixel of the screen actually has 3 sub pixels: one red, one green and one blue. Together, they are illuminated at different levels to create a certain color in combination with each other. However, just because sub-pixels are logically grouped within a specific pixel doesn't mean they can't be used in conjunction with the sub-pixels of adjacent pixels. Suppose we have a straight line of pixels. Inside each pixel we will have a red, then green, then blue line. This means that the blue of pixel n. 2 is next to the red of pixel n. 3. Then you can combine the blue of pixel number 2 with the red and green of pixel number 3 to create a single “pixel” that crosses the formal boundaries of the pixels. This softens the screendoor effect because it allows you to break the paradigm of a rectangular grid of pixels and instead eliminate straight, darker lines across the display. To give you a practical example, imagine placing another identical metal mesh on top of a metal mesh, but positioned slightly offset from the first, and looking through it. The holes will obviously look smaller.

Motion sickness

It is a sensation that involves, albeit less and less frequently, some subjects who are particularly sensitive to VR and that involves nausea, dizziness, disorientation, cold sweat. The most common cause of motion sickness is caused by a mismatch between the perceived motion in VR and the absence of body motion. The brain in fact expects that a movement perceived as real corresponds to a movement of the body and this generates confusion, causing this state. Modern viewers, if used in combination with software programmed in the right way, are able to avoid this inconvenience; It was also observed that the higher the display update frequency, the lower the impact of this effect on users. The new 70-80 and even 144hz viewers such as Valve's Index, should almost completely avoid this effect.

FOV

Acronym of Field of View, or Field of View, represents the extension of the image observable at a given moment and from a fixed point through a VR viewer. The wider and closer it is to the human field of view (about 210 °), the more realistic the perception of the scene will be. But the Fov, or visual field, also has an important function in the perception of the distance of objects. In fact, the brain has three ways to understand the distance of an object from an observation point: 1) knowing the original size from previous experience, it processes the current perceived size (a distant object is obviously smaller) and in this way it can understand a what distance it is. 2) The second method is through a calculation of the speed with which an object moves on our retina, a distant object moves more slowly than a nearby object (a car at a distance seems slower than a nearby car this way). 3) The third method by which the brain elaborates the concept of distance is given by the fact that our eyes are placed at a distance of about 60-64 mm from each other. This allows the brain to obtain two images from the two eyes at a different angle and then to capture a "3D model" so to speak. But this is true only if the object is close enough; in fact, if an object is sufficiently distant we will no longer have two angles of the object and therefore it will appear flat. So the Fov in addition to greater immersion, in a viewer is also a fundamental concept also to allow the brain to perceive distances.

Each manufacturer has different technology to improve the FOV. The most used techniques involve the use of lenses fresnel, lenses with concentric circles that allow to improve the visual field up to about 90 °, but have the defect of generating the so-called "God Rays”, Of the refractions that form as rays of light when looking at the outermost parts of the lens, especially in darker scenes. In any case, whichever lens is used, the image is almost always better when looking at the image in the center. This point, where the image is clearest and free of defects, is defined SweetSpot. The wider the sweet spot, that is the point from which it is possible to observe the image without distortion of any kind, the better the quality of the lens.

Mura effect - Clouding

The Mura effect, also called Clouding, is a term generally used to describe an altered image caused by imperfect lighting of the display. These effects can manifest themselves in single areas or single pixels that are darker or lighter than all others, show less contrast, or simply deviate from others in the adjacent area. As a rule, these problems of color inconsistency are particularly noticeable in the reproduction of dark images and in darker environmental conditions. In general, however, the wall effect is a problem related to the design of current LCD screens, but it can occur although more rarely even with OLED displays where it becomes even more evident than an LCD display. To be clearer, no display that works with backlighting or with the now almost completely obsolete side illumination - it is devoid of Mura effects, however a better quality of the LCD panel can noticeably reduce this effect.

Reprojection Rate or Asynchronous Reprojection
Asynchronous reprojection is a graphics technology that helps improve rendering performance in the VR environment especially when the system load is very high. This technology allows you to have moving images with constant framerates and without judder in those occasions when the frame drop is much more frequent, for example when the user rotates his head. This technique is mainly aimed at preventing the user from suffering from motion sickness (see above).

The operating mechanism in the official definition is as follows: "the system adjusts the position of the rendered frame just before it is seen by the user, to take into account the likely rotational movement of the user's head during the rendering of the frame". Asynchronous means that the technology will always use the most recent rendered frame, ensuring that even if the application loses a frame, the user-visible frame rate never decreases. In short, the technology anticipates the user's movements by reprojecting the already rendered frame a second time (and which therefore does not require new computational power): in this way the frame rate increases or remains constantly high.

Asynchronous reprojection has three main effects: 1) it ensures that the frame rate experienced by the user remains high, which is fundamental for comfort in VR; 2) reuse the frames already processed when the application is not able to process them due to a lack of system performance or because the scene is too complex; 3) ensures that the visual movement within the application is fluid and there is no judder (graphic artifacts that manifest themselves as a vibration of the image).

In the next article, we will get to the heart of the matter, with a guide to choosing the best VR headset for those who are about to enter this world for the first time.

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