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What happens to the VR head that makes you dizzy?

Date: 2016-04-19
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 With virtual reality gradually rise, the domestic manufacturers are doing virtual reality also increased again. But I often hear those who have indicated to me that experience, he put on foreign manufacturers such as Oculus, Sony and Valve's VR head was when the experience is very good, but the head was put on domestic VR, when fixed okay a move will make extreme vertigo, why? according to general opinion, VR head was simply worn on the head of the display, What is motion sickness?


In fact people for VR largely vertigo, motion sickness and seasickness lacks distinction: Human ear vestibular system felt motion and visual systems are inconsistent, in the brain will "fight" - people will feel dizzy honking. Overall, motion sickness divided into three types.

You see the move, the feeling did not move: for instance, to play FPS games is the case.

You feel moved, but did not move to see, for instance, motion sickness sickness is the case.

Movement and sensation to see the movement of a mismatch: the astronauts do centrifuge training time will encounter this situation.

For VR, the situation may be more complicated. All the user's field of vision are covered VR head was, VR head was already trying to cheat you to another world, then the visual system felt something more than just a piece of the picture on the View screen, but the entire field of view in this case, the VR resulting from motion sickness will have a variety of factors.

The first factor is of course not match your body movements and your vision of the observed movement.

For example, you sit VR roller coaster when visually in high-speed motion, but the vestibular system has said that you are not moving, then it will lead to dizziness. Logically, which caused dizziness playing FPS games is no different, everyone has different feelings.

The second very important reason, is no match head movement and visual observations of head movement.

Today we are saying this is the focus, but also can be part of the VR head significantly improved technically. Wherein it comprises two parts: the inter-frame delay and delay.

Inter-frame delay is well understood. Human head rotation variations and the corresponding field of view is extremely sensitive. If the user's head turned, while the opposite, vision rotational delay, only a very slight delay can feel it. How small it? Studies have shown that head movement and vision delay should not exceed 20ms, otherwise it will be very obvious.

Do not underestimate 20ms: actually speed the movement of the human head can easily be reached on the second Baidu. Suppose head rotation speed is 100 ° / sec (in fact, just turned around and we can achieve this speed), rotation 20ms delay means a delay in the View object is 2 degrees, the handle straight, thumbs up, thumbs width probably 2 degrees.

20ms delay time for VR head is significant in terms of a very big challenge. First, devices need to be sufficiently accurate way to measure the rotation of the head speed, angle and distance, which you can use inertial gyroscope (responsive but poor accuracy) or optical methods. The computer then the need for timely render screen display also shows the need for timely picture, all this needs to be done in less than 20ms. Accordingly, if the time for each frame is displayed on a distance of more than 20ms, the human eye will also feel the delay. Therefore, VR head was a screen refresh rate should be more than 50FPS, 60FPS for now is a baseline. To get all this in such a short period of time, in itself is a huge challenge.

It is not simply to increase the frame rate of 60FPS or more, desperate to do within 20ms delay on it? In fact, in addition to the inter-frame delay outside dizziness caused by VR also more complex causes. Now I use a few schematic presentation.


We can see this picture on the left is the real world, an object moving from left to right eye sees the situation: With the passage of time, the trajectory of the object is a line; and the right plan is any species display out: the image of an object at a point after each show for some time, skip to the next point; it is not a continuous motion.

But as soon as the person's head movement, the human eye will be displayed relative to the object relative motion, then in the eyes of the trajectory of the object becomes a different look.

Turn left when right in the head, the trajectory of the original object becomes a stationary right like this, it is no longer a point, but at the end of each frame, it jumps back to "should" in s position. However, the human eye's persistence of vision phenomenon will be retained on a frame and the image, so the image will cause streaking, causing dizziness.

If we still assume 60hz refresh rate, head rotation speed of 120 degrees / sec, then a rotation of the head frame of 2 degrees to DK2 resolution, a frame delay of 19 pixels, this time the head was displayed the images will be rather vague. The higher the resolution, the more serious the problem. In the theoretical limit of resolution of the human eye to calculate a frame delay will reach 600 pixels.

The problem how to solve it? Want to make more continuous image of an object moves, the best and easiest way is to increase the refresh rate and crude.

From 60 to 90, to 120, to 200 ...... might finally to 1000hz, when our visual system completely not tell the real or virtual. But obviously we can not increase the refresh rate to 1000hz, currently Oculus Rift CV1 and HTC Vive using 90hz refresh rate, while Sony Project Morpheus uses a 120hz refresh rate.

Another way is to reduce afterglow (Persistence).

Twilight (Persistence) is a concept in the CRT display period. CRT monitors are excited by electron beam emitting phosphors on the screen, so this is actually a CRT display only a short period of time of the pixels within each frame is light, and the rest of the pixel is dark, the diagram below.


You can see the liquid crystal display, each pixel frame is always light, so the liquid crystal display was called "full afterglow" (Full Persistence) display.


Intermediate This figure only half the time for lighting pixels; and the right of this picture is in the ideal case only a very short time the screen light, which is "zero afterglow" (Zero Persistence). Since the human eye's persistence of vision effect, the refresh rate is high is sufficient not notice the screen is only a very short time frame of each light. But in order to make up for the lack of brightness, luminous intensity of the pixels within each frame to be greatly enhanced.

Low afterglow display the VR head was the significance of that head movement trajectory when the trajectory of objects closer to the real physical world.


Then head movement to bring smear will be greatly reduced. Assume that the same rotation of the head 120 degrees / sec, was first refresh rate 60hz, a luminescent screen frame 2ms, resolution and viewing angle to DK2 meter, then in the arc of rotation of the head of the human eye 2ms observed visual delay only 2 pixels, vertigo just gone.

But we all know the basic principles of the LCD display: LCD flip by making selectively permeable to light. This means that the LCD display is difficult to use low afterglow.


(TN-LCD display basic principles)

Flip LCD response time is also the fastest 2-4ms, but also led to the principle backlight LCD can not do all black. In contrast traditional CRT monitors are natural low afterglow display.

Want to solve this problem, VR head was self-luminous display must be used, for example, OLED. Since each pixel is its self-luminous, so OLED screen can achieve low afterglow.

In fact, Oculus and Valve uses low afterglow AMOLED display, Sony will use its own OLED display. Currently domestic VR head was mostly traditional LCD screen, so it is very natural cause dizziness.

But the low afterglow is also not anything good. It will also have a side effect: As the image display time per frame is very short, so moving objects shows the trajectory is discontinuous, the visual system will be considered as different objects, and in the full light of the display case under visual systems have enough time to be defined for the same object. In this sense, it is a trade-off: while addressing the motion sickness and smear, low afterglow display object can cause head movement when flashing and jumping.

Well, I said so much, I think we all understand: virtual reality is not in front of the eyes put two screens plus a gyroscope able to get. In the hope that domestic manufacturers can make products sink in the heart, the basic virtual reality experience to do, we can be like smart phones from Apple, as Google to millet Huawei.

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