VR, which means “virtual reality”, is a computer simulation system that can create and experience virtual world. It uses the computer to generate a simulation environment, which is a multi-source information fusion, interactive three-dimensional dynamic scene and entity behavior system simulation, so that users immerse in the environment.
In the field of VR, the most well-known is VR glasses. VR glasses is the abbreviation of “virtual reality head mounted display device”, also known as VR head display.
Principle of VR glasses
The most important configuration of VR glasses is two lenses. The surface of the lens is designed with Plano convex (aspheric), biconvex and concave convex effects. The edge is thin and the center is thick. Convex lens can correct the light source angle of the lens, which leads to the re reading of the human eye, so as to increase the viewing angle, enlarge the picture and enhance the three-dimensional sense, so as to make people feel immersive.
Display technology is the core of VR glasses, including: interleaved display, picture exchange, parallax fusion.
1. Staggered display
The working principle of interleaving display is to divide a picture into two fields, that is, the odd scan line field formed by odd trace line or the even scan line field formed by single trace line and even trace line. When using the interleaved display mode for stereoscopic imaging, we can divide the left eye image and the right eye image into single image field and even image field (or in reverse order), which is called the stereoscopic interleaved mode.
If you use shutter stereoscopic glasses and interleave mode, you only need to use the field vertical synchronization signal as the shutter switching synchronization signal. That is to say, when a single field (i.e. left eye picture) is displayed, the stereoscopic glasses will cover one eye of the user, and when an even field is displayed, the other eye will be covered. This way, you can achieve the purpose of stereoscopic imaging.
Because interleaving mode is not suitable for long-time and close operation, as far as computer display peripheral technology is concerned, interleaving mode needs dual support of display hardware and driver. With the development of related display peripheral technology, non interleaved mode has completely replaced interleaved mode as standard equipment.
2. Picture exchange
The working principle of picture interaction is to display the left and right eye images on the screen interactively. Using stereo glasses and this kind of stereo display mode, we only need to use the vertical synchronization signal as the shutter switch synchronization signal to achieve the purpose of stereo imaging. With other stereoscopic imaging devices, the left and right eye images (pictures separated by vertical synchronization signals) can be distributed to the left and right eye display devices.
Image exchange provides full resolution image quality, so its visual effect is the best of the four stereo display modes.
3. Disparity fusion
The reason why people can see three-dimensional scenery is that both eyes can see things independently, and there is a gap between the left and right eyes, resulting in some subtle differences in the perspective of the two eyes, and such differences will make the scenery that the two eyes see a little bit of displacement. The difference between the images of the left eye and the right eye is called parallax. The human brain skillfully fuses the images of the two eyes to produce a stereo visual effect with a sense of space in the brain.
Because there is only one computer screen and people have two eyes, it is necessary to separate the images seen by the left and right eyes independently in order to have stereo vision. At this time, the parallax can be continuously displayed on the screen through 3D glasses. By controlling the IC to send stereo signals (left eye, right eye, left eye, right eye) to the screen and send synchronous signals to the 3D glasses at the same time, it can switch the left and right eye images synchronously. In other words, the left eye can see the scene that the left eye should see, and the right eye can see the scene that the right eye should see.
3D stereoscopic glasses is a penetrating liquid crystal lens. Through the circuit to control the on and off of the liquid crystal glasses, the on can control the glasses to be completely black, so as to cover the image of one eye; the off can control the glasses to be transparent, so that the other eye can see the image that the other eye should see. 3D glasses can imitate the real situation, so that the pictures of left and right eyes can be displayed alternately on the screen, and the real 3D images can be seen simultaneously with 3D glasses and the physiological characteristics of human vision.
VR glasses effect parameters
The two lenses of VR glasses can be regarded as two identical magnifying glasses, as shown in the light path section in the figure below. The human eye is in the focus of the left solid line and the optical axis, the middle one is the biconvex lens, and the right one is the screen. Because the lens has the function of bending light, the human eye can see the x position at the top of the screen, and it gives the feeling that it enters the human eye from a higher position along the dotted line, as if it were When the screen is enlarged to the size of X ‘, the immersion feeling emphasized by VR helmet depends on the size of the screen on the one hand and the ability of lens to bend light on the other. Therefore, the concept of FOV is introduced, which is also a parameter often publicized by VR manufacturers.
At present, the FOV Statistics announced by major manufacturers are as follows:
It can be seen that the FOV of most manufacturers is about 100 degrees, but some scholars believe that this parameter has no great practical value. First of all, unlike the size of memory or hard disk, there is no clear way to measure FOV, and users can’t be sure whether the manufacturer will make a false mark. Second, when people observe objects that are not in the center of vision, they tend to turn their heads rather than squint. It can be seen from a schematic diagram of the human eye perspective of Baidu that the maximum visual angle of the human eye level can even exceed 180 degrees, while the monocular comfortable visual angle is only about 60 degrees. In this position, the human eye vision is the most sensitive, and beyond this range, people will instinctively turn around. So you don’t have to worry too much about the size of the parameter, but should pay more attention to the clarity of the image in the 60 degree center area, as well as the feeling of actual use. Generally speaking, the larger the size of the lens, the less likely it is for the human eye to notice the edge of the lens, and the better the sense of immersion. Users can compare this parameter more intuitively. At present, the size of most VR helmet lenses is about 50 mm.
As the benchmark of VR, the clarity of DK1 lens edge image is often criticized by users in the early stage, which is due to the off-axis aberration in optical design. A little further away from the central region, the image quality will be greatly reduced. This is just like when we use a magnifying glass of ordinary quality, we will find that the edge image will become blurred because of the influence of the off-axis aberration. At present, the processing method of most companies is to make aspherical design on both front and back sides of the lens, as shown in the figure below, so as to reduce the off-axis aberration as much as possible and improve the image quality of the edge image.
At present, the molding process of plastic lens is very mature, which can produce plastic lens with very high optical quality. Therefore, DK2 and others adopt double-sided aspherical design, so users can pay attention to the lens of VR glasses. If aspherical design is adopted, the clarity will be much higher than that of aspherical. Of course, only excellent optical design is not enough, and manufacturing technology also has a high impact on the stability of lens quality. Therefore, if VR manufacturers can seek large OEM to process lenses for them, it is also one of the guarantees for product quality.
3. Color difference
Nowadays, when most VR glasses are used, red, green and blue color edges will appear in the edge area, which is called dispersion phenomenon. It is easy to appear when using high refractive index materials, just like white light will be divided into colorful light after passing through the prism. In DK1, this phenomenon is particularly obvious. From the perspective of optical design, two or more materials are needed to eliminate the dispersion. Therefore, in principle, a single lens (one material) can not solve the problem. In order to solve the problem, DK2 uses software to make a reverse color compensation before image display, as shown in the figure below.
This method is only modified at the software level, but it will have a certain impact on the clarity of the image, and every pixel on the image needs to do a reverse dispersion processing, which increases the hardware burden and reduces the frame rate of the image. The best way is to use a group of achromatic lenses with different materials to eliminate the color edge. This has been very mature in lens design, such as apochroma ticobjec tive, which can completely eliminate the chromatic aberration of the image.
In popular terms, distortion is image distortion, giving people a bulge in the middle (barrel distortion) or a depression (negative distortion), which is also a kind of aberration. It is caused by the different positions of the entrance pupil (that is, the human eye) in the optical system. For the spherical mirror, the aberration is inevitable, as shown in the figure on the left, and with the increase of FOV The edge image distortion will be more obvious. In order to solve this problem, the common method is to use the aspheric design, as shown in the right figure below. It can greatly reduce the image distortion, and also greatly reduce the weight of the lens. This is also the reason why the current height eyeglass is no longer as thick as the bottom of the bottle.
5. Myopia adjustment
At present, the incidence of myopia in China is as high as 70% in the population above high school, and the users of VR glasses are concentrated in the highly educated people. If they don’t wear glasses, they often can’t see the text on the screen clearly, let alone experience the so-called immersion, so it will reduce the popularity of VR devices.
At present, VR glasses in China claim that they will not affect the myopia of 400 degrees or even higher. Oculus’s solution to this problem is to match three groups of lenses with different degrees, which is equivalent to matching three groups of glasses with different eyesight from the optical principle. However, most domestic VR glasses have only one group of lenses, and often use two methods to solve the problem of myopia: one is to wear glasses, and then wear VR glasses. This method does solve the problem of myopia, but the sense of immersion is greatly weakened People can’t get close to the lens, and the scheme of wearing two pairs of glasses at the same time is very clumsy. The other is to put the screen or mobile phone close to the lens without wearing glasses, which is equivalent to nearsighted people getting close to the screen to see things on the screen, which will also reduce the sense of immersion, because the image on the edge of the screen is equivalent to being moved out of the picture, nearsighted people can only see the image in the middle part, and the image on the outside is cut off. Another defect of this method is that nearsighted people are more likely to see pixels, and D appears In addition, if the user’s left and right eyes have different degrees of myopia, this method can not solve the problem.
In fact, in many SLR cameras, there is a good solution. There is a knob called diopter adjustment beside the eyepiece of the SLR. By turning the knob, the nearsighted users within a certain range can see the image in the viewfinder without wearing glasses. Its principle is shown in the figure below. The viewfinder is composed of several independent lenses. Rotating the knob is equivalent to adjusting the distance between two lenses, so as to change the divergence or convergence degree of light entering people’s eyes. It can be equivalent to a zoom lens. The design method of this type of lens has a history of nearly one hundred years, and it is quite mature. If the lens of VR glasses is also composed of more than one lens, the diopter can be adjusted by design, and users can use it without wearing glasses.
Application of VR glasses
1. Visualization of drug molecular structure
There are many applications of VR glasses, and health care is an important field in the application of VR glasses. There are many new VR technologies about health care. For example, VR is used to visualize the molecular structure of drugs to help the development of new drugs. There’s a technology called 4sight that’s designed to help chemists visualize the structure of complex molecules.
The largest children’s Hospital in the UK, with hololens for surgery. Hospital staff plan to use hololens in the operating room to view the patient’s treatment during the operation. At the same time, the staff will also use surfacehub to realize the electronic storage of files such as charts, tests and medical images.
3. Analysis of brain health
A neurotechnology company used VR to analyze the brain health of athletes. Provide users with interactive VR content, support real-time feedback, and optional hierarchical training examples. The main purpose of these functions is to carry out early screening for physical defects that put athletes at an additional risk of injury. In addition, it will support professionals in the rehabilitation process.
According to digi capital, VR will become a $30 billion market by 2020. Although the technology still needs to be tested and improved, many companies have invested a lot of human and financial resources to develop product technology in VR.
Editor in charge: CC