preface
We often see the word 4K in the promotion of scientific information and electronic products such as computer monitors and digital televisions. Bilibili has recently upgraded its HTML5 player and video cloud services to provide users with Ultra High Definition (UHD) video experience. We take this opportunity to popularize the relevant knowledge of 4K resolution to you and answer the relevant questions you may encounter.
What is 4K?
The 4K resolution, or THE 4K (Kilo) for short, is the horizontal resolution of the display device or the display content is around 4000 pixels. Video at 4K resolution, we call it 4K video. Common 4K resolutions are 3840 × 2160 and 4096 × 2160, and different standards are used in different multimedia fields. Some 4K resolutions, such as 3840 × 2160, are also often referred to as 2160P (Progressive Scan), which refers to 2160 horizontal scan lines in the vertical direction of the display device or display content.
Resolution is a term that describes how dense those dots (pixels) are on the display. Individual pixels come together to make up the image you see on the screen, so the higher the number of pixels, the higher the resolution.
In the TV industry, different brands use different expressions or vocabulary to say the same thing, like Ultra HD, 4K TV, Ultra High Definition TV, and 4K UHD TV. Over time, the TV industry found the term “4K” simple and intuitive for consumers, and brands started using it more and more. Today, most manufacturers and distributors are using the term 4K TV or 4K ultra HD TV.
Why do you need 4K video?
As TV screens and computer monitors become larger and more high-resolution, we believe THAT 4K display devices will eventually become a popular choice, so the importance of 4K video is self-evident. 4K has four times the total number of pixels than 1080p (Full HD FHD: Full High Definition), and more pixels means more detail. While 1080p video works fine on a 65-inch TV or 34-inch computer monitor, 4K video works better on these devices and is even better than 1080p video on larger screens. Even on the same size display device, the picture quality of 4K display device will become more vivid and delicate due to the improvement of pixel density.
The increasing availability of 4K has greatly improved the display of videos and images in large-screen applications and significantly reduced the amount of pixel noise that users see, which means smoother edges for text and images and an immersive viewing experience unless you are very close to the display device.
Video resolution and number of pixels
| Video resolution | Number of pixels (in million pixels) |
|---|---|
| 1080 p (1920 x 1080) | ~ 2.1 megapixels |
| 4 k (4096 x 3840 x 2160 or 2160) | ~ 8.5 megapixels |
| 8 k (7680 x 4320) | ~ 33.2 megapixels |
Other aspects
4K video sources and 4K display devices are indeed of great help to improve the experience of watching videos. In addition, network speed, video decoding performance, color consistency, contrast, brightness, screen size, ambient light and other important factors affect the experience of watching videos. All of these factors need to be taken into account if you want to create or find a satisfying environment to watch videos.
Q&A
- What is the bit rate/bit rate of the video?
Bit rate refers to the number of bits transmitted or processed per unit of time. Bit rate is often used in telecommunications as a synonym for connection speed, information transfer rate, and digital bandwidth capacity. The common units are BIT /s(informal abbreviation BPS: Bit per second), KBPS (Kilo, 1Kbps = 1000Bps), Mbps(Mega, 1Gbps= 1000Kbps), and Gbps(Giga, 1Gbps=1000Mbps). The higher the bitrate of video, the more data it transmits per second and the sharper the picture, but this is not always true.
- What’s the frame rate in the video?
The frames per second or FPS is a measure of the number of frames displayed. Generally used to describe film, video shooting, computer graphics, games, and animation systems, usually in FPS or Hz units. For example, common video frame rates are 24FPS, 25FPS, 30FPS, and 60FPS, and common monitor refresh rates are 60Hz, 120Hz, and 144Hz. In general, the higher the frame rate, the smoother the motion.
- What is the physical and logical resolution of the display device?
Physical resolution refers to the inherent resolution of the display device, which cannot be changed. Physical resolution is also called hardware resolution, native resolution and so on. The 4K resolution advertised by display device manufacturers refers to the physical resolution of the device, and the pixels under the physical resolution are also called physical pixels. Often said display bad spots, dark spots, bright points are some physical pixels have defects, do not work properly.
Logical pixels are virtual pixels that software (usually an operating system) redivides physical pixels to a certain scale. Logical resolution is formed from these logical pixels. Since the partition ratio of physical pixels is variable, the logical resolution is also variable. Because the physical pixel is the smallest display unit of the display device (the physical pixel is not the smallest luminous unit), the logical resolution is usually less than or equal to the physical resolution. When the UI designer points to the screen and says there’s an extra pixel, the pixel is the logical pixel.
- What are PPI and DPI?
PPI (pixels per iNCH) represents the number of physical pixels contained per inch and is used to measure the pixel density of a display device, commonly used in electronic display devices. The higher PPI is, the better the display effect is, and the less likely pixel noise is to appear. Users have different requirements for different types of display devices. Commonly used PPI calculation formula
DPI (dots per inch) indicates the number of dots per inch that can be displayed or output and is commonly used in the printer world.
- What if the ratio of the physical resolution to the logical resolution of the display device is not an integer?
Imagine a display device with a physical resolution of 6×6 and a logical resolution of 3×3. At this point, the ratio of physical resolution to logical resolution is an integer 2 (only the horizontal ratio is calculated here), and a black letter “Y” is drawn on the display device (without anti-aliasing), resulting in the image above.
If the ratio of physical resolution to logical resolution was 1.5, would the drawing result look like this? The answer is no, as we mentioned earlier that “a physical pixel is the smallest display unit on a display device”, so a physical pixel cannot be both white and black.
The final result is something like the image above, where the grey between black and white fills in pixels that are intended to show both white and black. To display a text requires many physical pixels to work at the same time, so in actual use, users will not feel obvious color difference, but there will be obvious discomfort such as blurring and blurring, especially on display devices with low PPI.
- What if the refresh rate of the display device and the ratio of video FPS is not an integer?
Assume the refresh rate of the display device is 60Hz and the FPS of the video is 24. The first and second frames of the display device display the first frame of the video, the third, fourth and fifth frames of the display device display the second frame of the video, and the sixth and seventh frames of the display device display the third frame of the video, and so on. This method is called 2:3 pull down in the field of TV and film, and the result is as shown in the figure below. There is another less commonly used 3:2 pull down, which works the same way as 2:3 pull down and is not restated.
- What are the common YUVs in video parameters?
YUV (brightness Y, chroma U, concentration V) is a color coding method. Often used in various image processing components. YUV allows for reduced chromaticity bandwidth, taking human perception into account when encoding photos or movies. In layman’s terms, using YUV to encode video can reduce the size of the video.
| Color coding (8bits format) | Bit rate per pixel (bpp: bits per pIxel) |
|---|---|
| YUV444 | 32 bits |
| YUV422 | 16 bits |
| YUV420 | 16 bits |
| YUV420 | 12 bits |
| RGB | 24 bits |
| RGBA | 32 bits |
- What is the relationship between video resolution, frame rate and bit rate?
For a 30FPS 1080p (1920×1080) video in YUV422 format, the bit rate is
- Will 4K video be a better viewing experience with a 1080p display?
It’s hard to tell. It depends on how each person looks, the source of the video, the peripheral devices, and so on. We’ve mentioned some of the factors that affect the viewing experience, and all of them can affect the viewing experience, and not everyone’s screen sensitivity is exactly the same. Try it yourself and choose the definition that suits your current device based on your network.
- What is panoramic video?
Panoramic video, also known as 360° video, is a kind of video that simultaneously records the panoramic view of the picture. The material is usually captured by a panoramic camera, which simultaneously shoots objects in the same space from multiple angles and combines the panoramic images into videos. When the panoramic video is played, the user’s perspective is in the position of the camera, and the user can switch the perspective to watch pictures in other directions.
- What does panoramic video have to do with Web XR (AR and VR)?
Augmented Reality (AR), translated as Augmented Reality, refers to a technology that combines and interacts the virtual world on the screen with the real world scenes through actuarial calculation of the position and Angle of the camera image and image analysis technology.
VR(Virtual Reality) translated as Virtual Reality, is the use of computer simulation to produce a three-dimensional Virtual world, to provide users with visual and other sensory simulation, so that users feel as if they are in the environment, can observe things in three-dimensional space immediately, without restrictions.
Web XR is a hybrid of WEB-side AR and VR. By definition, the evolution of Web XR may be closer to Mixed Reality (where the product is Microsoft HoloLens).
For now, panoramic video doesn’t have much to do with Web XR.
- Why IS 4K video more likely to stall?
4K video typically has a higher bit rate than 1080p, which means more data per second and requires more network bandwidth. Network lag occurs when your network bandwidth is insufficient for 4K video, at which point we recommend switching to 1080p or automatic resolution. A stable bandwidth of at least 15Mbps is required for 4K@30Hz videos.
In addition, with the improvement of video bit rate, the video decoding performance of user machine is also higher. This can also occur when your machine’s video decoding performance is inadequate.
- Why does the Web side of station B not use H.265(HEVC, High Efficiency Video Coding) codec?
Until recently, H.264 (also known as AVC: Advanced Video Coding) was the first-choice codec for optimizing quality and reducing file size. Upgrading to HEVC requires more computing power than AVC, but is much more efficient and provides better video quality at a lower bit rate. The H.265 video codec reached a critical point of global influence at WWDC 2017, when Apple announced the HEVC codec as the “next generation Video Codec.” Because of the commitment to HEVC and the fact that the hardware in most mobile chipsets already supports HEVC video encoding at the time of release, video providers are aware that THE HEVC codec has become the new video compression standard for streaming video.
HEVC is about 40% more efficient than AVC. This means that users will see a 40 percent improvement in startup speed, as well as a 40 percent improvement in content quality when the player fully ADAPTS to its approach. With the HEVC codec, higher video quality can be obtained at the same bandwidth as AVC, or the same video quality can be provided at half the bandwidth of AVC.
So much for the benefits, why not the Web side?
The figure above shows the support status of Web browsers, where green indicates that HEVC is supported, red indicates that HEVC is not supported, and light green indicates that HEVC is supported by hardware. It can be seen that only some IE11, Edge and Safari browsers support HEVC natively. , in the case of browser does not support, can only through other means to realize decoding software, software decoding 4 k HEVC video slow, takes up high CPU, increase the power consumption of mobile devices, reduce the battery can use long, on mobile notebook or configure lower equipment performance is poorer, therefore temporarily no large-scale use of HEVC Web side. We will be experimenting with HEVC codec on native HEVC browsers, so stay tuned!
This article reprinted from: www.bilibili.com/read/cv5843…
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