Basic video knowledge

Bit rate is the number of digits transmitted per unit time during data transmission. Generally, the unit is kbps, that is, a thousand bits per second. The general understanding is the sampling rate. The larger the sampling rate per unit time, the higher the accuracy. The closer the processed file is to the original file, but the file volume is proportional to the sampling rate, therefore, almost all encoding formats focus on how to use the lowest Bit Rate to achieve the least distortion, and the CBR (fixed bit rate) and VBR (Variable Bit Rate) derived from this core ), all the articles are made in this regard, but things are not absolute. In terms of audio, the higher the bit rate, the smaller the compression ratio, and the smaller the sound quality loss, the closer the sound quality is to the source. "Bit Rate" is distortion. The higher the bit rate, the clearer it is. On the contrary, the picture is rough and multi-Mosaic. The information in the computer is represented by binary 0 and 1. Each 0 or 1 is called a single bit, which is expressed by lowercase B, that is, bit. uppercase B indicates byte, that is, byte. one byte = eight bits, that is, 1B = 8B; the upper-case k indicates the meaning of a thousand, that is, a thousand bits (Kb) or a thousand bits (KB ). Indicates the size unit of the file. Generally, bytes (Kb) are used to indicate the file size. Kbps: First, you need to know that PS refers to/s, that is, every second. Kbps indicates the network speed, that is, the number of kilobytes of information transmitted per second (k indicates the kilobytes and kb indicates the number of kilobytes ), in order to intuitively display the fast transmission speed of the network, companies generally use KB (kilobytes) to indicate the number of kilobytes transmitted per second if it is kbps. 1 kbps = 8 kbps. The Internet access speed of ADSL is 512 kbps. If it is converted into bytes, It is 512/8 = 64 Kbps (that is, 64 kilobytes per second ). In short, the number of frames is the number of frames transmitted in one second. It can also be understood as the number of frames that a graphic processor can refresh several times per second. FPS (frames per second) is usually used). Each frame is a static image, and the rapid continuous display of frames forms the illusion of motion. A higher frame rate allows you to get smoother and more realistic animations. The more frames (FPS) each second, the smoother the displayed action. Generally, 30 FPS is acceptable, so the minimum FPS to avoid lD is 30. Apart from 30 FPS, some computer video formats, such as Avi, can only provide 15 frames per second. The reason why we can use cameras to see continuous images is that image sensors constantly capture images and transmit them to the screen. When the transmission speed reaches a certain level, the human eye cannot identify the time gap between pictures, so you can see continuous dynamic pictures. The number of frames per second (FPS) or frame rate indicates the number of times a graphic processor can update each second. A higher frame rate allows you to get smoother and more realistic animations. In general, 30fps is acceptable, but improving the performance to 60fps can significantly improve the interaction and authenticity, but generally it is not easy to notice that there is a significant increase in smoothness when it exceeds 75fps. If the frame rate exceeds the screen refresh rate, only the graphic processing capability is wasted, because the display cannot be updated at such a fast speed, so that the frame rate exceeding the refresh rate is wasted. Generally, "FPS (frame per second, number of screen updates per second)" is used to represent this indicator. When watching TV and movie images, as long as the screen refresh rate reaches 24 frames/Second, it can satisfy people's needs. Similarly, when playing a normal game, you only need to update it at a rate of 24 frames/second, but in some high-speed games (such as shooting games ), if the screen update rate is still only 24 frames per second, the screen will feel dull and not smooth enough. Although theoretically, the faster the image refresh rate, the better, the higher the refresh rate has no practical significance-when the image FPS reaches 60 frames/Second, it can already meet the needs of most applications. Under normal circumstances, if the average FPs of the game screen can reach 30 frames/Second, the screen is basically smooth; 50 frames/second is supported, you can basically feel the feeling of the cloud flow. Generally, it is difficult to tell the difference between 60 frames/second and 100 frames/second. (2) Why does it affect FPS? Since the higher the refresh rate, the better? Why do we need to emphasize that there is no need to pursue a high refresh rate? The reason is that when the display resolution remains unchanged, the higher the FPS, the higher the processing capability of the video card. The screen displayed on the computer is output by the video card. Therefore, the filling of each pixel on the screen must be calculated and output by the video card. When the screen resolution is 1024 × 768, the screen refresh rate must reach 24 frames/second, the number of pixels that the video card needs to process in one second reaches "1024 × 768 × 24 = 18874368 ". If the screen update rate is required to reach 50 frames per second, the data volume will suddenly increase to "1024 × 768 × 50 = 39321600 ". The relationship between FPS and resolution and video card processing capability is as follows: processing capability = resolution × refresh rate. This is why when playing the game, the larger the resolution settings, the image is not smooth reason for the basic idea of MPEG-1 compression: Intra-frame compression and inter-frame compression. Second, statistical analysis of time correlation: the statistical results show that the interval is 1 ~ In a two-frame image, each pixel has only less than 10% points, and its brightness difference changes more than 2%, while the color difference changes only less than 1%. Compression Method: Grouping: divides several frames into one group (GOP). To prevent motion changes, the number of frames should not be too large. 1. definition frame: defines three types of images for each frame in each group, namely, I frame, B frame, and P frame. 2. prediction frame: I frame is used as the base frame, I frame is used to predict P frame, and then I frame and P frame are used to predict B frame; 3. data transmission: Finally, the difference between the I-frame data and the prediction is stored and transmitted. I frame: in-frame encoding frame I frame features: 1. It is a full-frame compression encoding frame. It compresses and transmits the full frame image information in JPEG format. the complete image can be reconstructed with only the data of the I frame during decoding. 3. I frame describes the details of the image background and motion subject; 4. I frames are generated without reference to other images; 5. I frame is the reference frame of P frame and B frame (its quality directly affects the quality of each frame in the same group); 6. an I frame is the basic frame (the first frame) of the frame group Gop. In a group, there is only one I frame. motion vectors are not required for an I frame. I frame occupies a large amount of data information. P frame: Forward prediction Encoding Frame. P Frame Prediction and Reconstruction: P frame uses I frame as the reference frame to find the predicted value and Motion Vector of P frame "a certain point" in I frame, transmits the prediction difference value along with the motion vector. The receiver finds the predicted value of "A point" of the P frame from the I frame based on the Motion Vector and obtains the "A Point" value of the P frame with the difference value, the complete P frame can be obtained. P frame features: 1. P frames are separated by 1 ~ 2 encoding frames; 2. P frame transmits the difference between P frame and I or P frame and the motion vector (prediction error) by motion compensation. 3. A complete P-frame image can be reconstructed only when the predicted values and prediction errors in I frame are summed up during decoding. 4. P frames are pre-predicted inter-frame encoding. It only refers to the I frame or P frame closest to it; 5. P frame can be the reference frame of P frame after P frame, or the reference frame of B frame before and after P frame; 6. because p frame is a reference frame, it may cause the spread of decoding errors; 7. because it is a difference transfer, P frames are compressed relatively high. B frame: bidirectional prediction interpolation Encoding Frame. B Frame Prediction and reconstruction of B frame I or P frame before B and P frame after B as the reference frame, "find" B frame "A Point" predicted value and two motion vectors, and transmits the prediction difference value and the motion vector. Based on the motion vector, the acceptor "finds (computes)" the predicted value in the two reference frames and sums the difference value to obtain the "A Point" sample value of the B frame, so as to obtain the complete B frame. B frame features 1. frame B is predicted by the first I or P frame and the next P frame; 2. frame B transmits the prediction error and motion vector between it and the previous I or P frame and the next P frame; 3. frame B is a bidirectional prediction Encoding Frame. 4. frame B has the highest compression ratio, because it only reflects the changes of the motion subject between frames of C, and the prediction is more accurate. frame B is not a reference frame and will not cause the spread of decoding errors. Note: I, B, and P frames are based on the needs of the compression algorithm and are manually defined. They are all real physical frames. As for which frame in the image is an I frame, it is random. Once I frame is determined, the frames in the future will be arranged strictly in the specified order.