What is bit rate control? In video coding, what is the concept of bitrate control, and what is it achieved by what?

Bit rate control is actually a coding optimization algorithm, which is used to control the video stream size.
So what is the purpose of its control? We can imagine that the same video encoding format, the code flow is large, it contains more information, then the corresponding image is clearer, and vice versa. The two current control modes are VBR and CBR,VBR are dynamic bit rate, CBR is fixed bit rate.
So how did they come to realize it? We first look at the purpose of video encoding, it is in order to transmit in a limited amount of bandwidth as clear as possible video, we have 25/frames per second image example, 25 frame image defined in the GOP group, currently mainly has i,b,p frame three frame format, I frame is a key frame, you can imagine it is a JPEG compressed image, and B , p frames are based on the existence of I-frames, if I frame is lost, b,p frame is not visible, the b,p frame is not the actual image pixel content, but the amount of change of each relevant pixel, they will be very small relative to the I frame information.
The GOP group refers to the length of the group where a keyframe I frame is located, with only 1 I frames per GOP group. Let's look at the size of the stream in a set of images. When the compression of video encoding is the same, the definition requirements are the same, the length format of the GOP group determines the size of the stream, for example: 25 frames per second, the GOP group length is 5, then the frame format is IBPBP, then 1 seconds there are 5 I-frames, 10 B-frames, 10 p-frames, if the GOP group length 15
, the frame format is IBBPBBPBBPBBPBB, then there will be 2 I-frames and 16 B-frames and 7 p-frames in 1 seconds, so the length format of the GOP group also determines the size of the code stream when 5 I-frames occupy more data than 2 I-frames. Finally, we look at the VBR and CBR, for VBR, it can be a special algorithm to control the length of a GOP group, that is, the number of I frames, when the image of the motion image, I can increase the number of I frames, then the image will be very clear, if the elements in the image is basically static, It can even tune the GOP group to a length of 25.
So according to the previous description, it can be imagined to ensure that the image quality control means, it is very flexible, not rigid, but the requirements of the control algorithm is very accurate, imprecise will lead to many problems, such as the code stream after the sudden increase in the non-degradation, even if the bandwidth is sufficient, can cause problems in the image. CBR In contrast, its bandwidth is constant, the code stream does not change, has always been the same size (there will be a little change), the image movement of more than the capacity of the code flow, the image will appear edge Blur phenomenon, its purpose is to stabilize the bandwidth, its implementation is relatively simple, the code stream will need to balance the relationship between IBP, Dynamic video big time I frame small, bp large, motion video hour I frame large, BP small.
In short, it is realized by the algorithm of the format of the solution. To summarize, the VBR bitstream control mode can reduce the bandwidth of the image dynamic picture, CBR control mode is stable and the image state is stable. They are designed to address different applications under different requirements.