The principle of code control
Code control, as the name suggests, is the code rate control, it is an important module of the encoder, the main function is to use the algorithm to control the size of the encoder output code stream. Although it is a very important part of the encoder, it is not part of the coding standard, that is to say, the standard does not set rules for code control. The code control of the encoder we usually use is realized by the encoder program itself.
What is the principle of code control? In fact, code control is the process of selecting an appropriate QP value for each frame of encoded image.
We know that when the picture of a frame of image is determined, the complexity and QP value of the picture almost determine its size after encoding. Since the encoder cannot determine the complexity of the picture, the goal of code control is to select an appropriate QP value to control the size of the coded stream. Of course, some code control algorithms can directly specify which QP value to use for encoding, so there is no need for the code control algorithm of the encoder to make decisions. But the final principle is the same. Then let's take a look at what code control algorithms are there.
Code control type
The commonly used code control algorithms mainly include: VBR (dynamic code rate), CQP (constant QP), CRF (constant code rate factor) and CBR (constant code rate).
VBR
VBR refers to the continuous change of the encoder output bit rate as the complexity of the original video picture changes. Usually, when the picture is complex or there is a lot of movement, the bit rate used will be higher; and when the picture is relatively simple, the bit rate used will be lower. The main goal of VBR is to ensure the quality of video images, so it is more suitable for video on demand and short video scenarios.
CQP
CQP is very simple, that is, every picture uses the same QP value from beginning to end