What is GOP in the mpeg file?

The so-called GOP refers to the image group. The frame sequence in the mpeg lattice is divided into three types: I, P, and B, such as arranging ibbpbbpbbpbbpbbp... style. This continuous frame image combination is GOP (image group, group of picture), which is the most basic unit for future access by MPEG, the order of the image will be repeated until the end of the image. A GOP is a set of consecutive IPB images. MPEG encoding divides an image (I .e. frame) into three types: I, P, and B. I is an internal Encoding Frame, P is a Forward prediction frame, and B is a bidirectional interpolation frame. To put it simply, I frame is a complete screen, while P frame and B frame record changes relative to I frame. Without I frame, P frames and B frames cannot be decoded. This is the reason why the MPEG format is difficult to accurately edit. It is also the reason why we need to adjust the header and tail.

MPEG-2 Frame Structure

The frame structure of MPEG-2 compression has two parameters, one is the length of the GOP (Group of picture) image group, generally can be encoded from 1-15; the other is the number of B frames between I frames and P frames, usually 1-2. Theoretically, the former records N, that is, the number of frames in which I frames appear. The latter describes the number of frames in which P frames appear, and the record is M. The GOP in the figure is n = 12, M = 3.

We usually think that the longer the length of the MPEG-2 GOP, the higher the image compression efficiency, that is, the higher the quality of the restored image under the same code stream with the same encoding format. In this experiment, we set two sets of different GOP lengths for the video server, but the results are different from the original concept. It is also a Kiel harbor sequence. Under the MP @ ML encoding and 8 Mbps conditions, GOP = 15 restored image quality pqr = 5.59; while GOP = 9's image pqr = 5.49, better than GOP = 15.

Of course, we do not think that the shorter the GOP, the higher the image quality. This may also be a curve between first rising and then falling. Under certain conditions, GoP has a best value. In addition, the IBP frame structure will also affect the quality of restored images. There is a correlation between the two. As this experiment has not been fully performed, the relationship between the two cannot be analyzed for the time being.