Multimedia development Coding GOP---what is GOP

The so-called GOP, meaning is the picture group, MPEG lattice frame sequence, divided into I, p, b three kinds, such as row into IBBPBBPBBPBBPBBP ... Style, this continuous frame picture combination is the GOP (picture group, Group of Pictures), is the most basic unit of MPEG future access, its order will be repeated until the end of the image. A GOP is a continuous set of IPB images. MPEG coding divides the screen (ie frame) into I, P, b Three, I is the internal encoding frame, P is forward prediction frame, B is bidirectional interpolation frame. In a nutshell, I-frames are a complete picture, while P-frames and B-frames record changes relative to the I-frame. Without I-frames, p-frames and B-frames cannot be decoded, which is why the MPEG format is difficult to precisely edit, and why we need to fine-tune the head and tail.

MPEG-2 compressed frame structure has two parameters, one is the length of the GOP (group of picture) image group, generally can be encoded from 1-15, and the other is the number of B frames between I and P frames, typically 1-2. The former is theoretically recorded as N, that is, how many frames appear once I-frames, the latter describes how many frames appear once P-frames, recorded as M. The GOP shown is n=12,m=3.
We usually think that the longer the GOP length of MPEG-2, the higher the image compression efficiency is, the higher the image quality is in the same code stream and encoding format. In the experiment, we tested the video server with two different GOP lengths, but the result was different from the original idea. Also is the Kiel Harbor sequence, under the [email protected] encoding and 8Mbps conditions, the gop=15 image quality pqr=5.59, and gop=9 image pqr=5.49, better than gop=15.

Of course, we don't think the shorter the GOP, the higher the image quality. It may also be a curve relationship that rises and falls first, and the GOP will have an optimal value under certain conditions. In addition, the IBP frame structure also has an effect on the quality of the restored image. There is a certain correlation between the two interactions. As the experiment is not sufficient, it is not possible to analyze the relationship between them for the time being.

What is close GOP, and open GOP?

In the video coding software generally have "whether to turn off the GOP" option, wherein the closed GOP refers to a GOP series of pictures only associated with this group of pictures, and adjacent to the GOP is not linked, the advantage is that time information can be accurately positioned to the GOP, the disadvantage of less video associated information, image quality than open GOP. Open the GOP, opposite the above.

Multimedia development Coding GOP---what is GOP