1. What is the background of the emergence of GOP? What is the full name of GOP?

  • Statistical results show that: in the continuous several frames of images, generally only within 10% of the pixel difference, the difference in brightness is not more than 2%, and the difference in chroma is only within 1%.
  • A sequence Of similar frames can be grouped into a Group Of Pictures (GOP).

2. In the GOPThe I frameWhat is? How many I frames can a GOP have? How do you code it? How to decode it?

  • I Picture, I Frame, Intra Coded Picture,In-frame encoding image, also called a Keyframe
  • It’s the first frame of the video, and the first frame of the GOP, and there’s only one I frame for a GOP, right?
  • Code:Encode the whole frame of picture data
  • Decoding:The entire image can be decoded using only the encoding data of the current I frame
  • It is an independent frame with all information and can be decoded independently without reference to other images. It can be simply understood as a static image.

3. In the GOPP frameWhat is? How do you code it? How to decode it?

  • P Picture, P Frame, Predictive Coded PicturePredictive coded image
  • Code:① The whole frame of image data will not be encoded ②In the fronttheI frame or P frameAs a reference frame, only encode the current P frame withReference frame difference data
  • Decoding:The previous reference frame needs to be decoded first, and then the complete image of the current P frame is decoded by combining the differential data

4. In the GOPB frameWhat is? How do you code it? How to decode it?

  • B Picture, B Frame, Bipredictive Coded PicturePre - and post-predictive coded images
  • Code:① The whole frame of image data will not be encoded ②At the same time with front, backtheI frame or P frameAs a reference frame, only encode the current B frame withDifference data between before and after reference frames③ Because there are more frames to refer to, only better differential data needs to be stored
  • Decoding:The reference frames in front and behind need to be decoded first, and then the complete image of the current B frame is decoded by combining the differential data

5. In GENERAL, how are the data sizes of I, P and B frames arranged in GOP? Combined with the following picture, understand whyEncoding order is not equal to display order?

  • Frame I > P > B

6. What does the length of GOP mean? Is the length of GOP as long as possible?

  • The length of GOP represents the number of GOP frames.
  • Disadvantages of too long GOP: Increasing the LENGTH of GOP is conducive to reducing the size of the video file, but it should not be set too large, which will lead to the distortion of the rear GOP frame and affect the video quality
  • Disadvantages of too long GOP: Because the complexity of P and B frames is greater than that of I frames, the GOP value is too large, and too many P and B frames will affect the coding efficiency and reduce the coding efficiency
  • Disadvantages of too long GOP: The length of GOP is also a key factor affecting the response speed of video SEEK. In SEEK, the player needs to locate the last I frame nearest to the specified position. If the GOP is too large, it means that the distance from the specified position may be longer (more reference frames need to be decoded) and the response time of SEEK (buffer time) will be longer

7. Which frames are intra-frame codes? Which frames are interframe codes?

  • I Frame uses Intra Frame encoding and deals with spatial redundancy.
  • P Frame and B Frame adopt inter-frame coding and deal with time redundancy.