Video is beneficial ⽤⼈ The principle of eye vision persistence , By playing ⼀ Series of Graphs ⽚, send ⼈ Ophthalmic production ⽣ The feeling of movement . Simply transmit video paintings ⾯, Video volume ⾮ often ⼤, To the existing ⽹ Network and storage are unacceptable . In order to make the video easy to transmit and store ,⼈ We found that the video has ⼤ Measure repeated information , If the duplicate message is removed from the sender , At the receiving end , So you ⼤⼤ Reduces the amount of video data ⽂ Pieces of , So there is H.264 Video compression standard .

Definition

stay H.264 In the compression standard I frame 、P frame 、B frame ⽤ Yu Biao ⽰ Transmitted video painting ⾯. In video compression , Each frame represents a still image . In the actual video compression coding , Various algorithms will be adopted to reduce the capacity of data , among IPB Frame is the most common algorithm .

I‑frame (Intra-coded picture): That is, a complete picture

P‑frame (Predicted picture): The area different from the previous picture

B‑frame (Bidirectional predicted picture): The difference area from the front and back pictures

I frame

I frame ⼜ It is called intra coding frame , Also known as full frame compression coding frame , yes ⼀ Kind of ⾃ Unique with all information ⽴ frame ,⽆ You need to refer to other images ⽴ Into the ⾏ decode , It can be simply understood as ⼀ Zhang static painting ⾯. The... In the video sequence ⼀ Every frame is always I frame , Because it's a keyframe .I Frames are usually each GOP（MPEG A video compression technology used ） The first frame of , After moderate compression , As a reference point for random access , It can be used as a still image .

Now there is a film as follows :

The film totals 20 Pictures make up , Every complete picture we can call I frame . If each JPEG The size of the picture is 100KB, Then transmitting two pictures is 100 * 20 = 2000KB

You know, this film is less than a few seconds 2M 了 , If it takes a few hours, it's ok , So there is an algorithm for video compression

I Frame features :

1. It's a full frame compressed encoded frame . It takes the whole frame of image information JPEG Compression coding and transmission ;
2. Use only when decoding I The frame data can reconstruct the whole image ;
3.I The frame describes the background of the image and details of the moving subject ;
4.I Frame is generated without reference to other pictures ;
5.I Frames are P The frame and B Frame reference frame ( Its quality directly affects the quality of subsequent frames in the same group );
6.I Frames are frame groups GOP The base frame of ( First frame ), Only one in a group I frame ;
7.I Frames don't need to consider motion vectors ;
8.I The frame takes up a large amount of information .

P frame

P frame ⼜ It is called inter prediction coding frame , Also known as forward prediction coding frame , Before reference ⾯ Of I Frame can enter ⾏ code . surface ⽰ Is the current frame drawing ⾯ With the former ⼀ frame （ front ⼀ The frame may be I Frames can also be P frame ） The difference between . Decoding requires ⽤ Previously cached paintings ⾯ Superimpose the differences defined in this frame ,⽣ Become the final painting ⾯.

And I Frame phase ⽐,P Frames usually occupy ⽤ Fewer data bits , But I don't ⾜ yes , because P Frame before ⾯ Of P and I The reference frame has complex patience , So for transmission errors ⾮ Chang Mingan . Usually, the time redundancy information of the previous encoded frames in the image sequence is fully removed to compress the encoded image with the amount of transmitted data , Also known as prediction frame . Through observation , We can see that , In fact, there are only some subtle differences between each frame , As shown in the figure below 6 frame

When the first frame is transmitted , In fact, we only need to transmit part of the second frame , Then the other end carries out image algorithm to combine

In this way , When transmitting the second frame , Not the original 1/10, Only need to transmit the first frame 100KB, The subsequent ones are transmitted in this way , This kind of transmission only part of the picture , Namely P Frames . Finally, the size of the whole film is 100KB + 10KB * 19 = 290 KB, Much smaller than before .

P Frame features :

1.P Frames are I The frame is separated by 1~2 Frame encoding frame ;
2.P The frame is transmitted with motion compensation I or P Frame difference and motion vector ( Prediction error );
3. When decoding, you have to decode I Only when the predicted value in the frame is summed with the prediction error can the complete P Frame image ;
4.P The frame belongs to the inter frame coding of forward prediction . It only refers to the one closest to it in front of it I The frame or P frame ;
5.P The frame can be behind it P Frame reference frame , It can also be before and after B Frame reference frame ;
6. because P Frames are reference frames , It can cause the spread of decoding errors ;
7. Because it's a differential transmission ,P The frame compression ratio is high .

B frame

B frame ⼜ It is called bidirectional prediction coding frame , Also known as bidirectional prediction interpolation coding frame , That is to say B A frame records the difference between this frame and the preceding and following frames . That is to say, to decode B frame , Not only to get the previous cache drawing ⾯, And the picture after decoding ⾯, By drawing before and after ⾯ The final picture is obtained by superposition with the data of this frame ⾯.B Frame compression rate ⾼, However, the requirements for decoding performance are relatively ⾼. Consider the encoded frame in front of the source image sequence , The time redundancy information between the encoded frames behind the source image sequence is also considered , To compress the encoded image of the transmitted data , Also known as bidirectional prediction frame .B The frame is actually the difference between the two pictures before and after . If you understand P The frame and I frame , It's easy to understand .

B Frame ratio P Frame smaller , More space saving

Suppose there are three pictures , Here's the picture :

After coding , It's going to be like this :

B Frame features

1.B The frame is from the front I or P The frame and the following P Frame to predict ;
2.B The frame transmits it with the preceding I or P The frame and the following P Prediction errors between frames and motion vectors ;
3.B The frame is a bidirectional predictive coding frame ;
4.B The frame compression ratio is the highest , Because it only reflects the change of motion subject between C reference frames , The prediction is more accurate ;
5.B Frame is not a reference frame , It doesn't cause the spread of decoding errors .

I frame 、B frame 、P The basic process of frame coding ：

I The basic process of frame coding is ：

(1) Intra prediction , Determine the intra prediction mode used .
(2) Subtract the predicted value from the pixel value , Get the residuals .
(3) Transform and quantify the residuals .
(4) Variable length coding and arithmetic coding .
(5) Reconstruct the image and filter it , The obtained image is used as the reference frame of other frames .

P The frame and B The basic process of frame coding is ：

(1) Do motion estimation , Calculate the rate distortion function using the inter frame coding mode ( section ) value .P Frame refers only to the previous frame ,B The frame can refer to the following frame .
(2) Intra prediction , Select the intra mode with the lowest value of rate distortion function and compare it with the inter mode , Determine which encoding mode to use .
(3) Calculate the difference between the actual value and the predicted value .
(4) Transform and quantify the residuals .
(5) Entropy coding , If it's interframe coding mode , Coded motion vector

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Summary

I Frame only need to consider this frame ;P The frame record is with the previous ⼀ The difference between frames ;B The frame records the front ⼀ Frame and after ⼀ The difference between frames , Can save more space , video ⽂ Pieces of ⼩ 了 , But relatively speaking, when decoding ⽐ a ⿇ Bother . Because in decoding , It's not just about ⽤ Previously cached paintings ⾯,⽽ And you should know ⼀ individual I perhaps P Paintings ⾯, For no ⽀ a B Frame decoding player is easy to get stuck .