Video compression picture types

The three major picture types used in the different video algorithms are I, P and B.
They are different in the following characteristics:
I?frames are the least compressible but don't require other video frames to decode.
P?frames can use data from previous frames to decompress and are more compressible than I?frames.
B?frames can use both previous and forward frames for data reference to get the highest amount of data compression.

I幀獨立壓縮, 解碼時不需依賴其它幀.
P幀需要依賴前邊的I幀才能進(jìn)行解碼, 優(yōu)點是壓縮率比I幀高.

Summary

A sequence of video frames, consisting of two keyframes (I), one forward-predicted frame (P) and one bi-directionally predicted frame (B).

P幀相對于I幀, 僅有三個點的位置向左側(cè)發(fā)生了平移, P幀就是來記錄變化的這部分信息.

An I?frame is an 'Intra-coded picture', in effect a fully specified picture, like a conventional static image file.

conventional:平常的
I幀像一幅平常的靜態(tài)圖片.

P?frames and B?frames hold only part of the image information, so they need less space to store than an I?frame and thus improve video compression rates.
A P?frame ('Predicted picture') holds only the changes in the image from the previous frame.
For example, in a scene where a car moves across a stationary background, only the car's movements need to be encoded.
The encoder does not need to store the unchanging background pixels in the P?frame, thus saving space.
P?frames are also known as delta?frames.

P幀和B幀只包含了圖像的局部信息, 故相對I幀, 僅需更少的空間去存儲.

A B?frame ('Bi-predictive picture') saves even more space by using differences between the current frame and both the preceding and following frames to specify its content.

Intra coded frames/slices (I?frames/slices or Key frames)

• I-frames are coded without reference to any frame except themselves.
• May be generated by an encoder to create a random access point (to allow a decoder to start decoding properly from scratch at that picture location).
• May also be generated when differentiating image details prohibit generation of effective P or B-frames.
• Typically require more bits to encode than other frame types.

編碼時不需依賴其它幀, 一般來說編碼之后的size比較大.

Often, I?frames are used for random access and are used as references for the decoding of other pictures.
Intra refresh periods of a half-second are common on such applications as digital television broadcast and DVD storage.
Longer refresh periods may be used in some environments.
For example, in video conferencing systems it is common to send I-frames very infrequently.

在數(shù)字電視廣播和DVD存儲時, 一般I幀率的刷新周期在0.5秒左右.
有時, 刷新周期可以適當(dāng)增長, 比如視頻會議的時候, 由于相對來說畫面的變化幅度不會太大, 所以發(fā)送I幀的周期可以長一些.

Predicted frames/slices (P-frames/slices)

• Require the prior decoding of some other picture(s) in order to be decoded.
• May contain both image data and motion vector displacements and combinations of the two.
• Typically require fewer bits for encoding than I pictures do.

Bi-directional predicted frames/slices (B-frames/slices)

• Require the prior decoding of other frame(s) in order to be decoded.
• Typically require fewer bits for encoding than either I or P-frames.

小結(jié)

I&P&B 比較簡圖