In Video coding, a group of pictures
, or GOP structure
, specifies the order in which intra-
Intra-frame coding is used in video coding . It is part of group of pictures with inter frames.The term intra-frame coding refers to the fact that the various lossless and lossy compression techniques are performed relative to information that is contained only within the current frame, and not...
and inter-frames are arranged. The GOP is a group of successive pictures within a coded video stream. Each coded video stream consists of successive GOPs. From the pictures contained in it, the visible frames are generated.
A GOP can contain the following picture types:
- I-picture or I-frame (intra coded picture) - reference picture, which represents a fixed image and which is independent of other picture types. Each GOP begins with this type of picture.
- P-picture or P-frame (predictive coded picture) - contains motion-compensated
Motion compensation is an algorithmic technique employed in the encoding of video data for video compression, for example in the generation of MPEG-2 files. Motion compensation describes a picture in terms of the transformation of a reference picture to the current picture. The reference picture...
difference information from the preceding I- or P-frame.
- B-picture or B-frame (bidirectionally predictive coded picture) - contains difference information from the preceding and following I- or P-frame within a GOP.
- D-picture or D-frame (DC direct coded picture) - serves the fast advance.
A GOP always begins with an I-frame. Afterwards several P-frames follow, in each case with some frames distance. In the remaining gaps are B-frames. A few video codecs allow for more than one I-frame in a GOP.
The I-frames contain the full image and do not require any additional information to reconstruct it. Therefore any errors within the GOP structure are corrected by the next I-frame. B-frames within a GOP only propagate errors in H.264
H.264/MPEG-4 Part 10 or AVC is a standard for video compression, and is currently one of the most commonly used formats for the recording, compression, and distribution of high definition video...
, where B-frames can be referenced by other pictures in order to increase compression efficiency.
The more I-frames the video stream has, the more editable it is. However, having more I-frames increases the stream size. In order to save bandwidth and disk space, videos prepared for internet broadcast often have only one I-frame per GOP.
The GOP structure is often referred by two numbers, for example M=3, N=12. The first one tells the distance between two anchor frames (I or P). The second one tells the distance between two full images (I-frames): it is the GOP length
. For the example M=3 N=12, the GOP structure is IBBPBBPBBPBBI. Instead of the M
parameter one can use the maximal count of B-frames between two consecutive anchor frames.