Group of pictures

In Video coding, a group of pictures, or GOP structure, specifies the order in which intra- 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.

Description

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 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, 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 number tells the distance between two anchor frames (I or P): it is the GOP size. 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 the maximal count of B-frames between two consecutive anchor frames can be used.

Many codecs employ GOP size with another parameter called Intra-Period or Intra-Refresh Period. An Intra Period always specifies the length between two consecutive I-Frames. In this case, when Intra Period is specified, the GOP length indicates the length between an I-Frame and a P-Frame. For example, in a sequence with pattern IBBBPBBBPBBBPBBBIBBBPB, the Intra Period is equal to 16 (length between two I frames) and GOP Size is equal to 4 (length between I and P frames or length between two consecutive P Frames).