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.
A GOP can contain the following picture types:
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 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.