In MPEG encoding, a group of pictures, or GOP, specifies the order in which intra-frames and inter frames are arranged. The GOP is a group of successive pictures within a MPEG-coded video stream. Each MPEG-coded video stream consists of successive GOPs. From the MPEG 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, corresponds to a fixed image and 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. With the next I-frame a new GOP begins. 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 above example, the GOP structure is IBBPBBPBBPBB. Instead of the M parameter one can use the maximal count of B-frames between two consecutive anchor frames. The more I-frames the MPEG stream has, the more it is editable. 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 I-frames contain the full image, they don't require any additional information to reconstruct the image. Therefore any errors in the streams are corrected by the next I-frame (an error in the I-frame propagates until the next I-frame). Errors in the P-frames propagate until the next anchor frame (I or P). B-frames do not propagate errors.
