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 one 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 one can use the maximal count of B-frames between two consecutive anchor frames.
Many recent 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, consider a sequence with pattern IBBBPBBBPBBBPBBBIBBBPB.... In this case, 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).
