Group of images
The Group of Pictures ( GoP ) or picture (s) group is a group of inter-coded, successive individual pictures ( frames ) in the picture stream of a video.
General
Each MPEG-coded film or video stream consists of successive groups of images. The visible individual images are generated from the MPEG images contained therein. A group is in MPEG-2 , typically half a second, in MPEG-4 long often around ten seconds.
A picture group can contain the following types of pictures:
- Intra picture
- (English intra-coded picture , short I picture or I frame ) Reference image, corresponds to a complete still image, similar to a JPEG image, and is independent of other image types.
- P-picture
- (English predictive-coded picture , P picture or P frame for short ) contains difference information from the previous I or P picture.
- B-picture
- (English bidirectionally predictive-coded picture , short B picture or B frame ) contains difference information from the preceding and / or following I or P picture.
- D picture
- (English DC direct coded picture , short D picture or D frame ) is used for fast forwarding.
If a picture group can be decoded independently of a preceding or following picture group, it is called closed. Accordingly, it is called open if it is dependent on another group of images.
construction
For MPEG-1 and MPEG-2, a group of pictures must begin with an intra or B picture and end with an intra or P picture . In between there can be any number of Intra , P and B pictures. Overall, a group of images must contain at least one intra-image; however, it can only consist of intra-images. Since a subsequent intra- or P-picture is required for the decoding of a B-picture, the picture group is normally rearranged for storage in the coding sequence. This has the advantage that the images to be processed can be reached more quickly during decoding and a smaller buffer is required for the B-images. In the coding sequence, each group of images must always begin with an intra-image or an Instantaneous Decoding Refresh (IDR) (with MPEG-4 ).
Here is an example for rearranging a group of images:
Order of presentation:
| Picture no. | 0 | 1 | 2 | 3 | 4th | 5 | 6th | 7th | 8th |
| Image type | I. | B. | B. | B. | P | B. | B. | B. | P |
Possible coding order:
| Picture no. | 0 | 4th | 1 | 2 | 3 | 8th | 5 | 6th | 7th |
| Image type | I. | P | B. | B. | B. | P | B. | B. | B. |
MPEG-4 picture groups can contain up to 14 intra-pictures; A new group only begins with a new IDR.
Behavior in the event of failure
If a video that is encoded in this way is transmitted and the connection is briefly interrupted, images can be played back from the buffer so that the playback speed remains constant. If the connection remains poor, the transmission rate is so low that the buffer cannot be filled. In addition, individual images keep falling out. The following effects then occur:
If an I-picture fails, only changes can be seen in the reproduction that occur after the following P-picture. Missing image information is replaced by display systems with gray, for example. Contents that do not change with the next P-picture remain gray afterwards.
If a P-picture fails, it only affects the following P- and B-pictures up to the next I-picture. The consequences are similar, only less persistent.
Failures of B-pictures are manageable and are only noticed by jerking.