Group of pictures

The Group of Pictures ( GoP ) or image (s ) group is a group of coded in relationship to each other, successive frames (english frames) in the stream of images of a video.

Each MPEG encoded movie or video stream consisting of successive groups of images. From the MPEG images contained in the visible frames are generated. A group is typically in MPEG -2, one-half second, in the MPEG-4 often ten seconds.

A group of pictures can contain the following types of images:

If a group of images can be decoded independently of any preceding or subsequent group of images, they are called closed. Accordingly, it is called open if it is dependent on another group of images.

Construction

A group of images must, in MPEG -1 and MPEG-2, in the presentation order start with an intra - or B-frame and with an intra - or P-frame stop. In between may be any number of intra -, P- and B-pictures. Overall, a group of images must contain at least one intra-frame; but it may also consist of only intra- images. Since you when decoding a B picture requires a subsequent intra- or P-picture, the images group is usually re-sorted for storage in the encoding order. This has the advantage that it is timely enters in the decoding processing to the images and also required to have a smaller buffer memory for B-pictures. In the encoding order each group of pictures must begin ( MPEG -4) always with an intra-picture or a Instantaneous Decoding Refresh ( IDR).

Here is an example of the re-sorting a group of images:

Presentation order:

Two possible coding sequences:

MPEG -4 images groups can contain up to 14 intra- pictures and start using a new IDR starts a new group.