Description
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).
Read more about this topic: Group Of Pictures
Famous quotes containing the word description:
“A sound mind in a sound body, is a short, but full description of a happy state in this World: he that has these two, has little more to wish for; and he that wants either of them, will be little the better for anything else.”
—John Locke (1632–1704)
“Do not require a description of the countries towards which you sail. The description does not describe them to you, and to- morrow you arrive there, and know them by inhabiting them.”
—Ralph Waldo Emerson (1803–1882)
“The next Augustan age will dawn on the other side of the Atlantic. There will, perhaps, be a Thucydides at Boston, a Xenophon at New York, and, in time, a Virgil at Mexico, and a Newton at Peru. At last, some curious traveller from Lima will visit England and give a description of the ruins of St. Paul’s, like the editions of Balbec and Palmyra.”
—Horace Walpole (1717–1797)