|Newer page:||version 4||Last edited on Sunday, January 10, 2010 10:28:01 am||by LawrenceDoliveiro|
|Older page:||version 3||Last edited on Saturday, January 9, 2010 9:41:51 pm||by LawrenceDoliveiro||Revert|
@@ -7,9 +7,9 @@
But not all frames can be expressed as differences—you have to have a full frame as a starting point. This is called an ''I-frame'' (because it only uses ''intraframe'' compression). This can be followed by one or more ''P-frames'' (because each one is expressed as a difference from the ''previous'' frame). The sequence starting with an I-frame, and continuing with all its dependent P-frames, is called a ''Group Of Pictures'' or “GOP”.
To compress things even further, the GOP can contain ''B-frames'', which are expressed as a difference between ''both'' a preceding and following I- or P-frame.
-That is, preceding and following in time. However, to ease the job of the decoder, the B-frame occurs in the video stream ''after'' the frames that it depends on. Thus, the temporal order of display of an B-frame between an I-frame and a P-frame might be IBP, but the order in which they appear in the stream is IPB.
frame is still tagged with the appropriate [presentation time stamp|MPEGTerminology]
to ensure it is
the right time
+That is, preceding and following in time. However, to ease the job of the decoder, the B-frame occurs in the video stream ''after'' the frames that it depends on. Thus, the temporal order of display of an B-frame between an I-frame and a P-frame might be IBP, but the order in which they appear in the stream is IPB. frame is to displayed the .
The optimal length of a GOP is a tradeoff; the more frames, the better the compression (at least until you hit something like a scene change where the differencing is no longer effective). But if you want to do random access or trick play, where you skip frames forward or backward through the video, you can really only skip to the I-frames, since the other frames cannot be decoded without starting from them. Also sometimes you may have seen on a digital TV broadcast where part of the screen is suddenly filled with green squares that may take an appreciable fraction of a second to disappear; this is where the received signal has lost part or all of an I-frame, so the decoder substitutes the green squares for the missing data, until the next I-frame comes along.
For all these reasons, the usual length of a GOP is enough frames to make up 0.5 to 1 second of video.