MPEG-4: differenze tra le versioni

Versione delle 17:52, 8 ott 2005

Versione delle 01:02, 12 dic 2005

Riga 32:

[[MPEG-1]], [[MPEG-2]], [[MPEG-7]] e [[MPEG-21]] sono altri standard MPEG.

There are two ways to encode a video frame in an MPEG-4 Simple Profile codec: as an I-frame or as a P-frame4. An I-frame is a video frame that has been encoded without reference to any other frame of video. A video stream or recording will always start with an I-frame and will typically contain regular I-frames throughout the stream. These regular I-frames, also called intra frames, key frames or access points, are crucial for the random access of recorded MPEG-4 files, such as with rewind and seek operations during playback and the regularity of these I-frames is known as the I-frame interval. However, the disadvantage of I-frames is that they tend to be much larger than P-frames.

P-frames are motion-compensated frames: that is to say the encoder makes use of the difference between the current frame being encoded and a previous frame of video, ensuring that information that does not change, e.g. a static background, is not repeatedly transmitted. Unlike purely difference-based codecs, such as delta-MJPEG, MPEG-4 not only looks for differences but searches for, and makes use of, motion that has occurred in the video. This means that motion-compensated codecs will typically outperform simple difference-based codecs when there is motion. The process of searching for motion is known as motion estimation.

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