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Unformatted text preview: res the current frame with the previous one and encodes the
motion vectors (i.e., the change in coordinate values due to the motion) and pixel
differences after motion. MPEG uses both intraframe and interframe techniques
for data compression.
In MPEG, encoded frames are of the following three type:
1. I (Intraframe). An I-frame is coded independently of any other frames in the
video sequence. Hence it contains all of the necessary information required to
reproduce a complete frame. It basically applies the intraframe compression of the
basic JPEG DCT algorithm.
2. P (Predicted). A P-frame is predicted from the differences from the previous Ior P-frame in the sequence.
3. B (Bidirectional). A B-frame is bidirectional interpolated by the preceding and
the subsequent I- 01 P-frames.
Note that due to the interframe dependency in case of P-frames and B-frames, a
current P-frame can be decoded only if the preceding I- or P-frame has been
presented to be decoded. Similarly, without both the preceding am subsequent Ior P-frame in the decoder, it is impossible to generate a B-frame. For example, if
the presentation order of a video sequence is IBBPBBPBBI.....then the actual input
sequence of MPEG frames to a decoder should be IPBBPBBIBB.....(see Figure
19.12). Therefore, the patterns of video frame sequence in storage and during
transmission are different from that for presentation.
The basic steps of MPEG compression (shown in Figure 19.13) are as follows (the
decoder performs the inverse operations):
1. The individual frames of the video data to be compressed are preprocessed
and color subsampling is performed on them to carry out the first level of
compression. Recall that color subsampling is approach shrinks the data size by
downsampling the chrominance component (i.e., using less bits to represent the
chrominance component) while leaving the luminance component unchanged.
2. Interframe motion compensation is then performed for the frames of types P
and B to carry out the second level of compression. This step is not needed for the
frames of type I because only intraframe...
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This document was uploaded on 04/07/2014.
- Spring '14