MPEG-1 Main Components


MPEG uses I, P, and B pictures, as discussed in Section 6.4. They are arranged in groups, where a group can be open or closed. The pictures are arranged in a certain order, called the coding order, but are output, after decoding, and sent to the display in a different order, called the display order. In a closed group, P and B pictures are decoded only from other pictures in the group. In an open group, they can be decoded from pictures outside the group. Different regions of a B picture may use different pictures for their decoding. A region may be decoded from some preceding pictures, from some following pictures, from both types, or from none. Similarly, a region in a P picture may use several preceding pictures for its decoding, or use none at all, in which case it is decoded using MPEG’s intra methods.

The basic building block of an MPEG picture is the macroblock It consists of a 16×16 block of luminance (grayscale) samples (divided into four 8×8 blocks) and two 8×8 blocks of the matching chrominance samples. The MPEG compression of a macroblock consists mainly in passing each of the six blocks through a discrete cosine transform, which creates decorrelated values, then quantizing and encoding the results. It is very similar to JPEG compression (Section 4.8), the main differences being that different quantization tables and different code tables are used in MPEG for intra and nonintra, and the rounding is done differently. A picture in MPEG is organized in slices, where each slice is a contiguous set of macroblocks (in raster order) that have the same grayscale (i.e., luminance component). The concept of slices makes sense because a picture may often contain large uniform areas, causing many contiguous macroblocks to have the same grayscale.