This method is sometimes called “logarithmic,” which I consider to be a very poor term in this context.

The fractions 0.6, 0.3, and 0.1 are comparable to the SD luma coefficients of green, red, and blue; so, I use green, red, and blue to designate I-picture, P-picture, and B-picture data respectively in Figures 16.2, 16.3, and 16.4 (page 153).

the reference frame. For the large ranges of MPEG-2, full block matching is impractical.

• Pixel-recursive (or pel-recursive) methods start with a small number of initial guesses at motion, based upon motion estimates from previous frames. The corresponding coordinates in the reference frame are searched, and each guess is refined. The best guess is taken as the final motion vector.

• Pyramidal methods form spatial lowpass-filtered versions of the target macroblock, and of the reference frames; block matches are performed at low resolution. Surrounding the coordinates of the most promising candidates at one resolution level, less severely filtered versions of the reference picture regions are formed, and block matches are performed on those. Successive refinement produces the final motion vector. This technique tends to produce smooth motion-vector fields.

Rate control and buffer management

A typical video sequence, encoded by a typical MPEG-2 encoder, produces I-, P-, and B-pictures that consume bits roughly in the ratio 60:30:10. An I-picture requires perhaps six times the number of bits as two B-pictures.

Many applications of MPEG-2, such as DTV, involve a transmission channel with a fixed data rate. This calls for constant bit rate (CBR) operation. Other applications of MPEG-2, such as DVD, involve a channel having variable (but limited) data rate. Such applications call for variable bit rate (VBR) operation, where the instantaneous bit rate is varied to achieve the desired picture quality for each frame, maximizing storage utilization.

The larger the decoder’s buffer size, the more flexibility is available to the encoder to allocate bits among pictures. However, a large buffer is expensive. Each profile/level combination dictates the minimum buffer size that a decoder must implement.

An encoder effects rate control by altering the quantization matrices – the perceptually weighted matrix used for intra macroblocks, and the flat matrix used for nonintra macroblocks. MPEG-2 allows quantizer matrices to be included in the bitstream. Additionally, and more importantly, a quantizer scale code is transmitted at the slice level, and may be updated at the

Figure 47.9 Buffer occupancy in MPEG-2 is managed through an idealized video buffering verifier (VBV) that analyzes the output bitstream produced by any encoder. This graph shows buffer occupancy for a typical GoP.