Video Coding

A popular and effective video coding method is the one that uses block-based temporal prediction and transform coding. This method is essentially the core of all the international video coding standards.

* Block-Based Hybrid Video Coding
- Block-based motion estimation => MV
- Motion compensated predication => predicated block
- DCT on predication error block
- Quantization of DCT coeffs
- Run-length scanning and variable length coding on MV and quantified coeffs
- Loop filtering to reduce block artifacts on reconstructed picture
Thus the temporal, spatial and symbol redundancy are reduced as much as possible.

The MB is the basic unit of motion estimation and compensation. In general, three modes for MB are I-, P- and B-mode. Because MV and DC coeffs (only for I-mode MB) of adjacent MBs or blocks are similar, they are typically coded predictively within the same GOB or slice to avoid the error propagation. This is extended in H.264 and called intra predication. The quantization parameter or factor is also determined in the MB level.

A frame could be coded entirely in the intra-mode. This picture is called I-picture or intra-picture. A MB in P-picture could be coded in I- or P-mode while one MB in B-picture could be in I-, P- and B-mode based on the predication error. Both P- and B-picture are called inter-picture. Also the picture structure of them could be field and frame.

For ME/MC, the key pointer is block size, search range, search precision, MV predication, block predication, MV number, etc. The improvement includes unrestricted motion vector, OBMC, global MC, multiple references, etc. For DCT, more advanced transformation could be used, like integer transform, wavelet, etc. For quantization, the stepsize (QP) and non-linear quantization or vector quantization have been explored. Alternate scanning (Horizontal or vertical) is proposed and so are arithmetic VLC and 3-D VLC instead of Huffman coding. For improved performance, the choice between different parameters can be determined by a rate-distortion optimization approach.

One important issue in video coding is rate control, which refers to how to code a video so that the resulting bitstream satisfies a target bit rate. This leads to how to choose the coding parameters, like frame rate, MB mode, QP, etc., to meet the rate constraint. The desired target bit rate is the average rates over short intervals of time due to the VLC video coder and the complexity of the scene. the variability of the bit rate within each interval must be handled by a smoothing buffer following the encoder. The buffer size is determined by the time interval and delay requirement of the application. RC is typically accomplished in three steps:
- Update the target average bitrate for each short time interval, based on the bandwidth, delay requirement and buffer fullness
- Determine the coding mode for picture (I, B and P) and the target bit budget for each picture to be coded in this interval, based on the target average rate for the interval and the current buffer fullness
- Determine the coding mode and QP for each MB in a picture to meet the target rate for this frame