**2.2 H.264/AVC**

16 Video Compression

full of multicore systems, an approach is proposed to execute WZ decoding in a parallel way. On the other hand, at the same time WZ is decoding, some information could be gathered are sent to the H.264/AVC encoder in order to reduce the encoding algorithm complexity. In this work, the search area of the Motion Estimation (ME) process is reduced by means of Motion Vectors (MVs) calculated in the WZ decoding algorithm. In this way, the complexity of the two most complex tasks of this framework (WZ decoding and H.264/AVC encoding) are largely reduced making the transcoding process more efficient.

The first practical Wyner-Ziv framework was proposed by Stanford in (Aaron et al., 2002), and this work was widely referenced and improved in later proposals. As a result, in (Artigas et al., 2007) an architecture called DISCOVER was proposed which outperforms the previous Stanford one. This architecture provided a reference for the research community and finally it was later improved upon with the VISNET-II architecture (Ascenso et al., 2010), which is depicted in Figure 1. In this architecture, the encoder splits the sequence into two kinds of frames: Key Frames (K) and Wyner-Ziv Frames (WZ) in module (1). K frames are encoded by an H.264/AVC encoder in (2). On the other hand, WZ frames are sent to the WZ encoder, where the information is firstly quantized (3a), and BitPlanes (BPs) are extracted in (3b); in (3c) each BP is independently channel encoded and several parity bits, which are stored in a buffer (3d), are calculated. On the decoder side, initially K frames are decoded by an H.264/AVC decoder (4). From these frames, Side Information (SI) is calculated in (5), which represents an estimation for each non-present original WZ frame. For this estimation, the Correlation Noise Model (CNM) module (6) generates a Laplacian distribution, which models the residual between SI and the original frame. Afterwards, SI and CNM are sent to the turbo decoder, which corrects differences of SI and the original frame by means of iterative decoding (requesting several parity bits from the encoder through the feedback channel). Finally, decoding bitplanes are reconstructed in module (7c).

Fig. 1. Block diagram of the reference WZ architecture [Ascenso et al. 2010].

**2. Background** 

**2.1 Wyner-Ziv video coding** 

H.264/AVC or MPEG-4 part 10 Advanced Video Coding (AVC) is a compression video standard developed by the ITU-T Video Coding Experts Group (ITU-T VCEG) together with the ISO/IEC Moving Picture Experts Group (MPEG). In fact, both standards are technically identical (ISO/IEC, 2003).

The main purpose of H.264/AVC is to offer a good quality standard able to considerably reduce the output bit rate of the encoded sequences, compared with previous standards, while exhibiting a substantially increasing definition of quality and image. H.264/AVC promises a significant advance compared with the commercial standards currently most in use (MPEG-2 and MPEG-4). For this reason H.264/AVC contains a large amount of compression techniques and innovations compared to previous standards; it allows more compressed video sequences to be obtained and provides greater flexibility for implementing the encoder. Figure 2 shows the block diagram of the H.264/AVC encoder.

Fig. 2. H.264/AVC encoder diagram

The ME is the most time-consuming task in the H.264/AVC encoder. It is a process which removes the temporal redundancy between images, comparing the current one with previous or later images in terms of time (reference images), looking for a pattern that indicates how the movement is produced inside the sequence.

To improve the encoding efficiency, H.264/AVC allows the use of partitions resulting from dividing the MB in different ways. Greater flexibility for the ME and Motion Compensated (MC) processes and greater motion vector precision give greater reliability to the H.264/AVC encoding process. The ME process is thus carried out many times per each partition and sub-partition. This feature is known as variable block size for the ME.
