**4.3.1 Reducing motion estimation complexity**

Within the WZ decoding process, an important task is the SI generation stage, which is the first step in the process for generating the WZ frames from K frames. VISNET-II performs Motion Compensated Temporal Interpolation (MCTI) to estimate the SI. The first step of this method is shown in Figure 5, which consists in matching each forward frame MB with a backward frame MB inside the search area. The process checks all the possibilities inside the search area and chooses the MV that generates the lowest residual. The middle of this MV represents the displacement for the MB interpolated (more details about the SI generation process in (Ascenso et al., 2005)).

Fig. 5. First step of SI generation process.

Obviously, MVs generated in the WZ decoding stage contain approximated information about the quantity of movement of the frame. Following this idea, the present approach proposes to reuse the MVs to accelerate the H.264/AVC encoding stage by reducing the search area of the ME stage. Moreover, the present reduction is adjusted for every input DVC GOP to every H.264/AVC GOP in an efficient and dynamic way. As is shown in Figure 6, the search area for each MB is defined by a circumference with a radius dependent on the incoming SI MV (*Rmv*). This search area can oscillate between a minimum (defined by *Rmin*) and a maximum (limited by the H.264 search area). In particular, the length will vary depending on the type of frame and the length of the reference frame, as will be explained in section 4.2.2. Furthermore, a minimum area is considered since MVs are calculated from 16x16 MBs in the SI process, and H.264/AVC can even work with smaller partitions than 16x16. Besides, SI is an approximation of the frame, so some changes could occur when the fame is completely reconstructed. For these reasons, this minimum was set at 4 pixels.

Fig. 6. Search area reduction for H.264 encoding stage.
