1. Field
The invention relates to a video coding technique, and more particularly to a video coding device, a video decoding device and a video coding method which are applicable to an image coding system employing interframe prediction such as the MPEG (Moving Picture Experts Group) system.
2. Description of the Related Art
In a video coding technique such as the MPEG system, orthogonal transform such as DCT (Discrete Cosine Transform) is employed for reduction of spatial information quantity and motion compensation (MC) is employed for reduction of time information. General MC is performed by coding the frame prediction using a motion vector, i.e. a difference (predictive residual) between a present picture and a predicted picture (for example, Peter Pirsch, “VLSI architectures for Video Compression-A Survey”, PROCEEDINGS OF THE IEEE (USA), IEEE, February 1995, Vol. 83. No. 2, p. 220-246). Therefore, as the predictive residual increases, the quantity of codes such as a DCT coefficient increases, thus reducing the coding efficiency. Further, in MPEG 4, a shape coding technique is adopted which recognizes objects in the picture and codes the shape information of each of the objects (for example, Po-chih Tseng et al, “Advances in Hardware Architectures for Image and Video Coding-A Survey”, PROCEEDINGS OF THE IEEE, (USA), IEEE, January 2005, Vol. 93, No. 1, p. 184-197). In MPEG 4, the object and background constituting one scene are separated and different coding processes can be performed for the foreground and background. For the background, only the pattern (pixel value) indicative of the luminance and chrominance is coded like MPEG 2, whereas for the foreground, the shape information as well as the pattern information is coded.
However, according to a region of a foreground object (hereinafter referred to as a foreground region) moving on the background image in a video, a shadow region corresponding to the foreground region also moves on the background image. As a result, the predictive residual increases between the region on which the shadow region is newly reflected and the region on which the shadow region has not been reflected. Thus, owing to an increase in the predictive residual, the coding efficiency of the video is reduced. Not only where the foreground region and shadow region move in parallel, but also where the foreground region approaches the background image or a camera position is shifted, the coding efficiency of the video is reduced.