DEVICE AND METHOD FOR SMOOTHING PICTURE SIGNAL, DEVICE AND METHOD FOR ENCODING PICTURE AND DEVICE AND METHOD FOR DECODING PICTURE

A great decrease of a step, i.e., block distortion, of such as unnatural, abnormal or artifact brightness and color, on a block boundary, leads to much visual improvement of picture quality. Against a picture that has been encoded and decoded by block, the smoothing filter characteristics is changed according to quantizing parameter, block activity of a decoded picture, the activity of each pixel of the decoded picture, and the position with respect to a block boundary of a pixel. A smoothing filter utilizes a weighted mean whose weight is the inverse number of a difference between a target pixel and its surrounding pixels. The weighted mean is obtained by adding a certain number to a weight before taking its inverse number. The value to be added is controlled by the parameter. Similarly, the mixing ratio of a pixel value after performing the smoothing filter and a decoded pixel value is changed depending on the parameter. A filter having strong edge preserving property is utilized for the inside of a block, and a filter having weak edge preserving property is utilized for a block boundary. A filter having edge preserving property is utilized for the post/loop filter.