Method and apparatus for encoding and decoding an image using a reference picture

Abstract

A method and apparatus for encoding and decoding an image by performing motion prediction and compensation on pictures in a group of pictures by selectively using a high-quality key picture that is previously encoded and restored, and a second picture that is previously encoded and restored. The method of encoding an image improves the prediction efficiency of an image by storing a key picture, which is encoded and restored to a high quality by reducing a loss caused by a quantization error using a small quantization coefficient, in a first storage unit and storing a previously encoded and restored second picture in a second storage unit, and then performing motion prediction and compensation by selectively using the key picture stored in the first storage unit and the second picture stored in the second storage unit while encoding a next picture in a group of pictures.

What is claimed is: 
     
         1 . A method of decoding an image, comprising:
 decoding a first picture from among pictures included in an encoded group of pictures in a bitstream and storing the first picture in a first storage unit;   decoding a second picture from among pictures included in an encoded group of pictures in the bitstream and storing the second picture in a second storage unit; and   decoding a current picture by using at least one of the first picture stored in the first storage unit and the second picture of the group of pictures.   
     
     
         2 . The method of  claim 1 , wherein the first picture is a key picture that is initially decoded from among the pictures included in the group of pictures. 
     
     
         3 . The method of  claim 1 , wherein the decoding of the current picture comprises:
 extracting reference picture information of a current block of the current picture to be decoded from the bitstream;   determining a reference picture from among the first and second pictures by using the reference picture information, and generating a prediction block of the current block by performing motion compensation using the determined reference picture; and   decoding the current block by adding a residual in the bitstream and the prediction block of the current block.   
     
     
         4 . An apparatus for decoding an image, comprising:
 a first storage unit which stores a decoded first picture from among pictures included in an encoded group of pictures in a bitstream;   a second storage unit which stores a previously decoded second picture of the group of pictures; and   a decoder which decodes a current picture by using at least one of the decoded first picture and the decoded second picture.   
     
     
         5 . The apparatus of  claim 4 , wherein the first picture is a key picture that is initially decoded from among the pictures included in the group of pictures. 
     
     
         6 . The apparatus of  claim 4 , wherein the decoder comprises:
 an entropy decoder which extracts reference picture information of a current block of the current picture to be decoded from the bitstream;   a predictor which determines a reference picture from among the first and second pictures by using the reference picture information, and generates a prediction block of the current block by performing motion compensation using the determined reference picture;   a residual restorer which restores a residual in the bitstream by inverse quantizing and inverse transforming the residual; and   an adder which decodes the current block by adding the restored residual and the prediction block of the current block.