Image coding method and image decoding method

Abstract

An image coding method for coding a current block, by assigning two or more reference picture indexes to one or more reference pictures that are different from a current picture which includes the current block, and the image coding method comprises the following when the two or more reference picture indexes are used to code the current block: determining whether or not reference pictures identified by two or more reference picture indexes are identical to each other; and switching, based on a result of the determining, between whether or not a prediction direction for coding the current block in a predetermined coding mode is fixed.

1 . (canceled) 
     
     
         2 . (canceled) 
     
     
         3 . An encoder comprising:
 a memory; and   a processor coupled to the memory and configured to:
 determine whether to fix an inter-prediction mode for a frame to a first mode, the first mode using a single reference frame; 
 perform inter-prediction on blocks in the frame in the first mode when the inter-prediction mode is determined to be fixed to the first mode, the blocks having positions different from each other; and 
 store, in header information, a first flag indicating whether the inter-prediction mode is fixed to the first mode, wherein 
   when the inter-prediction mode is determined not to be fixed to the first mode, the processor is configured to:
 perform inter-prediction on each block in the frame in the first mode or a second mode, the second mode being an inter-prediction mode using two reference frames; and 
 store, in the header information, second flags for the respective blocks, each of the second flags indicating whether the first mode or the second mode is used to perform inter-prediction on a corresponding block which corresponds to the each of the second flags, and 
   the processor is configured not to store the second flags in the header information when the inter-prediction mode is determined to be fixed to the first mode.   
     
     
         4 . The encoder according to  claim 3 , wherein
 the first flag having a first value indicates that the inter-prediction mode is fixed to the first mode, and   the first flag having a second value indicates that the inter-prediction mode is selectable from among modes including the first mode and the second mode.   
     
     
         5 . The encoder according to  claim 3 , wherein the first mode is a direct mode in which a motion vector for a first block is predicted from a second block which is adjacent to the first block. 
     
     
         6 . A decoder comprising:
 a memory; and   a processor coupled to the memory and configured to:
 read, from header information, a first flag indicating whether an inter-prediction mode is fixed to a first mode, the first mode using a single reference frame; and 
 perform inter-prediction on blocks in the frame in the first mode when the first flag indicates that the inter-prediction mode is fixed to the first mode, the blocks having positions different from each other, wherein 
   when the first flag indicates that the inter-prediction mode is not fixed to the first mode, the processor is configured to:
 read, from the header information, second flags for the respective blocks, each of the second flags indicating whether the first mode or a second mode is used to perform inter-prediction on a corresponding block which corresponds to the each of the second flags, the second mode being an inter-prediction mode using two reference frames; and 
 perform inter-prediction on each block in the frame in the first mode or the second mode according to a corresponding second flag which corresponds to the each block, and 
   the processor is configured to read the header information without the second flags when the first flag indicates that the inter-prediction mode is fixed to the first mode.   
     
     
         7 . The decoder according to  claim 6 , wherein
 the first flag having a first value indicates that the inter-prediction mode is fixed to the first mode, and   the first flag having a second value indicates that the inter-prediction mode is selectable from among modes including the first mode and the second mode.   
     
     
         8 . The decoder according to  claim 6 , wherein the first mode is a direct mode in which a motion vector for a first block is predicted from a second block which is adjacent to the first block. 
     
     
         9 . A decoder comprising:
 a memory; and   a processor coupled to the memory and configured to:
 read, from header information, a first flag indicating whether an inter-prediction mode is fixed to a bi-prediction mode; and 
 perform inter-prediction on blocks in a frame in the bi-prediction mode when the first flag indicates that the inter-prediction mode is fixed to the bi-prediction mode, the blocks having positions different from each other, wherein 
   when the first flag indicates that the inter-prediction mode is not fixed to the bi-prediction mode, the processor is configured to:
 read, from the header information, second flags for the respective blocks, each of the second flags indicating whether the bi-prediction mode or a uni-prediction mode is used to perform inter-prediction on a corresponding block which corresponds to the each of the second flags; and 
 perform inter-prediction on each block in the frame in the bi-prediction mode or the uni-prediction mode according to a corresponding second flag which corresponds to the each block, and 
   the processor is configured to read the header information without the second flags when the first flag indicates that the inter-prediction mode is fixed to the bi-prediction mode.   
     
     
         10 . The decoder according to  claim 9 , wherein the bi-prediction mode is a direct mode in which a motion vector for a first block is predicted from a second block which is adjacent to the first block.