Image decoding method, image coding method, image decoding apparatus, image coding apparatus and integrated circuit for generating a code stream with a hierarchical code structure

Abstract

An image decoding method decodes a coded stream which includes processing units and a header of the processing units, and which is generated by coding a moving picture using inter prediction. The processing units includes at least one processing unit divided in a hierarchy, the hierarchy including a highest hierarchical layer in which a coding unit exists as a largest processing unit and a lower hierarchical layer in which a prediction unit exists. The method includes identifying, by parsing hierarchy depth information stored in the header and indicating a hierarchical layer higher than a lowest hierarchical layer in which a smallest prediction unit exits, a hierarchical layer which is indicated by the hierarchy depth information or a hierarchical layer higher than the indicated hierarchical layer. The hierarchical layer includes a prediction unit in which a reference index is stored. The prediction unit is decoded using the reference index.

The invention claimed is: 
     
       1. An image decoding method for decoding a coded stream, the image decoding method comprising:
 receiving a coded stream which includes a plurality of processing units and a header of the processing units, the coded stream having been generated by coding an image using inter prediction,
 wherein the processing units including at least one processing unit divided into a plurality of sub-processing units in a multiple-level hierarchy in which the size of a sub-processing unit at one level is larger than the size of a sub-processing unit at a lower level, 
 wherein the hierarchy including a prediction unit that is sub-processing unit at a level lower than the highest level, 
 wherein a reference index necessary for decoding said prediction unit is stored in either said processing unit that is divided or a sub-processing unit at a level higher than said prediction unit, and 
 wherein said header including hierarchy depth information indicating the unit where the reference index is stored; 
   parsing said header to obtain said hierarchy depth information;   using said hierarchy depth information to obtain said reference index; and   decoding said prediction unit using said reference index.   
     
     
       2. An image decoding apparatus for decoding a coded stream the image decoding apparatus comprising:
 a hardware processor; and   a non-transitory, computer-readable memory device coupled to said hardware processor, said hardware processor, using said memory device, perform the following steps:   receiving a coded stream which includes a plurality of processing units and a header of the processing units, the coded stream having been generated by coding an image using inter prediction,
 wherein the processing units including at least one processing unit divided into a plurality of sub-processing units in a multiple-level hierarchy in which the size of a sub-processing unit at one level is larger than the size of a sub-processing unit at a lower level, 
 wherein the hierarchy including a prediction unit that is a sub-processing unit at a level lower than the highest level, 
 wherein a reference index necessary for decoding said prediction unit is stored hi either said processing unit that is divided or a sub-processing unit at a level higher than said prediction unit, and 
 wherein said header including hierarchy depth information indicating the unit where the reference index is stored; 
   parsing said header to obtain said hierarchy depth information;   using said hierarchy depth information to obtain said reference index; and   decoding said prediction unit using said reference index.   
     
     
       3. An image decoding apparatus for decoding a coded stream, the image decoding apparatus comprising:
 a receiver that receives a coded stream winch includes a plurality of processing units and a header of the processing units, the coded stream having been generated by coding an image using inter prediction,
 wherein the processing units including at least one processing unit divided into a plurality of sub-processing units in a multiple-level hierarchy in which the size of a sub-processing unit at one level is larger than the size of a sub-processing unit at a lower level, 
 wherein the hierarchy including a prediction unit that is a sub-processing unit at a level lower than the highest level, 
 wherein a reference index necessary for decoding said prediction unit is stored in either said processing unit that is divided or a sub-processing unit at a level higher than said prediction unit, and 
 wherein said header including hierarchy depth information indicating the unit where the reference index is stored; 
   an identifier that parses said header to obtain said hierarchy depth information and uses said hierarchy depth information to obtain said reference index; and   a decoder that decodes said prediction unit using said reference index.   
     
     
       4. A decoding method for decoding a coded stream to generate a picture, the coded stream including a coded picture including a plurality of sub coded units, each of the sub coded units including a plurality of sub prediction units, the decoding method comprising:
 parsing a first flag which indicates whether a reference index is stored in a sub coded unit, the first flag being in a picture header of the coded picture;   determining whether or not the first flag indicates that the reference index is stored in the sub coded unit   when the first flag is determined to indicate that the reference index is stored in the sub coded unit, parsing the reference index stored in the sub coded unit and decoding the sub coded unit by performing inter prediction in which (i) each of sub prediction units included in the sub coded unit identify a prediction mode and (ii) the reference index stored in the sub coded unit is applied to all of the sub prediction units included in the sub coded unit; and   when the first flag is determined not to indicate that the reference index is stored in the sub coded unit, parsing reference indexes stored in sub prediction units included in the sub coded unit and decoding the sub prediction units by performing inter prediction in which (i) each of the sub prediction units included in the sub coded unit identify the prediction mode and (ii) the reference indexes stored in the sub prediction units are applied to the sub prediction units.    
     
     
       5. The decoding method according to claim 4, wherein the sub coded unit is a lowest coded unit.  
     
     
       6. The decoding method according to claim 4, wherein when the first flag is determined to indicate that the reference index is stored in the sub coded unit, the reference index stored in the sub coded unit is a combined reference index.