Method and apparatus for transcoding coded picture signals from object-based coding to block-based coding

DETAILED DESCRIPTION The invention provides a transcoder for transcoding a coded object-based picture signal that represents a picture to a coded block-based picture signal that also represents the picture.
The coded object-based picture signal may be an MPEG-4 picture signal, for example, and the coded block-based picture signal may be an MPEG-2 picture signal, for example.
The transcoder comprises a culling module, a picture composer and a partial encoder.
The culling module receives the coded object-based picture signal and culls signal portions from the coded object-based picture signal to generate a culled object-based picture signal.
The signal portions culled are those that represent objects not visible in the picture.
The picture composer receives the culled object-based picture signal, partially decodes selected portions of the culled object-based picture signal and generates from them blocks of a partially-coded block-based picture signal in which the blocks have different coding states.
The partial encoder receives the partially-coded block-based picture signal and encodes the blocks of the partially-coded block-based picture signal to generate the coded block-based picture signal in which the blocks have a uniform coding state.
The coded block-based picture signal is capable of being decoded by a conventional block-based decoder.
The culling module may include an object culling module that culls, from the coded object-based picture signal, signal portions that represent objects that are not present in the picture and signal portions that represent objects that are present in the picture but are hidden.
The object-based picture signal may include a scene descriptor that describes the arrangement of the objects in the picture and may additionally include a coded shape descriptor for each of the objects.
The object culling module may use the scene descriptor to identify the signal portions that represent the objects not present in the picture and may decode the coded shape descriptors of the objects identified as being present in the picture to identify the signal portions that represent the objects that are present in the picture, but are hidden