Method and apparatus for coding moving pictures

1. A moving picture coding method which uses intra-frame coding and forward motion compensation interframe predictive coding to a moving picture signal, in predetermined block units and at predetermined frame intervals and uses bidirectional motion compensation interframe predictive coding to plural B-frames between an I-frame and a P-frame, said I-frame being subjected to said intra-frame coding, said P-frame being subjected to said forward motion compensation interframe predictive coding, said method comprising:

reducing a magnitude of a forward motion vector in accordance with a frame distance between each B-frame and a reference frame (an I- or P-frame) used for said forward motion-compensated interframe prediction, said forward motion vector being obtained in a forward motion vector search of said P-frame, and then distributing a reduced vector as a forward motion vector to each of said B-frames;

performing a backward motion vector search to a B-frame following said I- or P-frame of said B-frames, with said I- or P-frame used as a reference frame;

reducing a magnitude of a backward motion vector in accordance with a frame distance between each B-frame and a reference frame (an I- or P-frame) used for said backward motion vector search, said backward motion vector being obtained in said motion vector search, and then distributing a reduced vector as a backward motion vector; and

using bidirectional motion compensation interframe predictive coding to each of said B-frames, based on said forward motion vector distributed and said backward motion vector distributed.

2. A moving picture coding method method which uses intra-frame coding and forward motion compensation interframe predictive coding to a moving picture signal, in predetermined block units and at predetermined frame intervals and uses bidirectional motion compensation interframe predictive coding to plural B-frames between an I-frame and a P-frame, said I-frame being subjected to said intra-frame coding, said P-frame being subjected to said forward motion compensation interframe predictive coding, said method comprising:

obtaining a forward motion vector by performing a forward motion vector search of said P-frame, with said I- or P-frame used as a reference frame, and then using forward motion compensation interframe predictive coding to said forward motion vector;

distributing a reduced vector as a forward motion vector to each of said B-frames, said reduced vector being obtained by reducing a magnitude of a motion vector, using said forward motion vector, and in accordance with a frame distance between a reference frame and each B-frame used for said forward motion vector search;

performing a backward motion vector search of a B-frame following said I- or P-frame of said B-frames, with said I- or P-frame used as a reference frame, and thus obtaining a backward motion vector;

distributing a reduced vector as a backward motion vector to each of said B-frames, said reduced vector being obtained by reducing a magnitude of said motion vector, using said backward motion vector, and in accordance with a frame distance between a reference frame and each B-frame used for said backward motion vector search; and

using a bidirectional motion compensation interframe predictive coding to each of said B-frames, based on said forward motion vector and said backward motion vector.

3. The method defined in claim 2 , further comprising:

changing a search range for shifting a motion vector search range by a forward motion vector from a reference position to obtain a shifted search range, said forward motion vector being obtained in said forward motion vector search to said reference frame, and thus determining said shifted search range as a current search range.

4. The method defined in claim 3 , wherein said search range changing comprises adaptively changing a search range so as to include an original search reference position when said forward motion vector is larger than a predetermined value.

5. The method defined in claim 2 , further comprising:

re-searching for obtaining a motion vector by performing a motion vector search over a predetermined range, using a bidirectional motion vector distributed to each B-frame, with a position corresponding to bidirectional motion vector within a reference frame used as a reference position, and determining said motion vector as a motion vector to each B-frame.

6. A moving picture coding apparatus, which uses intra-frame coding and forward motion compensation interframe predictive coding to a moving picture signal, in predetermined block units and at predetermined frame intervals and uses bidirectional motion compensation interframe predictive coding to plural B-frames between an I-frame and a P-frame, said I-frame being subjected to said intra-frame coding, said P-frame being subjected to said forward motion compensation interframe predictive coding, said apparatus comprising:

a frame rearranger for storing input frames and rearranging said input frames in the order of coding;

a motion searcher for obtaining a forward motion vector by receiving said P-frame and a reference frame (said I- or P-frame) and by performing a forward motion vector search, and for obtaining a backward motion vector by receiving a B-frame following said I- or P-frame of said B-frames and a reference frame (said I- or P-frame) and by performing a backward motion vector search;

a motion vector storage for holding each of motion vectors obtained by said motion searcher;

a vector distributor for distributing a reduced vector as a bidirectional motion vector to each B-frame, with said I- or P-frame used as a reference frame, said reduced vector being obtained by reducing the magnitude of each motion vector, using said forward motion vector and said backward motion vector and in accordance with a frame distance to said I- or P-frame;

a predictive error creator for computing a differential to a predictive picture using said each motion vector and said reference frame, in said P- or B-frame, and then outputting a predictive error signal;

a frequency converter/quantizer for frequency converting and quantizing said I-frame or said predictive error signal and then outputting quantized data;

a variable-length encoder for performing a variable-length coding of said quantized data and then outputting encoded data;

an inverse quantizer/inverse frequency converter for inverse quantizing and inverse-frequency converting said quantized data and then outputting decoded data;

a reference frame creator for creating a reference frame from said decoded data through motion-compensated interframe prediction; and

a frame memory for storing said reference frame.

7. The apparatus defined in claim 6 , wherein said motion vector searcher shifts a motion vector search range by said forward motion vector from a reference position to obtain a shifted search range and then determines said shifted search range as a current search range, said forward motion vector being obtained through a forward motion vector search of a reference frame used for said forward motion vector search.

8. The apparatus defined in claim 7 , wherein said motion vector searcher adaptively changes a search range so as to include an original search reference position when said forward motion vector is larger than a predetermined value.

9. The apparatus defined in claim 6 , wherein said motion vector searcher obtains a motion vector by performing a motion vector search of each of said B-frames over a predetermined range, using said bidirectional motion vector, with a position within a reference frame corresponding to said bidirectional motion vector used as a reference position, and then determines said motion vector as a motion vector in each B-frame.

10. A signal-bearing medium tangibly embodying a program of machine-readable instructions executable by a digital processing apparatus to perform a method of coding moving pictures, wherein said method uses intra-frame coding and forward motion compensation interframe predictive coding to a moving picture signal, in predetermined block units and at predetermined frame intervals and uses bidirectional motion compensation interframe predictive coding to plural B-frames between an I-frame and a P-frame, said I-frame being subjected to said intra-frame coding, said P-frame being subjected to said forward motion compensation interframe predictive coding, said method comprising:

reducing a magnitude of a forward motion vector in accordance with a frame distance between each B-frame and a reference frame (an I- or P-frame) in said forward motion-compensated interframe prediction, said forward motion vector being obtained in a forward motion vector search of said P-frame, and then distributing a reduced vector as a forward motion vector to each of said B-frames;

performing a backward motion vector search to a B-frame following said I- or P-frame of said B-frames, with said I- or P-frame used as a reference frame;

reducing a magnitude of a backward motion vector in accordance with a frame distance between each B-frame and a reference frame (an I- or P-frame) in said backward motion vector search, said backward motion vector being obtained in said motion vector search, and then distributing a reduced vector as a backward motion vector; and

using bidirectional motion compensation interframe predictive coding to each of said B-frames, based on said forward motion vector distributed and said backward motion vector distributed.

11. A signal-bearing medium tangibly embodying a program of machine-readable instructions executable by a digital processing apparatus to perform a method of coding moving pictures, wherein said method uses intra-frame coding and forward motion compensation interframe predictive coding to a moving picture signal, in predetermined block units and at predetermined frame intervals and uses bidirectional motion compensation interframe predictive coding to plural B-frames between an I-frame and a P-frame, said I-frame being subjected to said intra-frame coding, said P-frame being subjected to said forward motion compensation interframe predictive coding, said method comprising:

obtaining a forward motion vector by performing a forward motion vector search of said P-frame, with said I- or P-frame used as a reference frame, and then using forward motion compensation interframe predictive coding to said forward motion vector;

distributing a reduced vector as a forward motion vector to each of said B-frames, said reduced vector being obtained by reducing the magnitude of a motion vector, using said forward motion vector, and in accordance with a frame distance between a reference frame and each B-frame used for said forward motion vector search;

performing a backward motion vector search of a B-frame following said I- or P-frame of said B-frames, with said I- or P-frame used as a reference frame, and thus obtaining a backward motion vector;

distributing a reduced vector as a backward motion vector to each of said B-frames, said reduced vector being obtained by reducing the magnitude of said motion vector, using said backward motion vector, and in accordance with a frame distance between a reference frame and each B-frame used for said backward motion vector search; and

using a bidirectional motion compensation interframe predictive coding to each of said B-frames, based on said forward motion vector and said backward motion vector.

12. The signal-bearing medium defined in claim 11 , said method further comprising:

changing a search range for shifting a motion vector search range by a forward motion vector from a reference position to obtain a shifted search range, said forward motion vector being obtained in said forward motion vector search to said reference frame, and thus determining said shifted search range as a current search range.

13. The signal-bearing medium defined in claim 12 , wherein said changing said search range comprises:

adaptively changing a search range so as to include an original search reference position when said forward motion vector is larger than a predetermined value.

14. The signal-bearing medium defined in claim 11 , further comprising:

re-searching for obtaining a motion vector by performing a motion vector search search over a predetermined range, using a bidirectional motion vector distributed to each B-frame, with a position corresponding to said bidirectional motion vector within a reference frame used as a reference position, and determining said motion vector as a motion vector to each B-frame.

15. The signal-bearing medium of claim 10 , said signal-bearing medium comprising one of:

a standalone diskette containing said instructions, said diskette for inserting into a computer drive:

a hard drive in a computer; and

a program memory in one of a computer and a computer peripheral, said program memory storing instructions for a program currently being executed.