Video encoding method and system

Details Video encoding method and system Video encoding method and system

2000-04-26

2002-08-27

Britton, Howard (Department: 2713)

Pulse or digital communications

Bandwidth reduction or expansion

Television or motion video signal

C348S699000, C375S240150

Reexamination Certificate

active

06442204

ABSTRACT:

FIELD OF THE INVENTION
The invention relates to a method of encoding a source sequence of pictures comprising the steps of:
dividing a source sequence into a set of group of pictures, each group of pictures comprising a first frame, hereafter referred to as I-frame, followed by at least a pair of frames, hereafter referred to as PB-frames;
dividing each I-frame and PB-frame into spatially non-overlapping blocks of pixels;
encoding the blocks from said I-frame, hereafter referred to as I-blocks, independently from any other frame in the group of pictures;
deriving motion vectors and corresponding predictors for the blocks from the temporally second frame of said PB-frame, hereafter referred to as the P-blocks, based on the I-blocks in the previous I-frame or the P-blocks in the previous PB-frame;
deriving for each block from the first frame of said PB-frame, hereafter referred to as a B-block, a forward motion vector from said motion vector of the P-block with same location, allowing to obtain for each B-block an associated I-block in the previous I-frame or an associated P-block in the previous PB-frame, hereafter referred to as If-block or Pf-block, respectively;
deriving for each B-block of the first frame of said PB-frame, a backward motion vector from said motion vector of the P-block with same location, allowing to obtain for each B-block an associated P-block in the P-frame of said PB-frame, hereafter referred to as the Pb-block;
choosing a prediction mode for the encoding of each B-block;
predictively encoding the P-blocks of the second frame of said PB-frame based on the I-blocks in the previous I-frame or the P-blocks in the previous PB-frame;
predictively encoding the B-blocks following the chosen prediction mode.
The invention also relates to a system for carrying out said method.
The invention may be used, for example, in video coding at very low bit rate.
BACKGROUND ART
Standardization of low bitrate video telephony products and technology by the ITU (International Telecommunication Union) are compiled in the standards H.320 and H.324. These standards describe all the requirements to be satisfied for the different components audio, video, multiplexer, control protocol and modem. H.320 is dedicated to videoconferencing or videophony over ISDN (Integrated Services Data Network) phone lines. H.324 is aimed at videophony over GSTN (Global Switch Telephonic Network) analog phone lines. The two standards both support for video-coding the Recommendation H.263, which describes compression of low bitrate video signals. The H.263 Recommendation comprises four optional modes for a video coder. One of these optional modes is called the PB-frames mode, which gives a way of encoding a PB-frame. A second version of the H.263 Recommendation, called H.263+, was developed to improve the image quality and comprises some new options. Thus, an option called Improved PB-frames mode, which is an improvement of the original PB-frames mode, provides a new way of encoding a PB-frame.
A sequence of picture frames may be composed of a series of I-frames and PB-frames. A I-frame consists in a picture coded according to an Intra mode, which means that a I-frame is coded using spatial redundancy within the picture without any reference to another picture. A P-frame is predictively encoded from a previous P or I-picture. Thus, when coding a P-picture, temporal redundancy between the P-picture and a previous picture used as a picture reference, which is mostly the previous I or P-picture, is used in addition to the spatial redundancy as for a I-picture. A B-picture has two temporal references and is usually predictively encoded from a previous P or I-picture and the P-picture currently being reconstructed. A PB-frame consists of two successive pictures, a first B-frame and a subsequent P-frame, coded as one unit.
A method of coding a PB-frame in accordance to the PB-frames mode is illustrated in FIG.
1
. It is shown a PB-frame composed of a B-frame B and a P-frame P
2
. The B-frame B is surrounded by a previous P-picture P
1
and the P-picture P
2
currently being reconstructed. It is shown in this example a P-picture P
1
, P
1
may also be a I-picture and serves as a picture reference for the encoding of the P-picture P
2
and the B-picture B. A B-block from the B-frame, in the PB-frames mode, can be forward or bidirectionally predictively encoded. The encoding of a B-block being forward predicted is based on the previous I or P-picture P
1
and the encoding of a B-block being bidirectionally predicted is based on both the previous I or P-picture P
1
and the P-picture P
2
currently being reconstructed. A set of motion vectors MV is derived for the P-picture P
2
of the PB-frame with reference to the picture P
1
. In fact for each macroblock of P
2
, a macroblock of P
1
is associated by block matching and a corresponding motion vector MV is derived. Motion vectors for the B-block are derived from the set of motion vectors derived previously for P
1
. Therefore a forward motion vector MVf and a backward motion vector MVb are calculated for a B-block as follows:
MVf
=(
TRb×MV
)/
TRd
MVb
=((
TRb−TRd
)×
MV
)/
TRd
  (1)
MVb=MVf−MV
where
TRb is the increment in the temporal reference of the B-picture from the previous P-frame P
1
, and
TRd is the increment in the temporal reference of the current P-frame P
2
from the previous I or P-picture P
1
.
It is considered on
FIG. 1
a macroblock AB of the B-picture. This macroblock AB has the same location as a macroblock A
2
B
2
, Prec, of P
2
that was previously reconstructed. A forward motion vector MV is associated to the macroblock A
2
B
2
from a macroblock A
1
B
1
, which belongs to P
1
. A forward motion vector MVf and a backward motion vector MVb, both associated with AB, are derived from MV as shown in (1). The macroblocks of P
1
and P
2
associated with the AB macroblock by the forward vector MVf and by the backward vector MVb are respectively K
1
M
1
and K
2
M
2
as illustrated on FIG.
1
.
The choice between bidirectional prediction and forward prediction is made at the block level in the B-picture and it depends on where MVb points. Then a MB part of the B-block AB, for which MVb points inside Prec, is bidirectionally predicted and the prediction for this part of the B-block is:
MB(ij)=[A
1
M
1
(ij)+A
2
M
2
(ij)]/2 where i and j are the spatial coordinates of the pixels.
A AM part of the B-block AB, for which MVb points outside Prec, is forward predicted and the prediction for this part of the B-block AB is:
AM(i,j)=K
1
A
1
(i,j).
An improved method of encoding a PB-frame according to the PB-frames mode is described in the European Patent Application EP 0 782 343 A2. It discloses a predictive method of coding the blocks in the bidirectionally predicted frame, which method introduces a delta motion vector added to or subtracted from the derived forward and backward motion vectors respectively. The described method may be relevant when the motion in a sequence of pictures is non-linear, however it is totally unsuitable for a sequence of pictures where scene-cuts occur. Indeed, when there is a scene cut between a previous P-frame and the B-part of a PB-frame, bidirectional and forward prediction give an erroneous coding. Besides the implementation of the delta vector, which is costly in terms of CPU burden, may result in unnecessary expensive and complicated calculations.
SUMMARY OF THE INVENTION
It is an object of the invention to ameliorate efficiency of existing coding methods, while decreasing CPU burden, and more precisely an object of the invention is to provide an efficient strategy or method which permits to make the most suitable choice among prediction modes for the coding of a given macroblock of a B-frame.
Thus, the choice of the prediction mode for the encoding of each B-block comprises for each B-block in series the steps of:
deriving the sum of absolute difference between the B-block and a block with pixels values being the means of the

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