Facsimile and static presentation processing – Facsimile – Specific signal processing circuitry
Patent
1990-04-13
1992-01-21
Kostak, Victor R.
Facsimile and static presentation processing
Facsimile
Specific signal processing circuitry
358136, H04N 712
Patent
active
050832026
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The present invention concerns motion estimation, particularly, though not exclusively, in the context of video coders employing inter-frame differential coding.
BACKGROUND OF THE INVENTION
FIG. 1 shows a known form of video coder. Video signals (commonly in digital form) are received at an input 1. A subtractor 2 forms the difference between the input and a predicted signal from a predictor 3 which is then further coded in box 4. The coding performed here is not material to the present invention, but may include thresholding (to suppress transmission of zero or minor differencs) quantisation or transform coding for example. The input to the predictor is the sum, formed in an adder 2 of the prediction and the coded difference signal decoded in a local decoder 6 (so that loss of information in the coding and decoing process is included in the predictor loop).
The differential coding is essentially inter-frame, and the predictor 3 could simply consist of a one-frame delay; as shown however a motion estimator 7 is also included. This comapres the frame of the picture being coded with the previous frame being supplied to the predictor. For each block of the current frame (into which the picture is regarded as divided) it identifies that region of the previous frame which the block most closely resembles. The vector difference in position between the identified region and the block in question is termed a motion vector (since it usually represents motion of an object within the scene depicted by the television picture) and is applied to the predictor to shift the identified region of the previous frame into the position of the relevant block in the current frame, thereby making the predictor output a better prediction. This results in the differences formed by the substractor 2 being, on average, smaller and permits the coder 4 to encode the picture using a lower bit rate than would otherwise be the case.
The motion estimator must typically compare each block with the corresponding block of the previous frame and regions positionally shifted from that block position; this involves a considerable amount of processing and often necessitates many accesses to stored versions of both frames.
The present invention is defined in the claims.
BRIEF DESCRIPTION OF THE DRAWING FIGURES
One embodiment of the invention will now be described, with reference to the accompanying drawings, in which:
FIG. 1 is a known video coder.
FIG. 2 is a diagram of a television picture illustrating a co-ordinate system and search area;
FIG. 3 is a block diagram of aprt (P1 of FIG. 5) of a motion estimator according to the invention;
FIG. 4 is a diagram of the search area S.sub.N of FIG. 2;
FIG. 5 is a block diagram of a complete motion estimator; and
FIG. 6 is a block diagram of the sorter CSO of FIG. 5.
DETAILED DESCRIPTION OF THE DRAWING FIGURES
The motino estimator to be described regards a "current" frame of a television picture which is being coded as being divided into 8.times.8 blocks--that is, eight picture elements (pixels) horizontally by eight lines vertically. Although the principles are equally applicable to interlaced systems, for simplicity of description a non-interlaced picture is assumed. It is designed to generate for each block a motion vector which indicates the position of the 8.times.8 region, lying within a defined search area of the (or a) previous frame of the picture, which is most similar to the block in question. FIG. 2 illustrates a field with an 8.times.8 block N (shaded) and a typical associated 23.times.23 search area indicated by a rectangle S.sub.N. If the pixels horizontally and lines vertically are identified by coordinates x, y, with an origin at the top left-hand corner, then the search area for a block whose upper left hand corner pixel has coordinates x.sub.N, y.sub.N is the area extending horizontally from (x.sub.N -8) to (x.sub.N +14) and verticlaly from (y.sub.N -8) to (y.sub.N +14).
In order to obtain the motion vector it is necessary to conduct a search in which t
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British Telecommunications public limited company
Kostak Victor R.
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