Video coding device

Abstract: In a video-coding and video-decoding device, motion-compensated interframe prediction is achieved by determining motion vectors of respective pixels according to representative motion vectors with weighting, and determining accurate motion vectors between video frames, dividing each frame into areas optimal to an objective figure, size and location and performing effective encoding and decoding of the motion vectors. According to the invention a motion-compensated interframe predicting portion (217, 1217) generates a predicted video-frame by varying codable area according to a reference video-frame received from a frame memory portion (216) and an input video frame and obtains side-information. A motion vector searching portion (232) searches a motion vector. An effective-area selecting portion (231) selects a valid or invalid mask depending upon a position of a processable object, divides a processable area of the input video frame into suitable areas, a variable-area predicted-frame generating portion (233) generates a predicted frame by affine transformation and translational displacement. An area-diving pattern deciding portion (234) outputs side-information such as the predicted image, motion vectors and divided areas, a side-information coding portion (235) encodes an additional motion vector as a difference from an average basic-motion vector or predictively encodes a motion vector from a median value of three neighboring motion-vectors. A difference between the input video-frame and the predicted video-frame from the predicting portion (217, 1217) is encoded, transferred and stored.

Description: BACKGROUND OF THE INVENTION The recent progress of high-speed digital signal processing systems with advances in LSI technology and development of video processing technique has meet an increasing interest for more effective usage of video information. In the field of telecommunications, construction and development of digital networks that is represented by ISDN (Integrated Service Digital Network) have realized video communications services such as videophones, video conferencing services, video data base services and so on. Furthermore, the spread of mobile communications networks with advances in digital information technology has increased users' need to realize mobile video communications services. It is impractical to handle video information because the video information generally contains a very large amount of information. However, it is possible to reduce the amount of video information by reducing redundancy of the video information. High efficient video compression techniques are particularly important to the mobile communications networks that have a small amount of transmission line capacity. By this reason, international organizations ITU-T and ISO/IEC have energetically worked to settle international standards on video coding methods for encoding video signal at a very low bit-rate. A video signal contains time-sequential information such as a change of motion in picture and spatial information concerning a content of one frame (video frame or video field; both will be hereinafter called a frame), each of them has a redundancy. A motion compensative interframe prediction with orthogonal transform encoding method that has been preferably used is such that the temporal redundancy of information is first reduced through the interframe prediction with motion compensation. According to the principle of the motion compensative interframe prediction encoding method, the motion compensative interframe predicting portion prepares predicted value of an input video signal from an already encoded signal stored in a frame memory and outputs a difference between the predicted value and the input video signal as a prediction error signal. A prediction error signal encoding portion encodes the prediction error signal by orthogonal transforming method to further reduce spatial redundancy of the information. The encoded prediction error signal is locally decoded, then stored in the frame memory and is used for prediction of a proceeding frame. In very low bit-rate video encoding, it is necessary to represent a video-signal with a very small amount of information. An amount of information allocated to the orthogonal transform encoding (i.e., prediction error encoding), therefore, is considerably limited. Accordingly, it is very important to improve the efficiency of the interframe prediction by using an interframe prediction method that can more correctly predict a change of a video signal with time. In most recent years, many studies have been made on interframe prediction methods using affine transformation and bilinear transformation. While the motion compensative interframe prediction method represents a movement contained in a video as a translational motion by using a motion vector per unit-area, the method using the affine transformation and bilinear transformation can more accurately represent a movement in a video, i.e., with increased prediction efficiency since it can represent movement, rotation, enlargement and deformation. An interframe predicting portion of the conventional video-coding device comprises a frame memory portion for storing already coded video signals, a motion vector detecting portion for determining a representative motion vector per unit area from a difference between an input video signal and a video signal read from the frame memory portion, a motion vector interpolating portion for determining a motion vector per pixel from the representative motion vector and a pixel value predicting portion for preparing a predicted video signal from a video signal read from the frame memory portion by using the motion vector per pixel. The operation of the above-mentioned interframe predicting portion is as follows: The frame memory portion stores already coded video signals as reference video frames for interframe prediction. The motion vector detecting portion receives an input video-frame signal to be encoded and reads a reference video frame stored in the frame memory. The motion vector detecting portion divides the coding video frame into unit-areas and scans the reference video frame to find an area most similar to a current coding unit-area. A displacement of a unit-area of the coding video-frame from the area found in the reference video frame is outputted as a motion vector. The motion vector is a representative motion vector representing an interframe displacement of a representative point within a unit-area (usually, a center of the unit-area). The relationship between a representative point and a vector searched unit-area are specified. In searching a similar area in a reference video frame, a sum of differential absolute values or a square-sum of differential values of each pixel in a unit-area is used as a scale of similarity. Furthermore, a center portion of the unit-area may be weighted by multiplying a differential pixel value at the center portion of the unit-area by a large coefficient and a differential pixel value at the periphery thereof by a small coefficient prior to summation so as to more accurately determine a displacement of the representative point. The representative motion vector is transferred to the motion vector interpolating portion which in turn determines motion vectors for respective pixels using the received representative motion vectors. For affine transformation, a motion vector for each pixel existing within a triangle area surrounded by three neighboring representative points (hereinafter called transformable unit-area) is determined by solving an affine transformation expression from the representative motion vectors of the respective representative points. For bilinear transformation, a motion vector for each pixel existing within a quadrangular area surrounded by four neighboring representative points (hereinafter called transformable unit-area) is determined by solving a bilinear transformation expression from the representative motion vectors of the respective representative points. For a transformation unit-area being quadrangular or rectangular, it is equivalent that a motion vector value of each representative point is distributed in a vertical direction and a horizontal direction in proportion to a distance between a remarked pixel and a representative point. The pixel value predicting portion receives the (remarked) pixel motion vectors inputted pixel by pixel and, considering the motion vectors to be a displacement of the remarked pixel from its corresponding position in the reference frame, reads a pixel value at the corresponding position from the frame memory as a predicted value of the remarked pixel to compose a predicted frame. If the pixel value indicates a position in the reference frame, where no pixel exists, e.g., the pixel motion vector (displacement) is a decimal fraction, a neighboring pixel value in the reference frame is read-out from the frame memory and a predicted value of the remarked pixel is determined as an interpolating value according to a known bilinear interpolation method. In a simplified structure of an interframe predicting portion of a conventional video-decoding device using affine and bilinear transformation method, the interframe predicting portion of the conventional decoding device comprises a frame memory portion for storing already decoded video signals, a motion-vector interpolating portion for determining a motion vector of each pixel from a representative motion vector inputted for each unit-area, and a pixel value predicting portion for preparing a predicted video signal from a video signal read from a frame memory by using the pixel motion vectors. The conventional video coding and decoding devices may show an excellent performance if an entire area surrounded by representative points can be represented by the same parameters of affine transformation and bilinear transformation. However, in the case of positions of the representative points being different from changes between video frames, e.g., positions and movement of an object, representative motion vectors of the representative points represent different movements of different frame object resulting in that a motion vector of a pixel, obtained from the representative motion vector, can not represent an adequate displacement of the pixel. Consequently, the conventional art device in this case may suffer a considerably decreased efficiency of interframe prediction, i.e., coding efficiency. Video communication services, e.g., videophones and video conferencing have been realized over high-speed digital communication networks such as ISDN (Integrated Service Digital Network). Recently, with the spread of radio transmission networks represented by PHS (Personal Handyphone System), progress of data modulation/demodulation techniques for PSTN (Public Switched Telephone Network) and advance of image compressing techniques, there have been increasing demands for video communication services over lower bit-rate transmission lines. Video communication services like video telephones and a video conferencing system must transmit a very large amount of information over limited transmission lines. Accordingly, it is necessary to reduce an amount of information by compressing and to encode the video information in view of transmission speed and costs of usable transmission lines. As well known, H.261, MPEG-1 (Moving Picture Coding Expert Group) and MPEG-2 are internationally established standard coding methods for compressing video information. In addition, MPEG-4 is now promoted to be standardized as a method for coding at a very low bit-rate of not more than 64 kbps. The standardized video-coding methods adopt a hybrid video coding method performing interframe prediction coding in combination with intraframe prediction coding. The interframe prediction coding is to generate a predictive video-frame from a reference video-frame and encode a difference of the predictive frame from a current video-frame to reduce an amount of codes to be transmitted. This enables using effective use of transmission lines. The interframe predictive coding is known as being a block-matching method, an affine transformation method and a warping prediction method. A conventional video-coding and video-decoding device using the affine transformation will be described as follows: The operation of the conventional video-coding device is first explained below. It is assumed that a reference video frame usable for producing a predictive video-frame is stored in a frame memory portion in a normal operating condition of the conventional video-coding device for conducting the motion-compensated interframe predictive coding. An input video frame enters into a subtracting portion and a motion-compensated interframe-predicting portion. The motion-compensated interframe-predicting portion determines a predictive motion from the reference video-frame stored in the frame memory portion and the input video-frame and outputs a predicted video-frame to the subtracting portion. The subtracting portion subtracts from the input video-frame the predicted video-frame entered by the motion-compensated interframe predicting portion and outputs the subtraction result, i.e., predicted error information to a video coding portion. The video coding portion spatially transforms the input predicted-error-information, e.g., by using DCT (Discrete Cosine Transform), quantizes the transformed information and outputs coded video-information. At the same time, the coded video-information outputted from the video coding portion is locally decoded by a video-decoding portion and is then transferred to the adder portion. The adder portion adds the predicted video-information outputted from the video-decoding portion to the predicted error-information outputted from the motion-compensated interframe-predicting portion to form a new reference video-frame which is then transferred to the frame memory portion. The frame memory portion stores therein the new reference video-frame outputted from the adding portion. This new reference video-frame is given to the motion-compensated interframe predicting portion when encoding a succeeding input video-frame. The video coding device outputs a series of coded video information (predicted error information) and a series of coded side-information by repeating the above-mentioned sequence of operations. The operation of the conventional video-decoding device is now described below. It is assumed that a reference video-frame usable for producing a predictive video-frame is stored in a frame memory portion in a normal operating condition of the conventional video-coding device for conducting the motion-compensated interframe predictive coding. Coded video-information inputted into the video-decoding device enters into a video-decoding portion which decodes the coded-video information by performing the same operations as the video-decoding portion of the video-coding device and outputs the obtained differential video-frame to an adder portion. Coded side-information inputted into the video-decoding device enters into a motion-compensated interframe predicting portion. The motion-compensated interframe predicting portion decodes the received coded side-information and obtains motion vectors, then it produces a predicted video-frame by using the obtained motion vectors and a reference video-frame read from the frame memory portion and transfers the produced predicted video-frame to an adder portion. The adder portion obtains an output video-frame by adding the differential video-frame from the video-decoding portion to the predicted video-frame from the motion-compensated interframe predicting portion. The output video-frame is outputted from the video-decoding device and at the same time is transferred to the frame memory. The frame memory stores therein the video-frame inputted from the adder portion as a new reference video-frame that will be used by the motion-compensated interframe predicting portion when decoding a proceeding video-frame. The video decoding device realizes outputting a series of decoded video-frames by repeating the above-mentioned sequential operations. The operation of a motion-compensated interframe predicting portion in a video-coding device and a video-decoding device and prior arts applied therein will be described below. An exemplified structure of a motion-compensated interframe predicting portion and its operation will be first-described as follows: The motion-compensated interframe-predicting portion comprises a motion-vector searching portion, a fixed-area predicted-frame generating portion and a motion-vector coding portion. The motion-vector searching portion searches motion vectors in an input video-frame and a reference video-frame read from a frame memory and outputs the detected motion-vectors to the fixed-area-size predicted-frame generating portion. When searching motion vectors, the motion-vector searching portion searches a motion vector of a control grid point by previously weighting a center pixel of a processable area and outputs a motion vector of each control grid point to the fixed-area-size predicted-frame generating portion. According to the conventional method, a processable area has a fixed size with control grid points set one for 16 or 8 pixels. The fixed-area-size predicted-frame generating portion performs interframe prediction on each processable area having a fixed size (generally in a rectangular block of 16 × 16 pixels) by using motion vectors inputted from the motion vector searching portion and a preceding video-frame read from the frame memory portion. The interframe prediction is realized by determining affine parameters from positions of three vertex points of each objective triangular area, performing affine transformation on every pixel existing within the triangular area and repeating the same processing on all processable triangular areas to produce a predicted video-frame. The produced predicted video-frame is transferred to the before-described subtracting portion and adder portion. The motion vectors are transferred to the motion vector coding portion. The motion vector coding portion encodes the motion vectors received from the fixed-area predicted-frame generating portion and outputs them as coded side-information. The motion-vector coding device encodes the motion vectors by the following method: Motion vectors are generally encoded not directly but by predictive method. Namely, a predictive value of each codable objective motion vector is determined and a difference between the predicted value and the motion vector value is encoded. By doing so, the efficiency of coding can be improved with a saved amount of information. The most simple method of determining a predicted value is such that just coded vector value is adopted as a predicted value of a proceeding motion-vector. According to this method, differences of two successive neighboring motion vectors are encoded in turn. By reason of a high correlation between motion vectors, it is possible to effectively encode motion vectors particularly in the case when neighboring motion vectors have similar values. This prediction coding method is adopted in a motion-vector coding method defined by ITU-T Recommendation H.261. Another method is to determine a predicted value from a plurality of motion vectors including a just-preceding vector. In this case, a prediction value is determined by using values of three motion-vectors which are located, respectively, left, just above and above right from a coding objective motion-vector. The prediction value may be a mean value or a median of three values of the three neighboring motion-vectors. In comparison with the predicting method using only just-preceding motion vector this method can use a wider range of correlation, i.e., higher correlation of motion vectors, attaining a further improved coding efficiency. The video coding system defined by the ITU-T Recommendation H.263 adopts the prediction method for coding predicted a motion vector by using a median of the three motion-vector values, which is well known to be more effective. The structure and operation of a motion-compensated interframe-predicting portion of a conventional video-decoding device will now described as follows: The motion-compensated interframe predicting portion is composed of a fixed-area-size predicted-frame generating portion and a motion-vector coding portion. Coded side-information inputted into the motion-compensated interframe predicting portion is transferred to the motion-vector decoding portion which in turn decodes the received coded side-information, obtains motion vectors and outputs the motion vectors to the fixed-area-size predicted-frame generating portion. The fixed-area-size predicted-frame generating, portion performs interframe prediction processing by using the motion vectors received from the motion vector decoding portion and a reference video-frame received from the frame memory portion. Interframe prediction is to determine affine parameters from positions of three vertices of an objective triangular area and motion vectors and then to perform affine transformation of all pixels within the triangular area by using the determined affine parameters. This processing is repeated on all objective triangular areas to generate a predicted video-frame. The fixed-area-size predicted-frame generating portion outputs the obtained predicted video-frame to an adder portion. Some kinds of methods used for motion-compensated interframe prediction in the above-mentioned video-coding and video-decoding devices will be described below. A motion vector searching method is as follows: A so-called "pixel-matching" method that weights generally a center pixel in a processable area is used for determining a motion vector of a control grid point. The processable area is composed of 21 pixels for instance each in X-direction and Y-direction. The pixel matching is such that an area of a reference frame, which matches with the processable area of a current frame is determined by calculation and then motion vectors are determined as displacement values in X- direction and Y-direction. In matching calculation, a difference between the processable area of the current video-frame and the matched area of the reference frame processable is multiplied by a weighting coefficient. By doing so, motion vectors are searched putting a weight on a center pixel in the processable area. Motion vectors at control grid points located at the periphery of a video-frame are set as follows: Motion vectors at quadrangular control grid points: X- and Y-components of each vector are set both at 0. Motion vectors at the top and bottom control grid points: An X-component is determined as an X-component of a motion vector detected at a control grid point one inside from an objective control grid point. A Y-component is set at 0. Motion vectors at the left-side and right-side control-grid points: An X-component of each motion vector is set at 0. A Y-component is determined as a Y-component of a motion vector detected at a control grid point one inside from an objective control grid point. The affine transformation is described as follows: The affine transformation is conducted by representing a map from a video-frame to another video-frame by using six parameters. The affine transformation is conducted, for the sake of simplifying calculation of affine parameters, usually on a triangular area. Motion vectors of control grid points A, B, C and D of the current video-frame are detected at corresponding control grid points A', B', C' and D' of a reference video-frame. Three of four control grid points are first selected and an area is divided to determine affine parameters. For example, the area on the current video-frame is divided into two triangles ABC and BCD on the current video-frame and a corresponding area on the reference video-frame is divided into two triangles A'B'C' and B'C'D'. On the triangles into which the area is divided, affine parameters are determined from vertex positions of each triangle (vertex positions of one triangle and motion vectors of the other may be used). A predictive video-frame is generated by mapping thereto all pixels of all triangular sub-areas according to the obtained affine parameters. If a position of any referred pixel in the reference video-frame is not an integer, a predicted value is bilinearly interpolated to determine a pixel value of the predicted video-frame. A predicted video-frame is generated by performing the above-mentioned processing operations. The motion-compensated interframe prediction according to the above-mentioned prior art using affine transformation of a fixed area-size has such a drawback that the efficiency of motion prediction is decreased when an object to be mapped has an insufficient size or its periphery is divided by a plurality of areas to create a considerable difference between the object edge and the area dividing lines. The motion-compensated interframe prediction with the affine transformation may be accompanied by geometrical distortion peculiar to the affine transformation, resulting in that the prediction efficiency is decreased with deterioration of the quality of coded video frames. The motion-compensated interframe prediction may have a considerable decrease of the prediction efficiency if an input video contains a motion of an objective area by a parallel displacement that can not correctly be represented. High accuracy of setting motion vectors of control-grid points at the peripheral part of a video-frame may be impossible because they are set affinely from respective inside points of the control grid. In the case of conducting affine transformation of a variable in size area, the number of control grid points must be increased and therefore the number of motion vectors is increased as compared with the conventional affine transformation of a fixed-size area. Accordingly, motion vectors must be encoded at a high efficiency. In the affine transformation of the variable in size area, the number of control grid points varies with the size of a selected area, i.e., the number of motion vectors varies with the number of the control grid points. Consequently, motion vectors may be arranged dispersively in the video-frame. This makes it difficult to apply the conventional motion-vector coding method as it is. The coding efficiency is also decreased. It is necessary to use a highly efficient motion-vector coding system adapted to variable area-size affine transformation. SUMMARY OF THE INVENTION The present invention relates to a video coding device for encoding video information by reducing volume of information contained in a video signal and a video decoding device for decoding coded information into a video signal and, more particularly, to a coding device for conducting interframe prediction per pixel by determining a motion vector from a representative motion vector according to a motion-vector-interpolation predicting method, and to a corresponding video decoding device. The present invention was made to provide a video coding device and a video decoding device, which can achieve high efficiency of interframe prediction and coding of video signals by accurately determining pixel motion vectors with a minimized effect of motion vectors irrelevant to each remarked pixel by previously weighting the representative motion vectors to be used for determining therefrom motion vectors for respective pixels. The present invention provides: (1) a video-coding device comprising frame memory means for storing already encoded video signals, motion vector detecting means for determining a representative motion vector associated with a representative point per unit area from a difference between an input video signal and a video signal read from the frame memory means, motion vector interpolating means for determining a motion vector per pixel using the representative motion vector, pixel value predicting means for preparing a predicted video signal from a video signal read from the frame memory means by using the motion vector per pixel, characterised in that there is provided weighting coefficient control means operable in the determination of the motion vector for each respective pixel to determine, for said respective pixel, weighting coefficients for the set of representative motion vectors associated with a corresponding set of mutually neighbouring said representative points around said respective pixel according to the relationship between the representative motion vectors of said set of representative points, said weighting coefficient control means being arranged to instruct the motion vector interpolating means to weight said representative motion vectors determined by the motion vector detecting means in accordance with said weighting coefficients, and said motion vector interpolating means being arranged to determine said motion vector per pixel in accordance with the weighted set of representative motion vectors; (2) a video-coding device as defined in item (1), characterized in that the weighting coefficient control means is arranged to select one of a set of predetermined patterns of weighting coefficients for the representative motion vectors and to instruct the motion vector interpolating means to employ the selected predetermined pattern of weighting coefficients for weighting said set of representative motion vectors; (3) a video-coding device as defined in item (1) or item (2), characterized in that the weighting coefficient control means is arranged to determine the vector weighting coefficients according to the directions of the representative motion vectors; (4) a video-coding device as defined in item (1) or item (2), characterized in that the weighting coefficient control means is arranged to determine the vector weighting coefficients according to the vector values of the representative motion vectors; (5) a video-decoding device comprising frame memory means for storing already decoded video signals, motion vector interpolating means for determining a motion vector per pixel using a representative motion vector inputted per unit area and associated with a representative point, pixel value predicting means for preparing a predicted video signal from a video signal read from the frame memory means by using the motion vector per pixel, characterized in that there is provided weighting coefficient control means operable in the determination of the motion vector for each respective pixel to determine for said respective pixel weighting coefficients for the set of representative motion vectors associated with a corresponding set of mutually neighbouring said representative points around said respective pixel according to the relationship between the representative motion vectors of said set of representative points, said weighting coefficient control means being arranged to instruct the motion vector interpolating means to weight said inputted representative motion vectors in accordance with said weighting coefficients, and said motion vector interpolating means being arranged to determine said motion vector per pixel in accordance with the weighted set of representative motion vectors; (6) a video-decoding device as defined in item (5), characterized in that the weighting coefficient control means is arranged to select one of a set of predetermined patterns of weighting coefficients for the representative motion vectors and to instruct the motion vector interpolating means to employ the selected predetermined pattern of weighting coefficients for weighting said set of representative motion vectors; (7) a video-decoding device as defined in item (5) or item (6), characterized in that the weighting coefficient control means is arranged to determine the vector weighting coefficients according to the directions of the representative motion vectors; (8) a video-decoding device as defined in item (5) or item (6), characterized in that the weighting coefficient control means is arranged to determine the vector weighting coefficients according to the vector values of the representative motion vectors; (9) a video-decoding device as defined in any of items (5) to (8), characterized in that motion vector converting means is provided for calculating representative motion vectors for a skipped image frame which was not encoded from representative motion vectors inputted per unit area of an encoded image frame, the device being arranged to prepare an interpolating video signal corresponding to the skipped image frame using said calculated representative motion vectors. In an embodiment of the above-mentioned video-coding device, the frame memory means stores already coded video-signals as a reference video frame to be used for interframe prediction. The motion vector detecting means receives an input video frame signal to be encoded and reads a reference video frame stored in the frame memory and searches in said reference frame so that an area being most similar to a current processable unit-area of an input video frame is found. The displacement of the found similar area in the reference frame from the processable unit-area is outputted as a motion vector. The weighting coefficient control means determines a weighting coefficient for each motion vector of the unit-area and informs the motion vector interpolating portion of the weighting coefficients specified for respective motion vectors. The weighting coefficients are determined in such a way that the interfering effect of the respective representative motion vectors representing different motions of different objects in an image is reduced to obtain a correct displacement of a current remarked pixel. The motion vector interpolating means determines motion vectors of respective pixels by using the motion vectors outputted from the motion vector detecting means and the weighting coefficients specified by the weighting coefficient control means and outputs the calculated results. At this time, a combination of a devised method for weighting each motion vector and affine/bilinear transformation technique in comparison with the conventional method can more accurately determine a motion vector of each pixel, attaining a higher efficiency of interframe prediction and video-signal coding. In the pixel value predicting means, the motion vectors of the respective pixels outputted from the motion vector interpolating means are used as displacement values of the respective pixels from the corresponding pixels of the reference frames and the corresponding pixel values are read from the frame memory to compose a predicted frame. In the weighting coefficient control means, it is also possible to previously prepare several patterns of weighing coefficients, select a suitable one of the patterns and give only information about the selected pattern, instead of direct indication of the values, to the motion vector interpolating means. Furthermore, the weighting coefficient control means may also determine a weighting coefficient according to only a direction of each representative motion vector in such a way that a motion vector indicating a particular direction is weighted with a small weight or, if there is a large dispersion of directions of motion vectors, only motion vectors of nearest neighbors to a remarked pixel are weighted with a larger weight. This method is effective to easily determine correct weighting coefficients, improving the efficiency of interframe prediction. The weighting coefficient control means may also determine a vector weighting coefficient according to a vector value of each representative motion vector by discriminating a dispersion of motion vectors and separating a motion vector that has a particular value representing the motion of another object in an image. This is effective to obtain adequate weighting coefficients for determining accurate motion vectors of respective pixels. The efficiency of interframe prediction can be improved. The above-mentioned video-coding and video decoding devices can selectively use any adaptive one of two or more methods for motion-compensated interframe prediction, which may minimize the number of codes for representing side-information (e.g., motion vectors) or a prediction error. In the motion-compensated interframe-prediction by using the affine-transformation technique, it is possible to reduce the amount of side-information (e.g., motion vectors) or prediction error in conformity with the shape, size and location of an object to be mapped. This could not be realized by the conventional method. Owing to these advantages, the video-coding and video-decoding devices according to the present invention can achieve a more effective motion-compensated interframe prediction with an increased efficiency of video-coding and decoding. Application of the video-coding and video-decoding devices according to the present invention can realize video-communications over transmission lines and channels whose transmission bit-rate is very low. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a block diagram for explaining the principle of a motion compensated interframe prediction with orthogonal transform encoding method. Fig. 2 is a simplified block diagram showing an exemplified structure of an interframe predicting portion of a conventional encoding device. Fig. 3 is a view showing the correlation between a unit area to be transformed, a representative point and a representative motion vector. Fig. 4 is a simplified block diagram showing an exemplified structure of an interframe predicting portion of a conventional encoding device. Fig. 5 is a schematic illustration of a whole system of a video coding device according to a prior art or the present invention. Fig. 6 is a schematic illustration of a whole system of a video decoding device according to a prior art or the present invention. Fig. 7 is illustrative of a motion-compensated interframe predicting portion of a conventional video coding device. Fig. 8 is illustrative of a motion-compensated interframe predicting portion of a conventional video decoding device. Fig. 9 is view for explaining a weighting method used when searching a motion vector according to the present invention. Fig. 10 is a view for explaining a control grid and a motion vector according to a prior art. Fig. 11 is view for explaining interframe prediction using affine transformation in case of forwarding interframe prediction of deformation of an object. Fig. 12 shows an exemplified structure of a motion-compensated interframe-predicting portion of a conventional video coding device. Fig. 13 shows an exemplified structure of a motion-compensated interframe-predicting portion of a conventional video decoding device. Fig. 14 is a simplified block diagram of a video coding device embodying the present invention. Fig. 15 is a flow chart showing a first example of operations of a weighting coefficient control portion of a video coding device according to the present invention. Fig. 16 is a flow chart showing a second example of operations of a weighting coefficient control portion of a video coding device according to the present invention. Fig. 17 is a simplified block diagram of a video decoding device embodying the present invention. Fig. 18 is a simplified block diagram showing an exemplified structure of a video decoding device according to the present invention. PREFERRED EMBODIMENT OF THE INVENTION To begin with, prior art will be explained as a reference of the present invention. A motion compensative interframe prediction with orthogonal transform encoding method that has been preferably used is such that the temporal redundancy of information is first reduced through the interframe prediction. Fig. 1 shows the principle of the motion compensative interframe prediction with orthogonal transform encoding method. The motion compensative interframe predicting portion 1191 prepares predicted value of an input video signal from an already encoded signal stored in a frame memory 1193 and outputs a difference between the predicted value and the input video signal as a prediction error signal. A prediction error signal encoding portion 1192 encodes the prediction error signal by orthogonal transforming method to further reduce redundancy of the information. The encoded prediction error signal is locally decoded, then stored in the frame memory 1193 and is used for prediction of a proceeding frame. In very low bit-rate video encoding, it is necessary to represent a video-signal with a very small amount of information. An amount of information allocated to the orthogonal transform encoding (i.e., prediction error encoding), therefore, is considerably limited. Accordingly, it is very important to improve the efficiency of the interframe prediction by using an interframe prediction method that can more correctly predict a change of a video signal with time. In the latest years, many studies have been made on interframe prediction methods using affine transformation and bilinear transformation. While the motion compensative interframe prediction method represents a movement contained in a video as a translational motion by using a vector per unit-area, the method using the affine transformation and bilinear transformation can more accurately represent a movement in a video, i.e., with increased prediction efficiency since it can represent movement, rotation, enlargement and deformation. Fig. 2 is a schematic illustration of an interframe predicting portion of a conventional video-coding device that uses affine transformation and bilinear transformation. An interframe predicting portion of the conventional video-coding device comprises a frame memory portion 1131 for storing already coded video signals, a motion vector detecting portion 1132 for determining a representative motion vector per unit area from a difference between an input video signal and a video signal read from the frame memory portion 1131, a motion vector interpolating portion 1133 for determining a motion vector per pixel from the representative motion vector and a pixel value predicting portion 1134 for preparing a predicted video signal from a video signal read from the frame memory portion 1131 by using the motion vector per pixel. The operation of the above-mentioned interframe predicting portion is as follows: The frame memory portion 1131 stores already coded video signals as reference video frames for interframe prediction. The motion vector detecting portion 1132 receives an input video-frame signal to be encoded and reads a reference video frame stored in the frame memory 1131. The motion vector detecting portion 1132 divides the coding video frame into unit-areas and scans the reference video frame to find an area most similar to a current coding unit-area. A displacement of a unit-area of the coding video-frame from the area found in the reference video frame is outputted as a motion vector. The motion vector is a representative motion vector representing a interframe displacement of a representative point within a unit-area (usually, a center of the unit-area). Fig. 3(a) shows an example of relationship between a representative point and a vector searched unit-area are specified. In searching a similar area in a reference video frame, a sum of absolute differential values or a square-sum of differential values of each pixel in a unit-area is used as a scale of similarity. Furthermore, a center portion of the unit-area may be weighted by multiplying a differential pixel value at the center portion of the unit-area by a large coefficient and a differential pixel value at the periphery thereof by a small coefficient prior to summation so as to more accurately determine a displacement of the representative point. The representative motion vector is transferred to the motion vector interpolating portion 1133 which in turn determines motion vectors for respective pixels using the received representative motion vectors. For affine transformation, a motion vector for each pixel existing within a triangle area surrounded by three neighboring representative points (hereinafter called transformable unit-area) is determined by solving an affine transformation expression from the representative motion vectors of the respective representative points. For bilinear transformation, a motion vector for each pixel existing within a quadrangular area surrounded by four neighboring representative points (hereinafter called transformable unit-area) is determined by solving a bilinear transformation expression from the representative motion vectors of the respective representative points. For a transformation unit-area being quadrangular or rectangular, it is equivalent that a motion vector value of each representative point is distributed in a vertical direction and a horizontal direction in proportion to an a distance between a remarked pixel and a representative point as shown in Fig. 3(b). The pixel value predicting portion 1134 receives the (remarked) pixel motion vectors inputted pixel by pixel and, considering the motion vectors to be a displacement of the remarked pixel from its corresponding position in the reference frame, reads a pixel value at the corresponding position from the frame memory 1131 as a predicted value of the remarked pixel to compose a predicted frame. If the pixel value indicates a position in the reference frame, where no pixel exists, e.g., the pixel motion vector (displacement) is a decimal fraction, a neighboring pixel value in the reference frame is read-out from the frame memory and a predicted value of the remarked pixel is determined as an interpolating value according to a known bilinear interpolation method. Fig. 4 shows a simplified structure of an interframe predicting portion of a conventional video-decoding device using affine and bilinear transformation method. The interframe predicting portion of the conventional decoding device comprises a frame memory portion 1141 for storing already decoded video signals, a motion-vector interpolating portion 1142 for determining a motion vector of each pixel from a representative motion vector inputted for each unit-area, a pixel value predicting portion 1143 for preparing a predicted video signal from a video signal read from a frame memory by using the pixel motion vectors. Furthermore, the operation of the conventional video-coding device is first explained below. Fig. 5 shows an exemplified entire system of the conventional video-coding device. It is assumed that a reference video frame usable for producing a predictive video-frame is stored in a frame memory portion 1216 in a normal operating condition of the conventional video-coding device for conducting the motion-compensated interframe predictive coding. An input video frame enters into a subtracting portion 1211 and a motion-compensated interframe-predicting portion 1217. The motion-compensated interframe-predicting portion 1217 determines a predictive motion from the reference video-frame stored in the frame memory portion 1216 and the input video-frame and outputs a predicted video-frame to the subtracting portion 1211. The subtracting portion 1211 subtracts from the input video-frame the predicted video-frame entered by the motion-compensated interframe predicting portion 1217 and outputs the subtraction result, i.e., predicted error information to a video coding portion 1212. The video coding portion 1212 spatially transforms the input predicted-error-information, e.g., by using DCT (Discrete Cosine Transform), quantizes the transformed information and outputs coded video-information. At the same time, the coded video-information outputted from the video coding portion 1212 is locally decoded by a video-decoding portion 1214 and is then transferred to the adder portion 1215. The adder portion 1215 adds the predicted video-information outputted from the video-decoding portion 1214 to the predicted error-information outputted from the motion-compensated interframe-predicting portion 1217 to form a new reference video-frame which is then transferred to the frame memory portion 1216. The frame memory portion 1216 stores therein the new reference video-frame outputted from the adding portion 1215. This new reference video-frame is given to the motion-compensated interframe predicting portion 1217 when encoding a succeeding input video-frame. The video coding device outputs a series of coded video information (predicted error information) and a series of coded side-information by repeating the above-mentioned sequence of operations. The operation of the conventional video-decoding device is now described below. Fig. 6 is illustrative of an exemplified entire system of the conventional video-decoding device. It is assumed that a reference video-frame usable for producing a predictive video-frame is stored in a frame memory portion 1216 in a normal operating condition of the conventional video-coding device for conducting the motion-compensated interframe predictive coding. Coded video-information inputted into the video-decoding device enters into a video-decoding portion 1221 which decodes the coded-video information by performing the same operations as the video-decoding portion 1214 of the video-coding device and outputs the obtained differential video-frame to an adder portion 1222. Coded side-information inputted into the video-decoding device enters into a motion-compensated interframe predicting portion 1223. The motion-compensated interframe predicting portion 1223 decodes the received coded side-information and obtains motion vectors, then it produces a predicted video-frame by using the obtained motion vectors and a reference video-frame read from the frame memory portion 1224 and transfers the produced predicted video-frame to an adder portion 1222. The adder portion 1222 obtains an output video-frame by adding the differential video-frame from the video-decoding portion 1221 to the predicted video-frame from the motion-compensated interframe predicting portion 1223. The output video-frame is outputted from the video-decoding device and at the same time is transferred to the frame memory 1224. The frame memory 1224 stores therein the video-frame inputted from the adder portion 1224 as a new reference video-frame that will be used by the motion-compensated interframe predicting portion 1223 when decoding a proceeding video-frame. The video decoding device realizes outputting a series of decoded video-frames by repeating the above-mentioned sequential operations. The operation of a motion-compensated interframe predicting portion in a video-coding device and a video-decoding device and prior arts applied therein will be described below. An exemplified structure of a motion-compensated interframe predicting portion and its operation will be first described as follows: Fig. 7 shows a conventional example of a motion-compensated interframe predicting portion of a video-coding device. As Fig. 7 shows, the motion-compensated interframe-predicting portion 1217 comprises a motion-vector searching portion 1241, a fixed-area predicted-frame generating portion 1242 and a motion-vector coding portion 1243. The motion-vector searching portion 1241 searches motion vectors in an input video-frame and a reference video-frame read from a frame memory 1216 and outputs the detected motion-vectors to the fixed-area-size predicted-frame generating portion 1242. When searching motion vectors, the motion-vector searching portion 1241 searches a motion vector of a control grid point by previously weighting a center pixel of a processable area and outputs a motion vector of each control grid point to the fixed-area-size predicted-frame generating portion 1242. According to the conventional method, a processable area has a fixed size with control grid points set one for 16 or 8 pixels. Fig. 10 illustrates an example of a control grid whose points set one for 16 pixels, indicating motion vectors existing therein. The fixed-area-size predicted-frame generating portion 1242 performs interframe prediction on each processable area having a fixed size (generally in a rectangular block of 16 × 16 pixels) by using motion vectors inputted from the motion vector searching portion 1241 and a preceding video-frame read from the frame memory portion 1216. The interframe prediction is realized by determining affine parameters from positions of three vertex points of each objective triangular area, performing affine transformation on every pixel existing within the triangular area and repeating the same processing on all processable triangular areas to produce a predicted video-frame. The produced predicted video-frame is transferred to the before-described subtracting portion 1211 and adder portion 1215. The motion vectors are transferred to the motion vector coding portion 1243. The motion vector coding portion 1243 encodes the motion vectors received from the fixed-area predicted-frame generating portion 1242 and outputs them as coded side-information. The motion-vector coding device 1243 encodes the motion vectors by the following method: Motion vectors are generally encoded not directly but by predictive method. Namely, a predictive value of each codable objective motion vector is determined and a difference between the predicted value and the motion vector value is encoded. By doing so, the efficiency of coding can be improved with a saved amount of information. The most simple method of determining a predicted value is such that just coded vector value is adopted as a predicted value of a proceeding motion-vector. According to this method, differences of two successive neighboring motion vectors are encoded in turn. By reason of a high correlation between motion vectors, it is possible to effectively encode motion vectors particularly in the case when neighboring motion vectors have similar values. This prediction coding method is adopted in a motion-vector coding method defined by ITU-T Recommendation H.261. Another method is to determine a predicted value from a plurality of motion vectors including a just-preceding vector. In this case, a prediction value is determined by using values of three motion-vectors which are located, respectively, left, just above and above right from a codable objective motion-vector. The prediction value may be a mean value or a median of three values of the three neighboring motion-vectors. In comparison with the predicting method using only just-preceding motion vector this method can use a wider range of correlation, i.e., higher correlation of motion vectors, attaining a further improved coding efficiency. The video coding system defined by the ITU-T Recommendation H.263 adopts the prediction method for coding predicted a motion vector by using a median of the three motion-vector values, which is well known to be more effective. The structure and operation of a motion-compensated interframe-predicting portion 1223 of a conventional video-decoding device will now described as follows: Fig. 8 is illustrative of an exemplified structure of the motion-compensated interframe-predicting portion of the video-decoding device. As shown in Fig. 8, the motion-compensated interframe predicting portion 1223 is composed of a fixed-area-size predicted-frame generating portion 1251 and a motion-vector coding portion 1252. Coded side-information inputted into the motion-compensated interframe predicting portion 1223 is transferred to the motion-vector decoding portion 1252 which in turn decodes the received coded side-information, obtains motion vectors and outputs the motion vectors to the fixed-area-size predicted-frame generating portion 1251. The fixed-area-size predicted-frame generating portion 1251 performs interframe prediction processing by using the motion vectors received from the motion vector decoding portion 1252 and a reference video-frame received from the frame memory portion 1224. Interframe prediction is to determine affine parameters from positions of three vertices of an objective triangular area and motion vectors and then to perform affine transformation of all pixels within the triangular area by using the determined affine parameters. This processing is repeated on all objective triangular areas to generate a predicted video-frame. The fixed-area-size predicted-frame generating portion 1251 outputs the obtained predicted video-frame to an adder portion 1222. Some kinds of methods used for motion-compensated interframe prediction in the above-mentioned video-coding and video-decoding devices will be described below. A motion vector searching method is as follows: A so-called "pixel-matching" method that weights generally a center pixel in a processable area is used for determining a motion vector of a control grid point. Fig. 9 is an illustration of a weighted area of pixels. In Fig. 9, the processable area is composed of 21 pixels for instance each in X-direction and Y-direction. The pixel matching is such that an area of a reference frame, which matches with the processable area of a current frame is determined by calculation and then motion vectors are determined as displacement values in X- direction and Y-direction. In matching calculation, a difference between the processable area of the current video-frame and the matched area of the reference frame processable is multiplied by a weighting coefficient. By doing so, motion vectors are searched putting a weight on a center pixel in the processable area. Motion vectors at control grid points located at the periphery of a video-frame are set as follows: Motion vectors at quadrangular control grid points: X- and Y-components of each vector are set both at 0. Motion vectors at the top and bottom control grid points (triangles shown in Fig. 10): An X-component is determined as an X-component of a motion vector detected at a control grid point one inside from an objective control grid point. A Y-component is set at 0. Motion vectors at the left-side and right-side control-grid points (circles shown in Fig. 10): An X-component of each motion vector is set at 0. A Y-component is determined as a Y-component of a motion vector detected at a control grid point one inside from an objective control grid point. The affine transformation is described as follows: The affine transformation is conducted by representing a map from a video-frame to another video-frame by using six parameters. The affine transformation is conducted, for the sake of simplifying calculation of affine parameters, usually on a triangular area. Fig. 11 is a view for explaining a method for interframe prediction by using affine transformation in the case of forward predicting. Motion vectors of control grid points A, B, C and D of the current video-frame are detected at corresponding control grid points A', B', C' and D' of a reference video-frame. Three of four control grid points are first selected and an area is divided to determine affine parameters. For example, the area on the current video-frame is divided into two triangles ABC and BCD on the current video-frame and a corresponding area on the reference video-frame is divided into two triangles A'B'C' and B'C'D'. On the triangles into which the area is divided, affine parameters are determined from vertex positions of each triangle (vertex positions of one triangle and motion vectors of the other may be used). A predictive video-frame is generated by mapping thereto all pixels of all triangular sub-areas according to the obtained affine parameters. If a position of any referred pixel in the reference video-frame is not an integer, a predicted value is bilinearly interpolated to determine a pixel value of the predicted video-frame. A predicted video-frame is generated by performing the above-mentioned processing operations. The operation of a motion-compensated interframe predicting portion in a video-coding device and a video-decoding device and methods applied therein will be described below. An exemplified structure of a motion-compensated interframe predicting portion 1217 (shown in Fig. 5) and its operation will be first described as follows: As shown in Fig. 12, the motion-compensated interframe-predicting portion 1217 comprises a motion-vector searching portion 1341, a predicted-frame generating portion 1342 and a motion-vector coding portion 1343. The motion-vector searching portion 1341 searches motion vectors in an input video-frame and a reference video-frame read from a frame memory portion 1216 and outputs the detected motion-vectors to the predicted frame generating portion 1342. The predicted frame generating portion 1342 generates a predictive image by using any one of prediction methods, i.e., block-displacement method, affine transformation method and bilinear transformation method, etc. In the process of generating a predicted video-frame, side-information such as area-information and motion vectors can be obtained. The generated predicted video-frame is transferred to the subtracting portion 1211 and the adder portion 1215 while the side-information is transferred to the side-information coding portion 1343. The side-information coding portion 1343 encodes the side-information received from the predicted-frame generating portion 1342 and outputs the coded side-information. The structure and the operation of the motion-compensated interframe-predicting portion 1223 (shown in Fig. 8) of a conventional video-decoding device will be now described as follows: As shown in Fig. 13, the motion-compensated interframe predicting portion 1223 is composed of a predicted-frame generating portion 1351 and side-information decoding portion 1352. Coded side-information inputted into the motion-compensated interframe predicting portion 1223 is transferred to the side-information decoding portion 1352. The side-information decoding portion 1352 decodes the received coded side-information, obtains decoded side-information including area-information and motion vectors, etc., and outputs the decoded side-information to the predicted-frame generating portion 1351. The predicted-frame generating portion 1351 performs interframe-prediction processing by using the side-information received from the side-information decoding portion 1352 and the reference video-frame received from the frame memory portion 1324 and outputs the produced predicted video-frame to the adder portion 1222. An embodiment of the present invention will be explained below referring to the accompanying drawings. Fig. 14 shows an exemplified structure of an interframe predicting portion of a video coding device according to the present invention. As shown in Fig. 14, the video coding device comprises a frame memory portion 111 for storing already encoded video signals; a motion vector detecting portion 112 for determining a representative motion vector per unit area from a difference between an input video signal and a video signal read from the frame memory portion 111, a weighting coefficient control portion 113 for determining weighting coefficient for each representative motion vector and instructing the weighting of each representative motion vector, a motion vector interpolating portion 114 for determining a motion vector per pixel from a representative motion vector, a pixel value predicting portion 115 for preparing a predicted video signal from a video signal read from the frame memory portion 111 by using each motion vector per pixel. The frame memory portion 111, the motion vector detecting portion 112 and the pixel value predicting portion 115 are similar in function to those of the conventional video coding device. The video coding device according to the present invention differs from the conventional device by provision of the weighting coefficient control portion 113 for specifying a weighting coefficient for each representative motion vector and a vector interpolating portion 114 which determines a motion vector per pixel from a representative motion vector outputted by the motion vector detecting portion 112 and by using a weighting coefficient for each vector, said coefficient being outputted from the weight coefficient control portion 113. The operations of the weighting coefficient control portion 113 and the motion vector interpolating portion 114, both of which are essential portions of the embodiment, are as follows: In this embodiment, the motion vector interpolating portion 114 uses the bilinear transformation method for determining a motion vector of a pixel but it may also use the affine transformation method. Like in the above-mentioned conventional coding device, the motion vector interpolating portion 114 performs operation on each unit-area. Namely, a unit-area for bilinear transformation is an area enclosed by four neighboring representative points (three representative points in the case of the affine transformation) corresponding to representative motion vectors outputted from the motion vector detecting portion 112. The weighting coefficient control portion 113 decides weighting coefficients for respective motion vectors of respective vertices (representative points) for a unit-area to be transformed. At this time, it may be considered that pixels neighboring to the representative points are subjected to most strong effect of vectors of respective nearest representative points. Accordingly, a unit-area to be transformed is further divided into subareas which are given different weighting coefficients. Fig. 3(c) illustrates that a unit area is divided into four subareas. The weighting coefficient control portion 113 judges whether motion vectors of vertices of a unit-area for transformation represent a movement of the same object of an image, i.e., said unit-area exists on one object in the image or not. When having judged the motion vectors concern the same object, the weight coefficient control portion 113 instructs the motion vector interpolating portion 114 to evenly weight all representative motion vectors of vertices (e.g., all with the same coefficient of 1). When having judged the vertices of the unit-area existing on different objects, the weight coefficient control portion 113 decides weighting coefficients in such a way that a motion vector of a vertex existing on the same object as a remarked subarea represented by a vertex is weighted with a larger weighting coefficient and a motion vector of a vertex judged to exist on a different object is weighted with a smaller weighting coefficient. In the extreme case, the weighting coefficient control portion 113 instructs the motion vector interpolating portion 114 to weight the motion vector of the different object with a weighting coefficient of 0 so as not to use said vector. The determination of weighting coefficients is conducted subarea by subarea. Weighting is controlled in such a way that a motion vector which was given a large weighting coefficient in one subarea is given a small weighting coefficient in other subareas. This is illustrated in Fig. 3(d). Weighting coefficients outputted by weighting coefficient control portion 113 may be a series of real numbers or they may be determined by selecting the most approximates to an optimal value from discretely defined values, e.g., 0, 0.25, 0.5, 0.75 and 1. The motion vector interpolating portion 114 weights every representative motion vector with a weighting coefficient specified by the weighting coefficient control portion 113 and then calculates motion vectors of respective pixels in the subarea and outputs the calculation results. When a motion vector of a vertex of a unit-area for transformation is MV i (i= 1, 2, 3, 4) and a weight for the motion vector w ij (i,j=1, 2, 3, 4), a weighting method may have by way of example as follows: The usual bilinear transformation equation may be calculated by using the weighted motion vector WMV j . When a motion vector MVP xy of a pixel having coordinates (x, y) is determined from a base motion vector MV i by bilinear transformation and a weight w 1 (i=1, 2, 3, 4) is given to the motion vector, the following equation is described. A coefficient w indicates a weighting ratio as a whole. The pixel motion vector MVP xy is corrected to WMVP xy in view of weighting. The motion vector interpolating portion 114 performs the above-mentioned operations for each subarea by using the weighting coefficient specified for each subarea by the weighting coefficient control portion 113. The weight coefficient control portion 113 may have M sets of weighting coefficients previously prepared, for example: W = { Wm } (m = 1,2,...M) Wm = { Wmij } (i,j = 1,2,3,4) It may select one of the prepared coefficients as an optimal weighting pattern for each unit-area or subarea, which coefficient is nearest to a weighting coefficient determined for the unit-area or subarea by the above-mentioned weighting coefficient processing. In this case, the weighting coefficient control portion 113 gives information about a selected pattern to the motion vector interpolating portion 114. It is also possible to previously prepare weighting patterns of paired weighting coefficients, one of which is for one subarea and the other is for other subarea. In this case, it becomes possible to specify weighting coefficients for all subareas of a unit-area at a time by specifying only a weighting pattern for the unit-area. In this instance, the motion vector interpolating portion 114 holds the same weighting patterns as those held in the weighting coefficient control portion 113, and it determines a motion vector for each pixel in the similar way as described before by using weighting coefficients for each motion vector according to the specified weighting coefficient pattern. It outputs the calculation results. Referring to a flow chart shown in Fig. 15, a first example of the weighting coefficient control portion 113 of video coding device according to the present invention will be described as follows: The motion vector detecting portion 112 enters representative motion vectors into a weighting, coefficient control portion 113 (Step 1). The weighting coefficient control portion 113 forms a transformable unit area of four neighboring representative points (Step 2). Then, it examines signs of horizontal and vertical components of each representative point of each vertex and classifies directions of motion vectors by extracting only positive, negative or zero signs (Step 3). The examination results are compared with each other to determine whether motion vectors of four vertices have roughly matched directions (Steps 4 and 5). If the motion vectors were recognized to be of a small dispersion and sufficiently matched, they are considered to represent the same object of an image and the same weighting coefficients are outputted (Step 6, 9). The transformable unit-area, if there is found a large dispersion between the motion vectors, is judged to include a plurality of different objects in an image. A vector showing a particular direction is separated and put into another matched group (Step 7). Weighting coefficients are decided for each subarea of the transformable unit area in such a way that a nearest neighboring motion vector of a remarked subarea and a motion vector showing a matched direction is weighted heavy and the motion vector judged to belong to other group or to show a particular direction is weighted light (Steps 8 and 9). Referring to a flow chart shown in Fig. 16, a second example of the weight coefficient control portion 113 of video coding device according to the present invention will be described as follows: Like the first example, a motion vector detecting portion 112 enters representative motion vectors into a weighting coefficient control portion 113 (Step 1) whereby a transformable unit area is formed by four neighboring representative points (Step 12). Then, horizontal and vertical component values of four neighboring representative vectors of every vertex are compared with each other (Steps 13 and 14) and the motion vectors of the four vertices are examined whether they are matched in their directions and magnitudes (Step 15). When the examination result shows the motion vectors are satisfactorily matched to each other without a large dispersion, the motion vectors are considered to represent the same object in an image and a uniform weighting coefficient is .outputted for them (Steps 16 and 19). If there is found a large dispersion between the motion vectors, the unit-area to be transformed is judged to include a plurality of different objects in an image. A motion vector showing a particular direction and magnitude is separated and put into another matched group (Step 17). Weighting coefficients for each subarea of the transformable unit area are decided in such a way that a nearest neighboring motion vector of a remarked subarea and a motion vector matched thereto are weighted heavy and the motion vector judged to belong to other group or to show a particular direction is weighted light (Steps 18 and 19). The second example differs from the first example by using not only vector signs but also vector values and being therefore capable of more accurately determining a direction of each vector and discriminating two motion vectors directed in the same direction but having different magnitudes. Namely, more strict processing of the vectors can be accomplished. Fig. 17 shows a first example of a structure of an interframe predicting portion in a video decoding device according to the present invention. The video decoding device of Fig. 17 comprises a frame memory portion 121 for storing already decoded video signals, a weighting coefficient control portion 122 for determining and designating weighting coefficients for representative motion vectors inputted for every transformable unit-area, a motion vector interpolating portion 123 for determining a motion vector per pixel from the representative motion vectors and the weighting coefficients, a pixel value predicting portion 124 for preparing a predicted video signal from a video signal read from the frame memory portion 121 by using the motion vector per pixel. The frame memory portion 121 and the pixel value predicting portion 124 are similar in function to those of the conventional video decoding device. The video decoding device according to the present invention differs from the conventional video decoding device in that the weighting coefficient control portion 122 is provided for specifying a weighting coefficient for each representative motion vector and a motion vector interpolating portion 123 is used for determining and outputting motion vectors pixel by pixel by using representative motion vector and a weighting coefficient for each vector outputted by the weighting coefficient control portion 122. The operations of the weighting coefficient control portion 122 and the motion vector interpolating portion 123, both of which are essential portions of the embodiment, will be described below. However, both portions function similarly to those of the coding device according to the present invention. In other words, the decoding device performs the same processing operations that the coding device does. The weighting coefficient control portion 122 decides weighting coefficients for respective motion vectors of respective vertices (representative points) for a unit-area to be processed by the motion vector interpolating portion 123 and specifies the coefficients for each subdivision of the transformable unit area. The weighting coefficient control portion 122 also judges whether motion vectors of vertices of a unit-area for transformation represent a movement of the same object in an image, i.e., said unit-area exists on one object in the image or not. When having judged that the motion vectors concern the same object, the weight coefficient control portion 122 instructs the motion vector interpolating portion 123 to evenly weight all representative motion vectors of vertices. When having judged that any vertex of the unit-area exists on different objects, the weighting coefficient control portion 122 decides weighting coefficients in such a way that a motion vector of a vertex existing on the same object as a remarked subarea represented by a vertex is weighted with a larger weighting coefficient and a motion vector of a vertex judged to exist on a different object is weighted with a smaller weighting coefficient. The processing operations and outputs of the weighting coefficient control portion 122 are similar to those of the weighting coefficient control portion 113 of the video coding device. The motion vector interpolating portion 123 receives the designated weighting coefficients from the weighting coefficient control portion 122 and calculates motion vectors for every pixel in the subarea by using the received coefficients in the same way as described for weighting motion vectors by the motion vector interpolating portion 113 of the video coding device. The weighting coefficient control portion 122 may have M sets of weighting coefficients previously prepared and may select one of them as an optimal weight pattern for each unit-area or subarea, which coefficient is nearest to a weighting coefficient determined for the unit-area or subarea by the above-mentioned weighting coefficient processing. This feature is the same as described in the video coding device. In this case, the weighting coefficient control portion 122 gives information about the selected pattern to the motion vector interpolating portion 123. The motion vector interpolating portion 123 has the same weight patterns as held in the weighting coefficient control portion 122, determines motion vectors for pixels by using the corresponding weighting coefficients according to the specified weight pattern in the above-mentioned way and outputs the weighted motion vectors. Fig. 18 shows a second exemplified structure of an interframe predicting portion of a video decoding device according to the present invention. The video decoding device of Fig. 18 comprises a frame memory portion 181 for already decoded video signals, a motion vector converting portion 182 for calculating representative motion vectors for a dropped frame from representative motion vectors for a coded frame, a weighting coefficient control portion 183 for determining and indicating weighting coefficients of representative motion vectors inputted for a unit area, a motion vector interpolating portion 184 for calculating a motion vector for each pixel from the corresponding representative motion vector by using the weighting coefficients and a pixel value predicting portion 185 for preparing a predicted video signal from a video signal read from the frame memory portion 181 by using the motion vector per pixel. The frame memory portion 181, the weighting coefficient control portion 183, the motion vector interpolating portion 184 and the pixel value predicting portion 185 are similar in function to those of the first embodiment of the video decoding device. This second embodiment of the video decoding device differs from the first embodiment by provision of motion vectors of encoded video frame and of the motion vector converting portion 182 for outputting motion vectors of video frames omitted without having been encoded. The operation of the motion vector converting portion 182 is as follows: The motion vector converting portion 182 internally divides a representative motion vector for an input coded video frame according to a temporal distance between two neighboring video frames and a temporal position of an omitted video frame. For example, a first frame of a video sequence was first encoded and a fifth frame was next encoded. Namely, three frames (second, third and fourth frames) were skipped. Now, representative motion vectors for the fifth video frames are inputted. In this case, a temporal distance between the coded frames (i.e., the first frame and the fifth frame) is 4. As the motion vectors for the fifth frame may represent a movement for 4-frame time, the vector value is proportionally distributed to the dropped frames: motion vectors for the second frame, the third frame and the fourth frame are determined by multiplying the fifth-frame motion-vector value by 1/4, 1/2 and 3/4 respectively. The determined values are outputted as the motion vectors for respective dropped frames. The outputted motion vectors are transferred to the weighting coefficient control portion 183 and the motion vector interpolating portion 184, wherein the vectors are processed in the same way as in the first embodiment. Finally, predicted video-signals are prepared by the pixel value predicting portion 185 and outputted as interpolating video-signals of the dropped video-frames. As is apparent from the foregoing description, the video coding device and the video decoding device according to the present invention offers following advantages: A video coding device is featured in that weighting coefficient control means determines weighting coefficient for each representative vector obtained by vector detecting means and give an instruction to motion vector interpolating means which can, therefore, accurately determine a motion vector of each pixel with a minimized effect of unrelated motion vectors even in the case of mismatches existing between a transformable unit-area and a content or position and movement of an object, realizing the improvement of the efficiency of interframe prediction coding. Accordingly, the application of this video-coding device can realize a very highly efficient video-coding system that is desired for use in very low bit-rate video-communication. A video coding device is featured in that weighting coefficient control means selects a suitable one of previously prepared and stored patterns of weighting coefficients for each representative motion vector and gives an instruction to motion vector interpolating means which can, therefore, accurately determine a motion vector of each pixel with a minimized effect of unrelated motion vectors in each transformable unit area, achieving a considerable improvement of the efficiency of interframe prediction coding by applying the above-mentioned simplified method. Owing to the increased efficiency of video coding with a reduced data amount, this video-coding device can realize saving in hardware and software and be manufactured with a reduced size at a low cost. A video-coding device is featured in that its weighting coefficient control means determines a weighting coefficient according to only a direction of each representative motion vector, considerably simplifying processing operations of the weighting coefficient control means. Accordingly, the video-coding device, which has a high efficiency video-interframe prediction method giving an increased efficiency of video coding, has a small size and is inexpensive to manufacture. A video-coding device is featured in that weighting coefficient control means determines a vector weighting coefficient according to, a vector value of each representative motion vector to enable motion vector interpolating means to strictly discriminate motion vectors irrelevant to a pixel in a transformable unit area, achieving an improved quality of motion vectors of respective pixels. Accordingly, the video-coding device has a high efficiency video-interframe predicting system giving an increased efficiency of video coding enough to realize the encoding of video-signals at a further decreased bit-rate. A video-decoding device is featured in that weighting coefficient control means determines a weighting coefficient for each representative vector obtained by vector detecting means and gives an instruction to motion vector interpolating means which can, therefore, accurately determine a motion vector of each pixel with a minimized effect of unrelated pixel's motion vector even in the case of mismatches existing between a transformable unit-area and a content or position and movement of an object, achieving a considerable improvement of the efficiency of interframe prediction and an increased quality of decoded video-signals. Accordingly, this video-decoding device can obtain high-quality decoded video-signals suitable for use in a very low bit-rate video-communications. A video-decoding device is featured in that weighting coefficient control means selects a suitable one of previously prepared and stored patterns of weighting coefficients for each representative motion vector and gives an instruction to motion vector interpolating means which can, therefore, accurately determine a motion vector of each pixel with a minimized effect of unrelated motion vectors in each transformable unit area, achieving a considerable improvement of the efficiency of interframe prediction and an increased quality of decoded video-signals by using the above-mentioned simplified method. Accordingly, this video-decoding device can realize saving in hardware and software and be manufactured with a reduced size at a low cost. A video-decoding device is featured in that its weighting coefficient control means determines a weighting coefficient according to only a direction of each representative motion vector, considerably simplifying processing operations of the weighting coefficient control means. Accordingly, the video-decoding device, which has a high efficiency video-interframe prediction method giving an increased efficiency of video coding, has a small size and is inexpensive to manufacture. A video-decoding device is featured in that weighting coefficient control means determines a vector weighting coefficient according to a vector value of each representative motion vector to enable motion vector interpolating means to strictly discriminate motion vectors irrelevant to a pixel in a transformable unit area, achieving an improved quality of motion vectors of respective pixels. Accordingly, the video-decoding device has a high efficiency video-interframe prediction system method giving an increased efficiency of video coding sufficient to realize the encoding of video-signals at a further decreased bit-rate. A video-decoding device is featured in that motion vector converting means determines a motion vector for a dropped frame from a representative motion vector inputted for a transformable unit area of encoded frame and prepares an interpolating video-signal corresponding to a frame skipped without having been encoded. This enables obtaining interpolating video-signals according to motion vectors of pixels accurately determined with a minimized effect of motion vectors irrelevant to those in a transformable unit-area. Namely, quality and temporal resolution of restorable images can be remarkably improved. Accordingly, the decoding device can obtain decoded video signals giving a smooth motion picture of high image quality even in a very low bit-rate video-communications.

Claims: A video-coding device comprising frame memory means (111) for storing already encoded video signals, motion vector detecting means (112) for determining a representative motion vector associated with a representative point per unit area from a difference between an input video signal and a video signal read from the frame memory means (111), motion vector interpolating means (114) for determining a motion vector (MV) per pixel using the representative motion vector, pixel value predicting means (115) for preparing a predicted video signal from a video signal read from the frame memory means (111) by using the motion vector per pixel, characterised in that there is provided weighting coefficient control means (113) operable in the determination of the motion vector for each respective pixel to determine, for said respective pixel, weighting coefficients for the set of representative motion vectors (MV1 to MV4) associated with a corresponding set of mutually neighbouring said representative points around said respective pixel according to the relationship between the representative motion vectors of said set of representative points, said weighting coefficient control means (113) being arranged to instruct the motion vector interpolating means (114) to weight said representative motion vectors determined by the motion vector detecting means (112) in accordance with said weighting coefficients, and said motion vector interpolating means (114) being arranged to determine said motion vector per pixel in accordance with the weighted set of representative motion vectors. A video-coding device as defined in claim 1, characterized in that the weighting coefficient control means (113) is arranged to select one of a set of predetermined patterns of weighting coefficients for the representative motion vectors and to instruct the motion vector interpolating means (114) to employ the selected predetermined pattern of weighting coefficients for weighting said set of representative motion vectors. A video-coding device according to claim 1 or claim 2, characterized in that the weighting coefficient control means (113) is arranged to determine the vector weighting coefficients according to the directions of the representative motion vectors. A video-coding device according to claim 1 or claim 2, characterized in that the weighting coefficient control means (113) is arranged to determine the vector weighting coefficients according to the vector values of the representative motion vectors. A video-decoding device comprising frame memory means (121; 181) for storing already decoded video signals, motion vector interpolating means (123; 184) for determining a motion vector per pixel using a representative motion vector inputted per unit area and associated with a representative point, pixel value predicting means (124; 185) for preparing a predicted video signal from a video signal read from the frame memory means (121; 181) by using the motion vector per pixel, characterized in that there is provided weighting coefficient control means (122; 183) operable in the determination of the motion vector for each respective pixel to determine for said respective pixel weighting coefficients for the set of representative motion vectors associated with a corresponding set of mutually neighbouring said representative points around said respective pixel according to the relationship between the representative motion vectors of said set of representative points, said weighting coefficient control means (122; 183) being arranged to instruct the motion vector interpolating means (123; 184) to weight said inputted representative motion vectors in accordance with said weighting coefficients, and said motion vector interpolating means (123; 184) being arranged to determine said motion vector per pixel in accordance with the weighted set of representative motion vectors. A video-decoding device according to claim 5, characterized in that the weighting coefficient control means (122; 183) is arranged to select one of a set of predetermined patterns of weighting coefficients for the representative motion vectors and to instruct the motion vector interpolating means (123; 184) to employ the selected predetermined pattern of weighting coefficients for weighting said set of representative motion vectors. A video-decoding device according to claim 5 or claim 6, characterized in that the weighting coefficient control means (122; 184) is arranged to determine the vector weighting coefficients according to the directions of the representative motion vectors. A video-decoding device according to claim 5 or claim 6, characterized in that the weighting coefficient control means (122; 184) is arranged to determine the vector weighting coefficients according to the vector values of the representative motion vectors. A video-decoding device according to any of claims 5 to 8, characterized in that motion vector converting means (182) is provided for calculating representative motion vectors for a skipped image frame which was not encoded from representative motion vectors inputted per unit area of an encoded image frame, the device being arranged to prepare an interpolating video signal corresponding to the skipped image frame using said calculated representative motion vectors.

Inventors: EMA NOBUYUKI, HIBI KEIICHI, EMA, NOBUYUKI, HIBI, KEIICHI
Applicants: SHARP KK, SHARP KABUSHIKI KAISHA
Application year: 1996
Main classification: H04N7
IPC codes: G06T9, H04N7