Abstract:
An image coding apparatus which performs an adaptive coding process in macroblock units each comprising a predetermined number of pixels into which the image space is divided, comprises: a color motion vector coding unit for coding either a frame-basis color motion vector or a field-basis color motion vector of a target macroblock to be coded which is used in either the frame-by-frame motion compensation coding process or the field-by-field motion compensation coding process for an interlaced color signal, based on its prediction value; and a shape motion vector coding unit for coding a frame-basis shape motion vector of the target macroblock which is used in the frame-by-frame motion compensation coding process for an interlaced shape signal, based on a prediction value obtained from a color motion vector and a shape motion vector of a coded macroblock.

Description:
FIELD OF THE INVENTION 
     The present invention relates to an image coding apparatus, an image decoding apparatus, an image coding method, an image decoding method, and a data storage medium. More particularly, the present invention relates to an apparatus and a method for coding a motion vector (shape motion vector) of a shape signal indicating a shape of an object included in an image on a frame, an apparatus and a method for decoding a coded signal obtained by coding the shape motion vector, and a data storage medium for storing an image processing program for coding or decoding the shape motion vector by software. 
     BACKGROUND OF THE INVENTION 
     In recent years, we have greeted the age of multimedia in which audio, video and other data are integrally handled, and the conventional information media (i.e., means for transmitting information media), such as newspapers, magazines, radios, and telephones, have been adopted as the subjects of multimedia. Generally, “multimedia” means to represent, not only characters, but also diagrams, speeches, and especially images simultaneously in relation with each other. In order to handle the conventional information media as the subjects of multimedia, it is necessary to transform the information into a digital format. 
     When the quantity of data possessed by each information medium described above is estimated as the quantity of digital data, in case of characters, the data quantity for each character is 1˜2 byte. However, in case of speech, the data quantity is 64 Kbits per second (quality for telecommunication) and, in case of moving picture, it is more than 100 Mbits per second (quality for current television broadcasting). So, as for the information media described above, it is not practical to handle such massive data as it is in the digital format. For example, although visual telephones have already been put to practical use by ISDN (Integrated Services Digital Network) having a transmission rate of 64 Kbps-1.5 Mbps, it is impossible to transmit an image of a television camera as it is by the ISDN. 
     So, data compression technologies are demanded. In case of visual telephones, the moving picture compression technologies standardized as H.261 and H.263 by ITU-T (International Telecommunication Union-Telecommunication Sector) are employed. Further, according to the data compression technology based on MPEG1, it is possible to record image data, together with audio data, in an ordinary music CD (compact disk). 
     MPEG (Moving Picture Experts Group) is an international standard of data compression for data of a moving picture (an image signal of a moving picture). In MPEG1, data of a moving picture is compressed to 1.5 Mbps, i.e., data of a television signal is compressed to about 1/100. Since the transmission rate to which MPEG1 is directed is limited to about 1.5 Mbps, MPEG2 is standardized to meet the demand for higher image quality. In MPEG2, data of a moving picture is compressed to 2˜15 Mbps. 
     Under the existing circumstances, standardization of MPEG4 is now proceeded by the working group for standardization of MPEG1 and MPEG2 (ISO/IEC JTC1/SC29/WG11). MPEG4 enables coding and signal operation in object units, and thereby realizes new functions required in the age of multimedia. MPEG4 has originally aimed at standardization of a coding method at a low bit rate, but the aim of standardization is now extended to a more versatile coding process at a high bit rate adaptable to an interlaced image. 
     FIGS.  8 ( a )- 8 ( f ) are diagrams for explaining an object-by-object (object-based) coding process. 
     Shown in FIG.  8 ( a ) is an image space Ts obtained from a color signal comprising a luminance signal and a chrominance signal and including an object To corresponding to an image. Shown in FIG.  8 ( b ) is an image space Ss obtained from a shape signal indicating a shape of the object. In the image space Ss shown in FIG.  8 ( b ), a portion painted in black is an inside region of an object So. The fish image (object) To in the image space Ts is represented by a color signal corresponding to pixels composing the inside region (pixels inside the object). 
     According to MPEG4, an image signal comprising the shape signal and the color signal corresponding to an object is coded for each unit region (macroblock) composed of a predetermined number of pixels. While MPEG1 and MPEG2 have introduced a technique for coding the color signal for each macroblock, MPEG4 adopts a technique for coding the color signal and the shape signal for each macroblock. 
     FIG.  8 ( c ) schematically shows a blocking process in which the color signal is divided into signals corresponding to respective macroblocks Tmb composing the image space Ts. FIG.  8 ( d ) schematically shows a blocking process in which the shape signal is divided into signals corresponding to respective macroblocks Smb composing the image space Ss. 
     With a view to performing an efficient coding process for a frame (image space)including an object, according to MPEG4, only the inside of a rectangular region including the object in the frame rather than an image signal corresponding to the entire frame is coded. The rectangular region is called a “bounding box”. 
     To be specific, for the color signal, as shown in FIG.  8 ( e ), the bounding box Tb is divided into portions corresponding to respective macroblocks (blocking process). Also, for the shape signal, as shown in FIG.  8 ( f ), a bounding box Sb is divided into portions corresponding to macroblocks (blocking process). 
     The image signal comprising the color signal and the shape signal is subjected to a motion compensation coding process for each macroblock like a motion compensation coding process according to MPEG1 and MPEG2. 
     In this motion compensation coding process, there should be a match of motion change from frame to frame between the color and shape signals. In actuality, however, there is no match between real motion of the object and motion of the object of respective signals which improves coding efficiency. 
     The motion compensation coding process is to perform coding to a motion compensation error of the color or shape signal. If motion information giving the motion compensation error smaller than that of motion information (motion vector) indicating real motion of the object is obtained in the motion compensation coding process, it is more desirable to perform the motion compensation process based on the motion information giving the smaller error, because this increases coding efficiency. Since the color signal is a multi-valued signal, and the shape signal is a binary signal for distinguishing between pixels inside the object and pixels outside the object, their qualities differ from each other a little, and correspondingly the motion vectors thereof giving smaller errors differ from each other a little. 
     The motion compensation error is a difference value between a color or shape signal corresponding to a macroblock to-be-processed (target macroblock)i.e., macroblock to be coded, in a current frame (frame being coded) and a color or shape signal corresponding to a region equal to the macroblock to-be-processed in size (prediction macroblock) in a previous (past) frame (coded frame serving as a reference frame). The motion vector of each macroblock is position information indicating the positioning relationship between the macroblock to-be-processed on the frame and the prediction macroblock. 
     According to MPEG4, the color signal and the shape signal are subjected to the motion compensation coding process based on the motion vectors (hereinafter referred to as a color motion vector and a shape motion vector) obtained from the corresponding color and shape signals, respectively. 
     Although a match might not be always found between the color motion vector and the corresponding shape motion vector with respect to size or direction, there is high correlation between them. For this reason, in MPEG4, according to the shape motion vector of the coded macroblock in a current frame and the corresponding color motion vector, a prediction value of the shape motion vector of the target macroblock is generated, and a difference value between the prediction value and the value of the shape motion vector of the target macroblock, i.e., information of the shape motion vector of the target macroblock is coded. 
     A description will be given of a prior art image decoding apparatus according to MPEG4. 
     FIG. 9 is a block diagram for explaining an MPEG compliant image coding apparatus according to the prior art. 
     Turning now to FIG. 9, there is shown an image coding apparatus  1000 . The image coding apparatus  1000  comprises a color coding section  1100  which subjects the color signal to a coding process including the motion compensation coding process and a shape coding section  1200  which subjects the shape signal to a coding process including the motion compensation coding process. 
     The shape coding section  1200  includes a blocking unit  1210  which performs a blocking process in such a way that a shape signal Ss corresponding to an object input through an input terminal  1201  is divided into signals corresponding to macroblocks and outputs blocked shape signals Bs, and a memory  1260  which stores a shape signal (locally decoded shape signal) Lds of a coded frame (previous frame) as a reference shape signal MLds. 
     The shape coding section  1200  further includes a shape motion detector  1240  which detects a motion vector (shape motion vector) MVs of the target macroblock to-be-processed based on the reference shape signal MLds stored in the memory  1260  and the blocked shape signal Bs output from the blocking unit  1210 , and outputs a motion compensation error signal Dfs of the shape signal obtained in this detection process, a shape motion compensator  1250  which performs the motion compensation process to the blocked shape signal Bs based on the reference shape signal MLds and the shape motion vector MVs and outputs a prediction shape signal Ps for the target macroblock (shape signal of the prediction macroblock), and a mode decision unit  1280  which performs a coding mode decision process for the shape signal based on the blocked shape signal Bs output from the blocking unit  1210  and the motion compensation error signal Dfs output from the shape motion detector  1240  and outputs a shape coding mode signal Mo. 
     The shape coding mode signal Mos indicates that the target macroblock corresponding to the shape signal is a macroblock outside the object, an intra-macroblock which has been intra-frame coded using correlation between pixel values in a frame, or an inter-macroblock which has been inter-frame coded using correlation of pixel values between frames. The macroblock outside the object is a macroblock, the pixels of which are all positioned outside the object. The intra-macroblock and the inter-macroblock are macroblocks inside the object, and at least some of their pixels are positioned inside the object. 
     The shape coding section  1200  still further includes a shape encoder  1220  which performs an arithmetic coding process (inter-frame coding process) with reference to the prediction shape signal Ps or an arithmetic coding process (intra-frame coding process) without reference to the prediction shape signal Ps for the blocked shape signal Bs according to the shape coding mode signal Mos and outputs a coded shape signal Cs, and outputs a decoded signal (locally decoded shape signal) Lds of the coded shape signal Cs, a variable length encoder  1230  which performs a variable length coding process to the coded shape signal Cs and the shape coding mode signal Mos and outputs a coded shape signal Es, and a motion vector coding unit  1270  which performs coding to the shape motion vector MVs according to the coding mode signal Mos and information Mmvt and Mmot relating to the motion vector sent from the color coding apparatus  1100  and outputs a coded motion vector signal Emvs. 
     In this shape coding apparatus  1200 , the locally decoded shape signal Lds from the shape encoder  1220  is stored in the memory  1260  as the reference shape signal MLds, which is output to the shape motion detector  1240  and the shape motion compensator  1250  and used in the motion detection process and the motion compensation process for the shape signal, respectively. The locally decoded shape signal Lds is sent to the color coding section  1100  and used in the motion detection process and the motion compensation process for the color signal. 
     The color coding section  1100  includes a blocking unit  1100  which performs a blocking process in such a way that a color signal St corresponding to an object input through an input terminal  1101  is divided into signals corresponding to respective macroblocks and outputs blocked color signals Bt and a memory  1160  which stores a color signal (locally decoded color signal) Ldt of a coded frame (previous frame) as a reference color signal MLdt. 
     The color coding section  1100  further includes a color motion detector  1140  which detects a color motion vector MVt of the target macroblock based on the reference color signal MLdt stored in the memory  1160 , the locally decoded shape signal Lds from the shape coding section  1200 , and the blocked color signal Bt from the blocking unit  1100 , and outputs a motion compensation error signal DFt of the color signal, a mode decision unit  1180  which performs a coding mode decision process for the color signal based on the blocked color signal Bt from the blocking unit  1110 , the motion compensation error signal DFt from the color motion detector  1140 , and the blocked shape signal BS from the shape coding section  1200  and outputs a color coding mode signal Mot, and a color motion compensator  1150  which performs a motion compensation process to the blocked color signal Bt based on the reference color signal MLdt, the color motion vector MVt, and the locally decoded shape signal Lds from the shape coding section  1200 , and outputs a prediction color signal Pt of the target macroblock (color signal of the prediction macroblock). 
     The color coding mode signal Mot indicates the target macroblock is the macroblock outside the object, the intra-macroblock, or the inter-macroblock. 
     The color coding section  1100  still further includes a color encoder  1120  which performs a wave coding process with reference to the prediction color signal Pt(inter-frame coding process) or a wave coding process without reference to the prediction color signal Pt (intra-frame coding process) for the blocked color signal Bt and outputs a coded color signal Ct and outputs a decoded signal (locally decoded color signal) Ldt of the coded color signal Ct, a variable length encoder  1130  which performs variable length coding to the coded color signal Ct and the color coding mode signal Mot and outputs a coded color signal Et, a motion vector coding unit  1170  which perform coding to the color motion vector MVt and outputs a coded motion vector signal Emvt and outputs the information Mmvt and Mmot relating to the color motion vector to the motion vector coding unit  1270 . 
     In this color coding section  1100 , the locally decoded color signal Lds from the color encoder  1120  is stored in the memory  1160  as the reference color signal MLds, which is output to the color motion detector  1140  and the color motion compensator  1150  and used in motion detection and motion compensation processes for the color signal, respectively. 
     It should be remembered that the image signal according to MPEG includes a transparency signal indicating the composition ratio in which the color signal corresponding to the object and another color signal are composited, and the image coding apparatus includes a transparency coding section for coding the transparency signal in addition to the shape coding section  1200  and the color coding section  1100 , although this is not illustrated herein. The construction of the transparency coding section is almost identical to that of the color coding section  1100  and differs from the same in that the transparency signal is subjected to a motion compensation process according to the color motion vector MVt. That is, the transparency coding section includes no motion detector. 
     Subsequently, a description will be given of operation of such a constructed image coding apparatus  1000 . 
     When the shape signal Ss of the image signal corresponding to a given object is input to the input terminal  1201  of the shape coding section  1200 , the blocking unit  1210  divides the shape signal Ss into signals corresponding to respective macroblocks into which the rectangular region (bounding box) is divided, to generate blocked shape signals Ss. The blocked shape signal Bs is supplied to the shape motion detector  1240 , the shape encoder  1220 , and the mode decision unit  1280  in the shape coding unit  1200 , the mode decision unit  1180  in the color coding unit  1100 . 
     The shape motion detector  1240  detects the shape motion vector MVs of the target macroblock based on the blocked shape signal Bs and the reference shape signal MLds of the previous frame stored in the memory  1260 . The shape motion vector MVs is output to the shape motion compensator  1250  and the motion vector coding unit  1270 . 
     At this time, the shape motion detector  1240  also outputs the motion compensation error signal DFs of the shape signal which has been generated in the motion detection process to the mode decision unit  1280 . The mode decision unit  1280  performs the coding mode decision process for the target macroblock based on the blocked image signal Bs and the motion compensation error signal DFs and outputs the shape coding mode signal Mos. 
     The shape motion compensator  1250  performs motion compensation to the blocked shape signal Bs based on the shape motion vector MVs and the reference shape signal MLds, to generate the prediction shape signal Ps for the target macroblock. The shape motion vector coding unit  1270  performs coding to the shape motion vector MVs according to the shape coding mode signal Mos and the information Mmvt and Mmot relating to the motion vectors from the motion vector coding unit  1170 , and outputs the coded shape motion vector signal Emvs through the output terminal  1203 . 
     The shape encoder  1220  performs arithmetic coding to the blocked shape signal Bs with or without reference to the prediction shape signal Ps according to the shape coding mode signal Mos, to generate the coded shape signal Cs and performs arithmetic decoding to the coded shape signal Cs to generate the locally decoded shape signal Lds, which is stored in the memory  1260  as the reference shape signal MLds. 
     The variable length encoder  1230  transforms the coded shape signal Cs and the shape coding mode signal Mos into variable length codes and outputs the resulting coded shape signal Es through the output terminal  1202 . 
     When the color signal St of the image signal corresponding to the given object is input to the input terminal  1101  of the color coding section  1100 , the blocking unit  1110  divides the color signal St into signals corresponding to respective macroblocks into which the rectangular region (bounding box) is divided, to generate blocked color signals St. The blocked color signal Bt is supplied to the color motion detector  1140 , the color encoder  1120 , and the mode decision unit  1180 . 
     The color motion detector  1140  detects the color motion vector MVt of the target macroblock based on the blocked color signal Bt, the reference color signal MLdt stored in the memory  1160 , and the locally decoded shape signal Lds from the shape coding section  1200 . The color motion vector MVt is output to the color motion compensator  1150  and the motion vector coding unit  1170 . 
     At this time, the color motion detector  1140  also outputs the motion compensation error signal DFt of the color signal generated in the motion detection process to the mode decision unit  1180 . The motion decision unit  1180  performs the coding mode decision process for the target macroblock based on the blocked image signal Bt, the motion compensation error signal DFt, and the blocked shape signal Bs, and outputs the color coding mode signal Mot. 
     The color motion compensator  1150  performs motion compensation for the blocked color signal Bt based on the color motion vector MVt, the reference color signal MLdt, and the locally decoded shape signal Lds, to generate the prediction color signal Pt for the target macroblock. The color motion vector coding unit  1170  performs coding to the color motion vector MVt according to the color coding mode signal Mot. Thereby, the coded motion vector signal Emvt is output through the output terminal  1103  and the information Mmvt and Mmot relating to the color motion vector MVt is supplied to the motion vector coding unit  1270  in the shape coding unit  1200  from the color motion vector coding unit  1170 . 
     The color encoder  1120  performs wave coding to the blocked color signal Bt with or without reference to the prediction color signal Pt based on the color coding mode signal Mot and the locally decoded shape signal Lds, to generate the coded color signal Ct and performs wave decoding to the coded color signal Ct to generate the locally decoded shape signal Ldt, which is stored in the memory  1160  as the reference color signal MLdt. 
     The variable length encoder  1130  transforms the coded color signal ct and the color coding mode signal Mot into variable length codes and outputs the coded color signal Et through the output terminal  1102 . 
     A description will be given of an MPEG4 compliant image decoding apparatus. 
     Turning to FIG. 10, there is shown an image decoding apparatus  2000 . The image decoding apparatus  2000  is adapted to perform a decoding process for the coded image signal sent from the image coding apparatus  1000  shown in FIG. 9, and comprises a color decoding section  2100  which performs a decoding process including a motion compensation decoding process for the coded color signal Et sent from the color coding section  1100  and a shape decoding section  2200  which performs a decoding process including the motion compensation decoding process for the coded shape signal Es sent from the shape coding section  1200 . 
     The shape decoding section  2200  includes a variable length decoder  2210  which performs variable length decoding to the coded shape signal Es corresponding to an object input through an input terminal  2201  and outputs a decoded shape signal As and the shape coding mode signal Mos and a shape decoder  2220  which performs arithmetic decoding to the decoded shape signal As corresponding to the target macroblock with or without reference to a prediction shape signal Ps according to the coding mode signal Mos and outputs a decoded shape signal Ds. 
     The shape decoding section  2200  further includes a memory  2260  which stores the decoded shape signal Ds as a reference shape signal MDs of a decoded frame (previous frame), and an inverse blocking unit  2230  which integrates the decoded shape signals Ds in such a way that they correspond to respective macroblocks composing the bounding box and outputs the resulting regenerated shape signal Rs through an output terminal  2203 . 
     The shape decoding section  2200  still further includes a motion vector decoding unit  2240  which decodes the coded shape motion vector signal Emvs sent from the image coding apparatus  1000  according to the shape coding mode signal Mos and the color motion vector information Mmvs and Mmot from the color decoding section  2100 , to provide a decoded shape motion vector signal Dms, and a shape motion compensator  2250  which generates the prediction shape signal Ps for the target macroblock based on the decoded shape motion vector signal Dmvs and the reference shape signal MDs. 
     The color decoding section  2100  includes a variable length decoder  2110  which performs variable length decoding to the coded color signal Et corresponding to an object supplied through an input terminal  2101  and outputs a decoded color signal At and the color coding mode signal Mot, and a color decoder  2120  which performs wave coding to the decoded color signal At with or without reference to the prediction color signal Pt and outputs a decoded color signal Dt. 
     The color decoding section  2100  further includes a memory  2160  which stores the decoded color signal Dt as a reference color signal MDt of a decoded frame (previous frame), an inverse blocking unit  2130  which integrates the decoded color signals Dt in such a way that they correspond to respective macroblocks composing the bounding box and outputs the resulting regenerated color signal Rt through an output terminal  2103 , a motion vector decoding unit  2140  which decodes the coded color motion vector signal Emvt sent from the image coding apparatus  1000  according to the color coding mode signal Mot, to provide a decoded color motion vector signal Dmvt, and outputs the information Mmvt and Mmot of the color motion vector, and a color motion compensator  2150  which generates a prediction color signal Pt for the target macroblock based on the decoded color motion vector signal Dmvt, the reference color signal MDt, and the decoded shape signal Ds from the shape decoding section  2200 . 
     Subsequently, a description will be given of operation of such a constructed image decoding apparatus  2000 . 
     When the coded shape signal Es corresponding to the object is input through the input terminal  2201 , the variable length decoder  2210  performs variable length decoding to the coded shape signal Es in macroblock units to generate the decoded shape signal As and the shape coding mode signal Mos for each macroblock. 
     The coded shape motion vector signal Emvs is supplied to the motion vector decoding unit  2240  through the input terminal  2202 , which decodes the signal Emvs according to the shape coding mode signal Mos and the information Mmvs and Mmot from the color decoding section  2100  to provide the decoded shape motion vector signal Dmvs. The shape motion compensator  2250  generates the prediction shape signal Ps for the target macroblock based on the decoded shape motion vector signal Dmvs and the reference shape signal MDs stored in the memory  2260 . 
     The shape decoder  2220  performs arithmetic decoding to the decoded shape signal As for the target macroblock with or without reference to the prediction shape signal Ps to generate the decoded shape signal Ds, which is stored in the memory  2260  as the reference shape signal MDs, and output to the inverse blocking unit  2230 , which integrates the decoded shape signals Ds in such a way that they correspond to respective macroblocks composing the bounding box and outputs the resulting regenerated shape signal Rs through the output terminal  2203 . 
     When the coded color signal Et corresponding to the object is input through the input terminal  2101 , the variable length decoder  2110  performs variable length decoding to the coded color signal Et in macroblock units, to generate the decoded color signal At and the color coding mode signal Mot for each macroblock. 
     The coded color motion vector signal Emvt is input to the motion vector decoding unit  2140  through the input terminal  2102 , which decodes the signal Emvt according to the color coding mode signal Mot and outputs the decoded color motion vector signal Dmvt and the information Mmvt and Mmot relating to the color motion vector. The color motion compensator  2150  generates the prediction color signal Pt for the target macroblock based on the decoded color motion vector signal Dmvt, the reference shape signal MDt stored in the memory  2160 , and the decoded shape signal Ds from the shape decoder  2220 . 
     The color decoder  2120  performs wave decoding to the decoded color signal At corresponding to the target macroblock with or without reference to the prediction color signal Pt based on the color coding mode signal Mot and the decoded shape signal Ds from the shape decoding section  2000  to generate the decoded color signal Dt, which is stored in the memory  2160  as the reference color signal MDt and output to the inverse blocking unit  2130 . The inverse blocking unit  2130  integrates the decoded color signals Dt in such a way that they correspond to respective macroblocks composing the bounding box and outputs the resulting regenerated color signal Rt through the output terminal  2103 . 
     Hereinafter, a description will be given of a coding process for the color motion vector and the shape motion vector performed by the image coding apparatus  1000 . 
     Initially, a description will be given of the motion compensation process by the use of the motion vector. 
     According to MPEG4, in a frame-by-frame motion compensation process for the color signal (interlaced or non-interlaced color signal) forming one macroblock (image space composed of (16×16) pixels, as shown in FIG.  11 ( a ), four motion vectors MV 1 -MV 4  are used to generate a prediction color signal for a target macroblock to-be-processed MB. The motion vectors MV 1 -MV 4  correspond to four blocks (image spaces each composed of (8×8) pixels B 1 -B 4  composing the macroblock MB, respectively. When these motion vectors MV 1 -MV 4  are equivalent, the frame-by-frame motion compensation process for the macroblock MB is, as shown in FIG.  11 ( d ), equivalent to the motion compensation process by the use of one motion vector MV (MV=MV 1 =MV 2 =MV 3 =MV 4 ). 
     Meanwhile, in a field-by-field motion compensation process for the color signal (interlaced color signal) corresponding to one macroblock, as shown in FIG.  11 ( b ), first and second motion vectors MVf 1  and MVf 2  are used to generate the prediction color signal for the target macroblock MB in a frame to-be-processed. The first motion vector MVf 1  is a motion vector of a first sub-macroblock MBf 1  comprising pixels on scanning lines in an odd field in the frame, and the second motion vector MVf 2  is a motion vector of a second sub-macroblock MVf 2  comprising pixels on scanning lines in an even field in the frame. Each of the sub-macroblocks is an image space comprising (16×8) pixels, although they are shown as square regions for convenience in the illustrated example shown in FIG.  11 ( b ). When the first and second motion vectors MVf 1  and MVf 2  are equivalent, the field-by-field motion compensation process for the macroblock MB is, as shown in FIG.  11 ( d ), equivalent to the frame-by-frame motion compensation process by the use of one motion vector MV (MV=MV 1 =MV 2 =MV 3 =MV 4 ). 
     On the other hand, a frame-by-frame motion compensation process for the shape signal is different from the frame-by-frame motion compensation process for the color signal. As shown in FIG.  11 ( d ), only the motion vector MV is used to generate the prediction shape signal for the target macroblock MB. 
     Subsequently, a description will be given of a process for coding motion vectors. 
     In the process for coding the motion vector (color motion vector) of the color signal corresponding to the target macroblock (16×16 pixels), difference (difference value) between a prediction motion vector predicted from a motion vector of a reference macroblock which has been coded and is positioned adjacently to the target macroblock and a motion vector of the target macroblock is coded as motion vector information of the target macroblock. 
     Hereinafter, a description will be given of the process for predicting the color motion vector with reference to FIGS.  12 ( a )- 12 ( d ). 
     (1) Motion Vector Prediction Process in Frame-by-Frame Motion Compensation Process 
     (1a) When a target macroblock MBx has four motion vectors, as shown in FIG.  12 ( a ), the prediction value of a motion vector MVt 0   a  of a block B 0   a  in the target macroblock MBx is generated based on motion vectors MVt 1   a , MVt 2   a , and MVt 2   b  of reference blocks RB 1   a , RB 2   a , and RB 2   b  which are adjacent to the block B 0   a.    
     The prediction value of a motion vector MVt 0   b  of a block B 0   b  in the target macroblock MBx is, like the prediction value of the motion vector MVt 0   a , motion vectors MVt 0   a , MVt 2   b , and MVt 3  of reference blocks B 0   a , RB 2   b , and RB 3  which are adjacent to the block B 0   b . The prediction value of a motion vector MVt 0   c  of a block B 0   c  in the target macroblock MBx is, like the prediction value of the motion vector MVt 0   a , generated based on motion vectors MVt 1   b,  MVt 0   a , and MVt 0   b  of reference blocks RB 1   b , B 0   a , and B 0   b  which are adjacent to the block B 0   c.    
     The prediction value of a motion vector MVt 0   d  of a block B 0   d  in the target macroblock MBx is generated based on motion vectors MVt 0   c , MVt 0   a , and MVt 0   b  of reference blocks B 0   c , B 0   a , and B 0   b  which are adjacent to the block B 0   d.    
     (1b) When a target macroblock MB 0  has one motion vector MVt 0 , that is, when motion vectors MVt 0   a -MVt 0   d  of respective blocks B 0   a -B 0   d  in the target macroblock MBx are equivalent, as shown in FIG.  12 ( b ), the prediction value of the motion vector MVt 0  of the target macroblock MB 0  is generated based on motion vectors MVt 1 , MVt 2 , and MVt 3  of reference blocks RB 1 , RB 2 , and RB 3  which are adjacent to the target macroblock. 
     In the above description of the motion vector prediction process in the frame-by-frame motion compensation process, the reference macroblock adjacent to the target macroblock in the macroblock which has been subjected to the frame-by-frame motion compensation process and has four motion vectors. In a case where the reference macroblock has one motion vector, based on the assumption that four blocks composing the reference macroblock have the same motion vector, the prediction values of respective blocks composing the macroblock are generated. In another case where the reference macroblock is a macroblock which has been subjected to the field-by-field motion compensation process and has two motion vectors, based on the average of these motion vectors, the prediction values of respective blocks are generated. 
     (2) Motion Vector Prediction Process in Field-By-Field Motion compensation Process 
     The field-by-field motion compensation process is, as shown in FIG.  12 ( c ), for the case where a target macroblock MBy to-be-processed has first and second motion vectors MVt 01  and MVt 02 . The first motion vector MVt 01  is the motion vector of the sub-macroblock MBf 1  comprising (16×8) pixels on scanning lines in the odd field in the frame, and the second motion vector MVt 02  comprising (16×8) pixels is the motion vector of the sub-macroblock comprising pixels on scanning lines in the even field in the frame. In this case, the prediction values of the first and second motion vectors MVt 01  and MVt 02  of the target macroblock MBy are generated based on the motion vectors MVt 1 , MVt 2 , and MVt 3  of the reference blocks RB 1 , RB 2 , and RB 3  adjacent to the target macroblock MBy. 
     In the above description of the motion vector prediction process in the field-by-field motion compensation process, the reference macroblock adjacent to the target macroblock is the macroblock which has been subjected to the frame-by-frame motion compensation process and has four motion vectors. In a case where the reference macroblock has one motion vector, based on the assumption that four blocks composing the reference macroblock have the same motion vector, the prediction values of motion vectors MVt 01  and MVt 02  of the two sub-macroblocks MBf 1  and MBf 2  composing the target macroblock MBy are generated. 
     In another case where the reference macroblock is a macroblock which has been subjected to the field-by-field motion compensation process and has two motion vectors, the prediction value of the motion vector MVt 01  of the sub-macroblock MBf 1  corresponding to the odd field is generated based on the motion vector of the sub-macroblock corresponding to the odd field in the reference macroblock, and the prediction value of the motion vector MVt 02  of the sub-macroblock MBf 2  corresponding to the even field is generated based on the motion vector of the sub-macroblock corresponding to the even field in the reference macroblock. 
     Hereinafter, a description will be given of the shape motion vector prediction process in the shape signal motion compensation process. As described above, the target macroblock corresponding to the shape signal has always one motion vector. 
     In the motion vector (shape motion vector) coding process for the target macroblock (16×16 pixels), a difference value between the value of the prediction motion vector and the value of the shape motion vector of the target macroblock, i.e., the shape motion vector information of the target macroblock, is coded. 
     As already described, since there is high correlation between the motion vector of the shape signal and the motion vector of the color signal, the prediction value (prediction shape motion vector) of the shape motion vector of the target macroblock is generated based on both of the color motion vector of the coded reference macroblock and the shape motion vector of the coded reference macroblock. 
     The prediction value of the shape motion vector MVS 0  of the target macroblock MBs 0  shown in FIG.  12 ( d ) is generated by referring to the color motion vectors MVt 1 , MVt 2 , and MVt 3  of the blocks RB 1 , RB 2 , and RB 3  adjacent to the target macroblock MB 0  corresponding to the color signal shown in FIG.  12 ( b ) as well as the vectors MVs 1 , MVs 2 , and MVs 3  of the macroblocks RMBs 1 , RMBs 2 , and RMBs 3  adjacent to the target macroblock MBs 0  shown in FIG.  12 ( d ). 
     In the process for predicting the color motion vector and the shape motion vector, if macroblocks adjacent to the target macroblock are intra-macroblocks or macroblocks outside the object, they have no motion vectors, in which case, the prediction value of the motion vector of the target macroblock is generated using adjacent macroblocks which have motion vectors as reference macroblocks. 
     FIG. 13 is a block diagram for explaining a prior art motion vector coding unit included in the image coding apparatus  1000 . Shown in FIG. 13 is the motion vector coding unit (color motion vector coding unit )  1170  included in the color coding section  1100  and the motion vector coding unit (shape motion vector coding unit)  1270  included in the shape coding section  1200 . By way of example, a description will be given of a case where the target macroblock MB 0  has one color motion vector and the blocks RB 1 -RB 3  adjacent to the target macroblock MB 0  are used as reference blocks (see FIG.  12 ( b )). 
     The color motion vector coding unit  1170  is adapted to perform coding to the motion vector of the non-interlaced color signal, and includes an MV memory  102  which stores the motion vector MVt of the target macroblock input through an input terminal  1  and an MV memory  103  which stores the color coding mode signal Mot input through an input terminal  2 . The color coding mode signal Mot is the color motion vector effectiveness signal indicating that the target macroblock in the image space obtained from the color signal is one of the intra-macroblock and the macroblock outside the object or the inter-macroblock. According to this signal Mot, it is decided whether the reference macroblock adjacent to the target macroblock is one of the intra-macroblock and the macroblock outside the object, or the inter macroblock. 
     The color motion vector coding unit  1170  further includes an MV predictor  104  which generates an MV prediction value (the prediction value of the color motion vector of the target macroblock) Pmvt from the color motion vectors MVt 1 , MVt 2 , and MVt 3  of the reference blocks RB 1 , RB 2 , and RB 3  in the coded macroblock which have been stored in the MV memory  102  according to the color coding mode signal Mmot (Mot) from the MV memory  103 , and an MV encoder  105  which encodes the color motion vector MVt 0  of the target macroblock MB 0  with reference to the MV prediction value Pmvt from the MV predictor  104  and outputs the coded color motion vector signal Emvt. 
     The color motion vector coding unit  1170  still further includes a primary switch  100  provided between the input terminal  1  and the MV encoder  105 , and a secondary switch  101  provided between an output terminal  10  and the MV encoder  105 , the switches  100  and  101  being on-off controlled by the color coding mode signal Mot. 
     The shape motion vector coding unit  1270  is adapted to perform coding to the motion vector of the non-interlaced shape signal and includes an MV memory  202  which stores the shape motion vector MVs of the target macroblock input through an input terminal  4 , and an MV memory  203  which stores the shape coding mode signal Mos input through an input terminal  5 . The shape coding mode signal Mos is the shape motion vector effectiveness signal indicating that the target macroblock in the image space obtained from the shape signal is one of the intra-macroblock and the macroblock outside the object or the inter-macroblock. According to this signal Mos, it is decided whether each of the reference macroblock adjacent to the target macroblock is one of the intra-macroblock and the macroblock outside the object, or the inter macroblock. 
     The shape motion vector coding unit  1270  further includes an MV predictor  204  which generates an MV prediction value (the prediction value of the shape motion vector for the target macroblock) Pmvs from the shape motion vectors MVs 1 , MVs 2 , and MVs 3  in the coded macroblock which have been stored in the MV memory  202  according to the color coding mode signal Mot and the shape coding mode signal Mos, and an MV encoder  205  which encodes the shape motion vector MVS 0  of the target macroblock MBs 0  with reference to the MV prediction value Pmvs from the MV predictor  204  and outputs the coded shape motion vector signal Emvs. The MV predictor  204  is adapted to convert a 0.5 pixel-precision color motion vector from the MV memory  102  into a one pixel-precision color motion vector and perform the prediction process by the use of the one-pixel precision color motion vector. This is because the color coding section performs the motion compensation process by the use of the 0.5 pixel-precision while the shape coding section performs the motion compensation process by the use of the one-pixel precision. 
     The shape motion vector coding unit  1270  still further includes a primary switch  200  provided between an input terminal  4  and the MV encoder  205 , and a secondary switch  201  provided between an output terminal  11  and the MV encoder  205 , the switches  200  and  201  being on-off controlled by the shape coding mode signal Mos. 
     Subsequently, operation will be described. 
     The color motion vector coding unit  1170  performs coding to the color motion vector MVt and outputs the coded signal Emvt, and the shape motion vector coding unit  1270  performs coding to the shape motion vector MVs and outputs the coded signal Emvs. 
     When the color coding mode signal Mot is input to the input terminal  2  as the motion vector effectiveness signal, in accordance with this signal Mot, the switches  100  and  101  are on-off controlled. To be specific, when the color motion vector MVt 0  of the target macroblock MB 0  should be coded, the switches  100  and  101  are in the ON-state, or otherwise, they are in the OFF-state. 
     In other words, when the target macroblock is the intra-macroblock or the macroblock outside the object, and therefore the coding process for the motion vector is unnecessary, the switches  100  and  101  are in the OFF-state. On the other hand, when the target macroblock is the inter-macroblock, the switches  101  and  101  are in the ON-state. 
     The color motion vector MVt for each macroblock is temporarily stored in the MV memory  102 . To be more detailed, the color motion vectors (see FIG.  11 ( a )) of the blocks composing each macroblock are stored in the MV memory  102 , from which the motion vectors MVt 1 -MVt 3  of the reference blocks MB 1 -MB 3  adjacent to the target macroblock MB 0  are supplied to the MV predictor  104 . At this time, the signal Mot is temporarily stored in the MV memory  103 . 
     The color motion vector effectiveness signal Hot for the target macroblock is also input to the MV predictor  104 , which decides that each of the motion vectors MVt 1 -MVt 3  should be coded according to the signal Mot, that is, each of the reference blocks RB 1 -RB 3  is one of the intra-macroblock and the macroblock outside the object, or the inter-macroblock. According to the decision results, the prediction value (MV prediction value) Pmvt of the motion vector of the target macroblock MB 0  is generated. 
     When decided that the reference blocks RB 1 -RB 3  are the inter-macroblocks, the MV predictor  104  generates the prediction value of the motion vector of the target macroblock MB 0  from the motion vectors Mvt 1 -Mvt 3  of the coded reference blocks RB 1 -RB 3  and outputs the prediction value Pmvt to the MV encoder  105 . Note that the motion vectors of the reference blocks corresponding to the intra-macroblocks or the macroblocks outside the macroblock are not used for generating the prediction value. 
     The MV encoder  105  encodes the difference value (color difference motion vector) between the value of the motion vector MVt (MVt 0 ) of the target macroblock and the prediction value Pmvt as the value of the color motion vector to generate the coded signal Emvt of the color motion vector, which is output through the secondary switch  101 . 
     Likewise, when the shape coding mode signal Mos is input to an input terminal  5  as the shape motion vector effectiveness signal Mos, in accordance with this signal Mos, the switches  200  and  201  are on-off controlled. To be specific, when the color motion vector MVs 0  of the target macroblock MBs 0  is to be coded, the switches  200  and  201  are in the ON-state, or otherwise, they are in the OFF-state. 
     In other words, when the target macroblock is the intra-macroblock or the macroblock outside the object, and therefore the coding process for the corresponding motion vector is unnecessary, the switches  200  and  201  are in the OFF-state. On the other hand, when the target macroblock is the inter-macroblock, the switches  201  and  201  are in the ON-state. 
     The shape motion vector MVs for each macroblock is temporarily stored in the MV memory  202 . To be more detailed, the motion vectors MVs 1 -MVs 3  of the reference macroblock RMBs 1 -RMBs 3  adjacent to the target macroblock MB 0  are sent to the MV predictor  204  from the MV memory  102 . The motion vectors MBt 1 -MBt 3  of the blocks MBt 1 -MBt 3  adjacent to the target macroblock MB 0  shown in FIG.  12 ( c ) are also sent to the MV predictor  204  from the MV memory  102 . The signal Mmos is stored in the AV memory  203 . 
     The MV predictor  204  is supplied with the shape motion vector signal Mmos of the target macroblock from the MV memory  203  and the color motion vector effectiveness signal Mmot (Mot) of the target macroblock from the MV memory  103 . The MV predictor  204  decides whether or not the motion vectors MVs 1 -MVs 3  of the reference macroblocks RMBs 1 -RMBs 3  adjacent to the target macroblock should be coded, that is, decides whether or not each of the reference macroblocks is one of the intra-macroblock and the macroblock outside the object, or the inter-macroblock, according to the signal Mos. The MV predictor  204  decides whether or not the motion vectors MVt 1 -MVt 3  of the reference macroblocks RB 1 -RB 3  adjacent to the target macroblock should be coded, that is, decides whether or not each of the reference macroblocks RB 1 -RB 3  is one of the intra-macroblock and the macroblock outside the object, or the inter-macroblock, according to the signal Mmot. According to the decision results, the prediction value (MV prediction value) Pmvs of the motion vector of the target macroblock MBs 0  is generated. 
     When the reference macroblocks RMB 1 -RMB 3  and the reference blocks RB 1 -RB 3  are the inter-macroblocks, the MV predictor  204  generates the prediction value (MV prediction value) Pmvs of the motion vector of the target macroblock MBs 0  from the motion vectors MVs 1 -MVs 3  and the motion vectors MVt 1 -MVt 3 , and outputs the prediction value Pmvs to the MV encoder  205 . Note that the motion vectors of the reference blocks corresponding to the intra-macroblocks or the macroblocks outside the object are not used for generating the prediction value. 
     The MV encoder  205  encodes the difference value (shape difference motion vector) between the value of the motion vector MVs of the target macroblock and the prediction value Pmvs as the value of the shape motion vector to generate the coded signal Emvs of the shape motion vector, which is output through the secondary switch  201 . 
     FIG. 14 is a block diagram for explaining a prior art motion vector decoding unit included in the image decoding apparatus  2000 . In FIG. 14, there is shown a motion vector decoding unit (color motion vector decoding unit) included in the color decoding section  2100  and a motion vector decoding unit (shape motion vector decoding unit)  2240  included in the shape decoding section  2200 . Also in the description of the motion vector decoding unit, as in the case of the motion vector decoding unit, a description will be given of a case where the target macroblock MB 0  has one color motion vector and the blocks RB 1 -RB 3  of the target macroblock are used as reference blocks RB 1 -RB 3  (see FIG.  12 ( b )). 
     The color motion vector decoding unit  2140  is adapted to decode the coded color motion vector Em output from the color motion vector coding unit  1170 . 
     The color motion vector decoding unit  2140  includes an MV decoder  303  which decodes the coded color motion vector signal Emvt of the target macroblock to generate a decoded color motion vector signal Dmvt, and an MV memory  304  which temporarily stores the decoded color motion vector signal Dmvt. When the motion vector of the target macroblock MB 0  is decoded, the motion vectors MVt 1 -MVt 3  of the reference blocks RB 1 -RB 3  (see FIG.  12 ( c )) are output from the MV memory  304 . 
     The color motion vector decoding unit  2140  further includes an MV memory  302  which temporarily stores the color coding mode signal Mot for each macroblock input through an input terminal  6 , and an MV predictor  305  which generates the MV prediction value Pmvt of the motion vector of the target macroblock from the motion vectors MVt 1 -MVt 3  of the coded reference blocks RB 1 -RB 3 . 
     The color motion vector decoding unit  2140  still further includes a primary switch  300  provided between an input terminal  13  and an MV decoder  303 , and a secondary switch  301  provided between an output terminal  20  and an MV decoder  303 ,the switches  300  and  301  being on-off controlled by the coding mode signal Mot. 
     The shape motion vector decoding unit  2240  is adapted to decode the coded shape motion vector Emvs output from the motion vector coding unit  1270  shown in FIG.  13 . 
     The shape motion vector decoding unit  2240  includes an MV decoder  403  which decodes the coded shape motion vector signal Emvs of the target macroblock input to an input terminal  14  to generate a decoded shape motion vector signal Dmvs, and an MV memory  404  which temporarily stores the decoded shape motion vector signal Dmvs and outputs motion vectors MVs 1 -MVs 3  of the reference macroblocks RMB 1 -RMB 3  (see FIG.  12 ( d )). 
     The shape motion vector decoding unit  2240  further includes an MV memory  402  which stores the coding mode signal Mos of the target macroblock which is input through an input terminal  7 , and an MV predictor  405  which generates the MV prediction value Pmvs of the motion vector MVs 0  of the target macroblock MBs 0  from the motion vectors MVs 1 -MVs 3  of the reference macroblocks RMB 1 -RMB 3  and the motion vectors MVt 1 -MVt 3  of the reference blocks RB 1 -RB 3  according to the coding mode signal Mmos and the coding mode signal Mot. 
     The shape motion vector decoding unit  2240  still further includes a primary switch provided between an input terminal  14  and an MV decoder  403  and a switch  401  provided between an output terminal  21  and an MV decoder  403 , the switches  400  and  401  being on-off controlled by the coding mode signal MOS. 
     Subsequently, operation will be described. 
     The color motion vector decoding unit  2140  decodes the coded color motion vector signal Emvt sent from the color motion vector coding unit  1170  to generate the decoded color motion vector signal Dmvt. The shape motion vector decoding unit  2240  decodes the coded shape motion vector signal Emvs sent from the shape motion vector coding unit  1270  to generate the decoded shape motion vector signal Dmvs. 
     When the coding mode signal Mot decoded by the variable length decoder  2110  included in the color decoding section  2100  is input through the input terminal  6  as the motion vector effectiveness signal indicating whether or not the motion vector of the target macroblock is effective, the primary switch  300  and the secondary switch  301  is on-off controlled by the coding mode signal Mot. To be specific, when the coded color motion vector signal of the target macroblock is to be decoded, the switches  300  and  301  are in the ON-state, or otherwise the switches  300  and  301  are in the OFF-state. 
     In other words, when the target macroblock is the macroblock which has been intra-frame coded or the macroblock outside the object, and therefore the coding process for its motion vector is unnecessary, the switches are in the OFF-state, while the target macroblock is the macroblock which has been subjected to the motion compensation coding process (inter-frame coding process), the switches are in the ON-state. 
     The MV decoder  303  decodes the coded color motion vector signal Emvt with reference to the prediction value Pmvt of the color motion vector to generate the decoded color motion vector Dmvt, which is temporarily stored in the Mv memory  304 . 
     When the color motion vector MVt 0  of the target macroblock MB 0  is decoded, the decoded signals Dmv of the motion vectors MVt 1 , MVt 2 , and MVt 3  of the reference blocks RB 1 -RB 3  shown in FIG.  12 ( c ) are input to the MV predictor  305 . 
     The coding mode signal Mot is temporarily stored in the MV memory  302 . The MV predictor  305  decides whether or not each of the motion vectors MVt 1 -MVt 3  has been coded according to the coding mode signal Mmot from the MV memory  302 , that is, each of the reference blocks is either one of the intra-macroblock and the macroblock outside the object or the inter-macroblock. According to the decision results, the prediction value (MV prediction value) Pmvt of the motion vector MVt 0  of the target macroblock MB 0  is generated. The prediction value is generated as in the process performed by the color motion vector coding unit, and the resulting MV prediction value Pmvt is output to the MV decoder  303 . 
     When the coding mode signal Mos decoded by the variable length decoder  2210  included in the shape decoding section  2200  is input through the input terminal  7 , the switches  400  and  401  are on-off controlled. To be specific, when the coded shape motion vector signal of the target macroblock should be decoded, the switches  400  and  401  are in the ON-state, or otherwise the switches  400  and  401  are in the OFF-state. 
     In other words, when the target macroblock is the macroblock which has been intra-frame coded or the macroblock outside the object, and therefore the coding process for its motion vector is unnecessary, the switches  400  and  401  are in the OFF-state, while the target macroblock is the macroblock which has been subjected to the motion compensation coding process (inter-frame coding process), the switches are in the ON-state. 
     The MV decoder  403  decodes the coded shape motion vector signal Emvs with reference to the prediction value Pmvs to generate the decoded shape motion vector signal Dmvs, which is temporarily stored in the MV memory  404 . 
     When the shape motion vector MVs 0  of the target macroblock MBs 0  is coded, the decoded signals Dmvs of the motion vectors MVs 1 , MVs 2 , and MVs 3  of the reference macroblocks RMB 1 , RMB 2 , and RMB 3  shown in FIG.  12 ( d ) and the decoded signals Dmvt of the motion vectors MVt 1 , MVt 2 , and MVt 3  of the reference blocks RB 1 , RB 2 , and RB 3  shown in FIG.  12 ( c ) are input to the MV predictor  405 . 
     The coding mode signal Mos is temporarily stored in the MV memory  402 . The MV predictor  405  is supplied with the signal Mos from the MV memory  402  and the signal Mmot from the MV memory  302 . 
     The MV predictor  405  decides whether or not each of the shape motion vectors MVs 1 -MVs 3  of the reference macroblocks, that is, decides whether or not each of them is one of the intra-macroblock and the macroblock outside the object, or the inter-macroblock, according to the signal Mmos. Also, the MV predictor  405  decides whether or not each of the motion vectors MVt 1 -MVt 3  of the reference blocks, that is, decides whether or not each of the reference blocks is one of the intra-macroblock and the macroblock outside the object, or the inter-macroblock, according to the coding mode signal Mmot. According to the decision results, the prediction value Pmvs of the target macroblock Mbs 0  is generated. The prediction value is generated as in the process performed by the color motion vector coding unit. The resulting MV prediction value Pmvs is output to the MV decoder  403 . 
     In the image coding process according to MPEG4, an image signal to be processed can be extended to an interlaced image signal, like MPEG2. In this image coding process, field-by-field motion compensation can be performed to a color signal. 
     The field-by-field motion compensation according to MPEG4 is identical to that according to MPEG2. In the motion compensation process, as shown in FIG.  12 ( c ), motion vectors MVt 01  and MVt 02  corresponding to blocks MBf 1  and MBf 2  in the first and second fields composing a macroblock are assigned to each macroblock in one image (one frame). That is, two motion vectors exist for each macroblock. 
     The motion compensation process according to MPEG4 employs a vector coding method in which a difference motion vector between the motion vector of a target macroblock and a motion vector predicted from the motion vector of a macroblock adjacent to the target macroblock is encoded as information of the motion vector of the target macroblock, thereby providing higher coding efficiency than that of the motion vector coding based on MPEG2. 
     FIG. 15 is a block diagram illustrating a prior art motion vector coding unit for an interlaced color signal. 
     The motion vector coding unit  1170   a  includes a frame MV memory  102   a  which stores motion vectors used for the frame-by-frame motion compensation (hereinafter referred to as frame-basis motion vectors); a field MV memory  102   b  which stores motion vectors used for the field-by-field motion compensation (hereinafter referred to as field-basis motion vectors); a field MV converter  110   a  which converts a frame-basis motion vector to field-basis motion vectors; and a frame MV converter  110   b  which converts field-basis motion vectors to a frame-basis motion vector. 
     Further, the motion vector coding unit  1170   a  includes an MV memory  103  which stores a coding mode signal Mot indicating whether the motion vector of a target macroblock is effective or not; a frame MV predictor  104   a  which generates a prediction value of the frame-basis motion vector from the frame-basis motion vector of a reference macroblock which has already been coded, according to the stored coding mode signal Mmot; and a field MV predictor  104   b  which generates a prediction value of the field-basis motion vector from the field-basis motion vector of a reference macroblock which has already been coded, according to the stored coding mode signal Mmot. 
     The frame MV predictor  104   a  refers to the motion vector stored in the frame MV memory  102   a  when the motion vector of the already coded reference macroblock is a frame-basis motion vector, and refers to a frame-basis motion vector which has been obtained by converting the field-basis motion vectors stored in the field MV memory  102   b  by the frame MV converter  110   b  when the motion vector of the reference macroblock is a field-basis motion vector. The field MV predictor  104   b  refers to the motion vector stored in the field MV memory  102   b  when the motion vector of the already coded reference macroblock is a field-basis motion vector, and refers to field-basis motion vectors obtained by converting the frame-basis motion vector stored in the frame MV memory  102   a  by the field MV converter  110   a  when the motion vector of the already coded reference block is a frame-basis motion vector. 
     Moreover, the motion vector coding unit  1170   a  includes a frame MV encoder  105   a  which encodes the frame-basis motion vector by referring to the MV prediction value output from the MV predictor  104   a ; and a field MV encoder  105   b  which encodes the field-basis motion vector by referring to the MV prediction value output from the MV predictor  104   b.    
     Further, the motion vector coding unit  1170   a  includes a primary switch  100  having an input node S 1   i  connected to the input terminal  1 , a first output node S 1   a  connected to the inputs of the frame MV memory  102   a  and the frame MV encoder  105   a , a second output node S 1   b  connected to the inputs of the field MV memory  102   b  and the field MV encoder  105   b , and a third output node S 1   c  which is open. The primary switch  100  connects the input node S 1   i  to any of the first to third output nodes, according to the color coding mode signal Mot. The motion vector coding unit  1170   a  further includes a secondary switch  101  having an output node s 2   o  connected to the output terminal  10 , a first input node S 2   a  connected to the output of the field MV encoder  105   a , a second input node S 2   b  connected to the output of the field MV encoder  105   b , and a third input node S 2   c  which is open. The secondary switch  101  connects the output node S 2   o  to any of the first to third input nodes, according to the color coding mode signal Mot. 
     A description is given of the operation. 
     The color motion vector Mvt of the target macroblock is input to the input terminal  1 , while the color motion vector effectiveness signal Mot of the target macroblock is input to the input terminal  2 . The primary switch  100  and the secondary switch  101  are controlled according to the color motion vector effectiveness signal Mot. 
     That is, when the color motion vector MVt does not need to be coded, the input node S 1   i  of the primary switch  100  is connected to the open output node S 1   c , and the output node S 2   o  of the secondary switch  101  is connected to the open input node S 2   c.    
     When the color motion vector MVt is a frame-basis motion vector which needs to be coded, the input node S 1   i  of the primary switch  100  is connected to the first output node S 1   a , and the output node S 2   o  of the secondary switch  101  is connected to the first input node S 2   a.    
     Then, the motion vector MVt of the target macroblock, which has been input through the input terminal  1 , is temporarily stored in the MV memory  102   a . Further, in the field MV converter  110   a , the frame-basis motion vector stored in the MV memory  102   a  is converted to the field-basis motion vectors. 
     When the color motion vector MVt is a field-basis motion vector which needs to be coded, the input node S 1   i  of the primary switch  100  is connected to the second output node S 1   b , while the output node S 2   o  of the secondary switch  101  connected to the second input node S 2   b.    
     Then, the motion vector MVt of the target macroblock, which has been input through the input terminal  1 , is temporarily stored in the MV memory  102   b . Further, in the frame MV converter  110   b , the field-basis motion vectors stored in the MV memory  102   b  are converted to the frame-basis motion vector. 
     When the motion vector MVt of the target macroblock is a frame-basis motion vector, a prediction value of the frame-basis motion vector is generated in the frame MV predictor  104   a , and the motion vector MVt of the target macroblock is encoded in the frame MV encoder  105   a  with reference to the prediction value to generate a coded signal Emvt 1  of the frame-basis motion vector. The coded signal Emvt 1  is output as a coded motion vector Emvt through the secondary switch  101 . 
     Meanwhile, in the frame MV predictor  104   a , when the reference macroblock is a macroblock subjected to the frame-by-frame motion compensation, the motion vector stored in the frame MV memory  102   a  is referred to. When the reference macroblock is a macroblock subjected to the field-by-field motion compensation, a frame-basis motion vector obtained by converting the field-basis motion vectors stored in the field MV memory  102   b  by the frame MV converter  110   b  is referred to. 
     On the other hand, when the motion vector MVt of the target macroblock is a field-basis motion vector, a prediction value of the field-basis motion vector is generated in the field MV predictor  104   b , and the motion vector MVt of the target macroblock is encoded in the field MV encoder  105   b  with reference to the prediction value to generate a coded signal Emvt 2  of the field-basis motion vector. This coded signal Emvt 2  is output as a coded motion vector signal Emvt through the secondary switch  101 . 
     In the field MV predictor  104   b , when the reference macroblock is a macroblock subjected to the field-by-field motion compensation, the motion vector stored in the field MV memory  102   b  is referred to. When the reference macroblock is a macroblock subjected to the frame-by-frame motion compensation, field-basis motion vectors obtained by converting the frame-basis motion vector stored in the frame MV memory  102   b  by the field MV converter  110   a  is referred to. 
     FIG. 16 is a block diagram illustrating a prior art motion vector decoding unit for an interlaced color signal. 
     This motion vector decoding unit  2140   a  decodes the coded motion vector signal Emvt output from the motion vector coding unit  1170   a  shown in FIG. 15 to generate a decoded motion vector signal Dmvt. 
     The motion vector decoding unit  2140   a  includes a frame decoder  303   a  which decodes the coded signal of the frame-basis motion vector of the target macroblock by referring to the prediction motion vector obtained from the motion vector of an already decoded reference macroblock, to generate a decoded signal Dmvt 1  of the frame-basis motion vector; and a field decoder  303   b  which decodes the coded signal of the field-basis motion vector of the target macroblock by referring to the prediction motion vector obtained from the motion vector of an already decoded reference macroblock, to generate a decoded signal Dmvt 2  of the field-basis motion vector. 
     The motion vector decoding unit  2140   a  further includes a frame MV memory  304   a  which stores the decoded signal Dmvt 1  of the frame-basis motion vector; a field MV memory  304   b  which stores the decoded signal Dmvt 2  of the field-basis motion vector; a field MV converter  310   a  which converts the decoded signal of the frame-basis motion vector to the decoded signal of the field-basis motion vectors; and a frame MV converter  310   b  which converts the decoded signal of the field-basis motion vectors to the decoded signal of the frame-basis motion vector. 
     The motion vector decoding unit  2140   a  further includes an MV memory which stores a coding mode signal Mot indicating whether the motion vector of the target macroblock is effective or not; a frame MV predictor  305   a  which generates prediction value Pmvt 1  of the frame-basis motion vector from a frame-basis motion vector which has already been decoded, according to the stored coding mode signal Mmot; and a field MV predictor  305   b  which generates a prediction value Pmvt 2  of the field-basis motion vector from a field-basis motion vector which has already been decoded, according to the stored coding mode signal Mot. 
     The frame MV predictor  305   a  refers to the motion vector stored in the frame MV memory  304   a  when the motion vector of the already decoded reference macroblock is a frame-basis motion vector When the motion vector of the already decoded reference macroblock is a field-basis motion vector, the frame MV predictor  305   a  refers to a frame-basis motion vector which is obtained by converting the field-basis motion vectors stored in the field MV memory  304   b  by the frame MV converter  310   b . The field MV predictor  305   b  refers to the motion vector stored in the field MV memory  304   b  when the motion vector of the already decoded reference macroblock is a field-basis motion vector. When the motion vector of the already decoded reference macroblock is a frame-basis motion vector, the field MV predictor  305   b  refers to the field-basis motion vectors obtained by converting the frame-basis motion vector stored in the frame MV memory  304   a  by the field MV converter  310   a.    
     Further, the motion vector decoding unit  2140   a  includes a primary switch  300  having an input node S 3   i  connected to the input terminal  13 , a first output node  53   a  connected to the input of the frame MV decoder  303   a , a second output node S 3   b  connected to the input of the field MV decoder  303   b , and a third output node S 3   c  which is open. The primary switch  300  connects the input node S 3   i  to any of the first to third output nodes, according to the coding mode signal Mot. The motion vector decoding unit  2140   a  further includes a secondary switch  301  having an output node S 4   o  connected to the output terminal  20 , a first input node  54   a  connected to the output of the frame MV decoder  303   a , a second input node S 4   b  connected to the output of the field MV decoder  303   b , and a third input node S 4   c  which is open. The secondary switch  301  connects the output node S 4   o  to any of the first to third input nodes, according to the coding mode signal Mot. 
     A description is given of the operation. 
     The coded signal Emvt of the motion vector of the target macroblock is input to the input terminal  13 , and the motion vector effectiveness signal Mot of the target macroblock is input to the input terminal  6 . The primary switch  300  and the secondary switch  301  are controlled according to the motion vector effectiveness signal Mot. 
     Further, when the motion vector effectiveness signal Mot is stored in the MV memory  302 , it is decided whether the motion vectors MVt 1 , MVt 2  and MVt 3  of the macroblocks to be referred to by the MV predictors  305   a  and  305   b  are encoded or not, and whether these vectors are frame-basis motion vectors or field-basis motion vectors, by the frame MV predictor  305   a  and the field MV predictor  305   b , according to the effectiveness signal Mmot stored in the MV memory  302 . 
     When the coded signal Emvt of the motion vector does not need to be decoded, the input node S 3   i  of the primary switch  300  is connected to the open output node S 3   c , and the output node S 4   o  of the secondary switch  301  is connected to the open input node S 4   c.    
     On the other hand, when the coded signal Emvt of the motion vector of the target macroblock is a frame-basis motion vector which needs to be decoded, the input node S 3   i  of the primary switch  300  is connected to the first output node S 3   a , and the output node S 4   o  of the secondary switch  301  is connected to the first input node S 4   a.    
     Then, the coded signal Emvt of the motion vector of the target macroblock, which has been input through the input terminal  13 , is decoded in the frame MV decoder  303   a  with reference to the prediction motion vector Pmvt 1  of the target macroblock to generate a decoded signal Dmvt 1  of the motion vector. This decoded signal Dmvt 1  is temporarily stored in the MV memory  304   a . Further, in the field MV converter  310   a , the frame-basis motion vector stored in the MV memory  304   a  is converted to the field-basis motion vectors. 
     Further, the frame MV predictor  305   a  performs frame-by-frame motion compensation in which a prediction motion vector of the motion vector of the target macroblock is generated by referring to the motion vector of an already decoded reference macroblock adjacent to the target macroblock. When the reference macroblock is a macroblock subjected to the frame-by-frame motion compensation, the motion vector stored in the frame MV memory  304   a  is referred to. When the reference macroblock is a macroblock subjected to the field-by-field motion compensation, a frame-basis motion vector obtained by converting the field-basis motion vectors stored in the field MV memory  304   b  by the frame MV converter  310   b  is referred to. 
     Next, when the coded signal Emvt of the motion vector of the target macroblock is a field-basis motion vector which needs to be decoded, the input node S 3   i  of the primary switch  300  is connected to the second output node S 3   b , and the output node S 4   o  of the secondary switch  301  is connected to the second input node S 4   b.    
     Then, the coded signal Emvt of the motion vector of the target macroblock, which has been input through the input terminal  13 , is decoded by the field MV decoder  303   b  with reference to the prediction motion vector Pmvt 2  of the target macroblock to generate a decoded signal Dmvt 2  of the motion vector. The decoded signal Dmvt 2  is temporarily stored in the MV memory  304   b . Further, in the field MV converter  310   b , the frame-basis motion vector stored in the MV memory  304   b  is converted to the field-basis motion vectors. 
     On the other hand, the field MV predictor  305   b  performs field-by-field motion compensation in which a prediction motion vector of the motion vector of the target macroblock is generated with reference to the motion vector of an already decoded reference macroblock adjacent to the target macroblock, according to the motion vector effectiveness signal of the target macroblock. In this process, when the reference macroblock is a macroblock subjected to the field-by-field motion compensation, the motion vector stored in the field MV memory  304   b  is referred to. When the reference macroblock is a macroblock subjected to the frame-by-frame motion compensation, a frame-basis motion vector obtained by converting the field-basis motion vectors stored in the frame MV memory by the frame MV converter  310   b  is referred to. 
     By the way, an interlaced signal used for ordinary TV broadcasting and a non-interlaced color signal or shape signal used for internet or data base are expected to be widely utilized in various fields in the future. However, it is thought that the applicable field of an interlaced shape signal would not be so wide. 
     As for “Next Generation TV” which has been much talked about in recent years, there is a high possibility of adopting non-interlacing and, therefore, from the practical point of view, it is not desired that the structures of coding and decoding apparatuses are complicated due to coding of an interlaced shape signal whose applicable field is not so wide. 
     Therefore, in order to realize coding and decoding of an interlaced shape signal without complicating the structures of coding and decoding apparatuses, the circuit structure for coding or decoding an interlaced color signal may be combined with the circuit structure for coding or decoding a non-interlaced shape signal. 
     So, in order to realize coding and decoding of an interlaced shape signal without complicating the structures of coding and decoding apparatuses, the circuit structure for coding or decoding an interlaced color signal may be combined with the circuit structure for coding or decoding a non-interlaced shape signal. 
     However, if non-interlace coding is performed to an interlaced shape signal, motion compensation of the interlaced shape signal would be performed frame by frame in accordance with the non-interlacing. In this case, when the color signal is an interlaced signal, there are two kinds of motion vectors relating to the color signal, i.e., a field-basis motion vector and a frame-basis motion vector. Therefore, it is difficult to perform coding of the shape motion vector with reference to the color motion vector as in the prior art motion vector coding. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide an image coding apparatus and an image decoding apparatus, and an image coding method and an image decoding method, wherein coding and decoding of a motion vector of a non-interlaced shape signal can be performed with extension of a circuit structure minimized and together with coding and decoding of a motion vector of an interlaced color signal, and a data storage medium which stores an image processing program which makes a computer perform coding or decoding of a motion vector of an interlaced shape signal according to the image coding method or the image decoding method. 
     Other objects and advantages of the present invention will become apparent from the detailed description given hereinafter It should be understood, however, that the detailed description and specific embodiment are given by way of illustration only, since various changes and modifications within the scope of the invention will become apparent to those skilled in the art from this detailed description. 
     According to a first aspect of the present invention, an image coding apparatus which receives an interlaced image signal comprising a color signal for color display of an object and a shape signal indicating a shape of the object and corresponding to the object included in an image space as an input and performs an adaptive coding process including a frame-by-frame motion compensation coding process and a field-by-field motion compensation coding process for the image signal in macroblock units each comprising a predetermined number of pixels into which the image space is divided, comprises; a color motion vector coding unit for coding either a frame-basis color motion vector or a field-basis color motion vector of a target macroblock to be coded which is used in either the frame-by-frame motion compensation coding process or the field-by-field motion compensation coding process for an interlaced color signal, based on its prediction value; and a shape motion vector coding unit for coding a frame-basis shape motion vector of the target macroblock which is used in the frame-by-frame motion compensation coding process for an interlaced shape signal, based on a prediction value obtained from a color motion vector and a shape motion vector of a coded macroblock; wherein the color motion vector coding unit includes motion vector conversion means for converting field-basis color motion vectors of the coded macroblock into a frame-basis color motion vector of the coded macroblock, and the shape motion vector coding unit includes a shape motion vector predictor for generating the prediction value of the shape motion vector of the target macroblock based on the frame-basis shape motion vector of the coded macroblock and the frame-basis color motion vector of the coded macroblock output from the motion vector conversion means when the color motion vector of the coded macroblock is the field-basis motion vector. Therefore, the motion vector of the non-interlaced shape signal can be coded with extension of a circuit structure minimized and together with a coding process for the motion vector of the interlaced color signal. 
     According to a second aspect of the present invention, an image coding apparatus which receives an interlaced image signal comprising a color signal for color display of an object and a shape signal indicating a shape of the object and corresponding to the object included in an image space as an input and performs an adaptive coding process including a frame-by-frame motion compensation coding process and a field-by-field motion compensation coding process for the image signal in macroblock units each comprising a predetermined number of pixels into which the image space is divided, comprises: a color motion vector coding unit for coding either a frame-basis color motion vector or a field-basis color motion vector of a target macroblock to be coded which is used in either the frame-by-frame motion compensation coding process or the field-by-field motion compensation coding process for an interlaced color signal, based on its prediction value; and a shape motion vector coding unit for coding a frame-basis shape motion vector of the target macroblock which is used in the frame-by-frame motion compensation coding process for an interlaced shape signal, based on its prediction value; wherein the shape motion vector coding unit includes a shape motion predictor which generates the prediction value of the shape motion vector of the target macroblock based on a color motion vector and a shape motion vector of a coded macroblock when the color motion vector of the coded macroblock is the frame-basis motion vector, and generates the prediction value of the shape motion vector of the target macroblock based on a frame-basis shape motion vector of the coded macroblock when the color motion vector of the coded macroblock is the field-basis motion vector Therefore, when the prediction value of the shape motion vector is generated, the field-basis color motion vector of the coded macroblock is not ref erred to, and consequently, the process for converting the field-basis color motion vectors into the frame-basis color motion vector can be dispensed with in the process for coding the color motion vector when the color motion vectors of the target macroblock and the coded macroblock are the field-basis motion vectors. 
     According to a third aspect of the present invention, an image coding apparatus which receives either an interlaced image signal or a non-interlaced image signal comprising a color signal for color display of an object and a shape signal indicating a shape of the object and corresponding to the object included in an image space as an input and performs an adaptive coding process including a frame-by-frame motion compensation coding process and a field-by-field motion compensation coding process for the image signal in macroblock units each comprising a predetermined number of pixels into which the image space is divided, comprises: a color motion vector coding unit for coding either a frame-basis color motion vector or a field-basis color motion vector of a target macroblock to be coded which is used in either a frame-by-frame motion compensation coding process or a field-by-field motion compensation coding process for a color signal, based on its prediction value; and a shape motion vector coding unit for coding a frame-basis shape motion vector of the target macroblock which is used in the frame-by-frame motion compensation coding process for a non-interlaced shape signal, based on its prediction value; wherein the shape motion vector coding unit includes a shape motion vector predictor for generating the prediction value of the shape motion vector of the target macroblock based on a color motion vector and a shape motion vector of a coded macroblock when the non-interlaced image signal is received as the image signal, and generates the prediction value of the shape motion vector of the target macroblock based on a frame-basis shape motion vector of the coded macroblock when the interlaced image signal is received as the image signal. Therefore, when the input image signal is the interlaced image signal, the decision process for deciding whether the color motion vector of the reference macroblock is the field-basis motion vector or the frame-basis motion vector may be dispensed with in the process for coding the shape motion vector, and correspondingly the amount of signal processing for the decision process can be reduced. 
     According to a fourth aspect of the present invention, an image decoding apparatus which receives a coded image signal corresponding to an interlaced image signal comprising a color signal for color display of an object and a shape signal indicating a shape of the object and corresponding to the object included in an image space as an input and performs an adaptive decoding process including a frame-by-frame motion compensation decoding process and a field-by-field motion compensation decoding process for the coded image signal in macroblock units each comprising a predetermined number of pixels into which the image space is divided, comprises: a color motion vector decoding unit for decoding either a frame-basis color motion vector or a field-basis color motion vector of a target macroblock to be decoded which is used in either the frame-by-frame motion compensation decoding process or the field-by-field motion compensation decoding process for a coded signal of an interlaced color signal, based on its prediction value; and a shape motion vector decoding unit for decoding a frame-basis shape motion vector of the target macroblock which is used in the frame-by-frame motion compensation decoding process for a coded signal of an interlaced shape signal, based on a prediction value obtained from a color motion vector and a shape motion vector of a decoded macroblock; wherein the color motion vector decoding unit includes motion vector conversion means for converting field-basis color motion vectors of the decoded macroblock into a frame-basis color motion vector of the decoded macroblock, and the shape motion vector decoding unit includes a shape motion vector predictor for generating the prediction value of the shape motion vector of the target macroblock based on the frame-basis shape motion vector of the decoded macroblock and the frame-basis color motion vector of the decoded macroblock output from the motion vector conversion means when the color motion vector of the decoded macroblock is the field-basis motion vector. Therefore, the motion vector of the non-interlaced shape signal can be decoded with extension of a circuit structure minimized and together with a decoding process for the motion vector of the interlaced color signal. 
     According to a fifth embodiment of the present invention, an image decoding apparatus which receives a coded image signal corresponding to an interlaced image signal comprising a color signal for color display of an object and a shape signal indicating a shape of the object and corresponding to the object included in an image space as an input and performs an adaptive decoding process including a frame-by-frame motion compensation decoding process and a field-by-field motion compensation decoding process for the coded image signal in macroblock units each comprising a predetermined number of pixels into which the image space is divided, comprises: a color motion vector decoding unit for decoding either a frame-basis color motion vector or a field-basis color motion vector of a target macroblock to be decoded which is used in either a frame-by-frame motion compensation decoding process or a field-by-field motion compensation decoding process for a coded signal of an interlaced color signal, based on its prediction value; and a shape motion vector decoding unit for decoding a frame-basis shape motion vector of the target macroblock which is used in the frame-by-frame motion compensation decoding process for a coded signal of an interlaced shape signal, based on its prediction value; wherein the shape motion vector decoding unit includes a shape motion vector predictor which generates the prediction value of the shape motion vector of the target macroblock based on a color motion vector and a shape motion vector of a decoded macroblock when the color motion vector of the decoded macroblock is the frame-basis motion vector, and generates the prediction value of the shape motion vector of the target macroblock based on a frame-basis shape motion vector of the decoded macroblock when the color motion vector of the decoded macroblock is the field-basis motion vector. Therefore, when the prediction value of the shape motion vector is generated, the field-basis color motion vector of the decoded macroblock is not referred to, and consequently, the process for converting the field-basis color motion vectors into the frame-basis color motion vector can be dispensed with in the process for decoding the color motion vector when the color motion vectors of the target macroblock and the coded macroblock are the field-basis motion vectors. 
     According to a sixth aspect of the present invention, an image decoding apparatus which receives a coded image signal corresponding to either an interlaced image signal or a non-interlaced image signal comprising a color signal for color display of an object and a shape signal indicating a shape of the object and corresponding to the object included in an image space as an input and performs an adaptive decoding process including a frame-by-frame motion compensation decoding process and a field-by-field motion compensation decoding process for the coded image signal in macroblock units each comprising a predetermined number of pixels into which the image space is divided, comprises: a color motion vector decoding unit for decoding either a frame-basis color motion vector or a field-basis color motion vector of a target macroblock to be decoded which is used in either a frame-by-frame motion compensation decoding process or a field-by-field motion compensation decoding process for a coded signal of the color signal, based on its prediction value; and a shape motion vector decoding unit for decoding a frame-basis shape motion vector of the target macroblock which is used in the frame-by-frame motion compensation decoding process for a coded signal of a non-interlaced shape signal, based on its prediction value; wherein the shape motion vector decoding unit includes a shape motion vector predictor for generating the prediction value of the shape motion vector of the target macroblock based on a color motion vector and a shape motion vector of a decoded macroblock when the coded image signal of the non-interlaced image signal is received as the coded image signal, and generates the prediction value of the shape motion vector of the target macroblock based on a frame-basis shape motion vector of the decoded macroblock when the coded image signal of the interlaced image signal is received as the coded image signal. Therefore, when the input image signal is the interlaced image signal, the decision process for deciding whether the color motion vector of the reference macroblock is the field-basis motion vector or the frame-basis motion vector may be dispensed with in the process for decoding the shape motion vector, and correspondingly the amount of signal processing for the decision process can be reduced. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram showing an image coding apparatus according to a first embodiment of the present invention, wherein a motion vector coding unit included in the image coding apparatus is shown. 
     FIG. 2 is a block diagram showing an image decoding apparatus according to a second embodiment of the present invention, wherein a motion vector decoding unit included in the image decoding apparatus is shown. 
     FIG. 3 is a block diagram showing an image coding apparatus according to a third embodiment of the present invention, wherein a motion vector coding unit included in the image coding apparatus is shown. 
     FIG. 4 is a block diagram showing an image decoding apparatus according to a fourth embodiment of the present invention, wherein a motion vector decoding unit included in the image decoding apparatus is shown. 
     FIG. 5 is a block diagram showing an image coding apparatus according to a fifth embodiment of the present invention, wherein a motion vector coding unit included in the image coding apparatus is shown. 
     FIG. 6 is a block diagram showing an image decoding apparatus according to a sixth embodiment of the present invention, wherein a motion vector decoding unit included in the image decoding apparatus is shown. 
     FIGS.  7 ( a )- 7 ( b ) are diagrams for explaining a data storage medium which stores a program for performing the motion vector coding process or the motion vector decoding process according to the above embodiment, and FIG.  7 ( c ) is a diagram for explaining the computer system which performs the motion vector coding process and the motion vector decoding system. 
     FIGS.  8 ( a )- 8 ( f ) are diagrams for explaining an object-by-object coding process according to MPEG4, wherein FIG.  8 ( a ) shows an image space Tc obtained from a color signal, FIG.  8 ( b ) shows an image space Ss obtained from a shape signal, FIGS.  8 ( c ) and  8 ( e ) show a blocking process for the color signal, and FIGS.  8 ( d ) and  8 ( f ) show a blocking process for a shape signal. 
     FIG. 9 is a block diagram for explaining a prior art image coding apparatus according to MPEG4. 
     FIG. 10 is a block diagram for explaining a prior art image decoding apparatus according to MPEG4. 
     FIGS.  11 ( a )- 11 ( d ) are diagrams showing correspondence between macroblocks and motion vectors, wherein FIG.  11 ( a ) shows a macroblock having four motion vectors, FIG.  11 ( b ) shows a macroblock having two motion vectors, FIG.  11 ( c ) shows sub-macroblocks corresponding to respective fields in the macroblock, and FIG.  11 ( d ) shows a macroblock having one motion vector. 
     FIGS.  12 ( a )- 12 ( d ) are diagrams for explaining a process for predicting a motion vector, wherein FIG.  12 ( a ) shows a target macroblock to-be-processed corresponding to a color signal having four motion vectors, FIG.  12 ( b ) shows a target macroblock corresponding to the color signal having one motion vector, FIG.  12 ( c ) shows a target macroblock corresponding to the color signal having two motion vectors, and FIG.  12 ( d ) shows a process for predicting a motion vector of a shape signal. 
     FIG. 13 is a block diagram for explaining a prior art motion vector coding unit. 
     FIG. 14 is a block diagram showing a prior art motion vector decoding unit. 
     FIG. 15 is a block diagram showing a prior art motion vector coding unit for an interlaced image signal. 
     FIG. 16 is a block diagram showing a prior art motion vector decoding unit for the interlaced image signal. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Now, a description will be given of preferred embodiments of the present invention. 
     Embodiment 1 
     FIG. 1 is a block diagram for explaining a motion vector coding unit included in an image coding apparatus according to a first embodiment of the present invention. 
     The image coding apparatus of the first embodiment is an image coding apparatus which performs coding to an interlaced image signal according to MPEG4. The image coding apparatus of the first embodiment includes a motion vector coding apparatus  1071  for the interlaced image signal, which has replaced the color signal motion vector coding unit  1170  and the shape signal motion vector coding unit  1270  included in the prior art image coding apparatus  1000  shown in FIG.  9 . 
     The interlaced image signal comprises an interlaced color signal for color display of an object corresponding to a image on a frame, an interlaced shape signal indicating a shape of the object, and an interlaced transparency signal indicating a composition ratio in which the object and another image are to be composited. Therefore, the image coding apparatus comprises a color coding section which performs coding to the interlaced color signal, a shape coding section which performs coding to the interlaced shape signal, and a transparency coding section which performs coding to the interlaced transparency signal. The construction of the transparency coding unit is similar to that of the color coding unit, and is irrelevant to the present invention. Therefore, this will not be discussed herein. 
     Turning now to FIG. 1, a motion vector coding unit  1071  of the first embodiment comprises a color motion vector coding unit  1171  included in the color coding section and a shape motion vector coding unit  1271  included in the shape coding section. 
     In this first embodiment, the construction of the color coding section is almost identical to that of the color coding section  1100  in FIG. 9, and in a motion compensation coding process for the interlaced color signal, switching between the frame-by-frame motion compensation process and the field-by-field motion compensation process is adaptively performed. The construction of the color motion vector coding unit  1171  is identical to that of the color motion vector coding unit according to the prior art in FIG.  15 . 
     The field converter  110   a  and the frame converter  110   b  have not been discussed in the prior art description. In this embodiment, a field MV converter  110   a  included in the color motion vector coding unit  1171  is adapted to receive a frame-basis color motion vector (frame CMV) of a macroblock which has been subjected to the frame-by-frame motion compensation process from a frame MV memory  102   a  and output two motion vectors identical to the frame CMV as color motion vectors (field CMV) of respective fields composing the frame. A frame MV converter  110   b  included in the color motion vector coding unit  1171  is adapted to receive two field-basis color motion vectors (field CMV 1  and field CMV 2 ) of a macroblock which has been subjected to the field-by-field motion compensation process from a field MV memory  102   b  and output a motion vector resulting from averaging the field CMV 1  and the field CMV 2  as the color motion vector (frame CMV) of a frame composed of these fields. 
     While in the frame MV converter  110   b , the average of the motion vectors of the fields is converted into the frame-basis motion vector, when a sub-macroblock corresponding to one field is positioned outside the object, a motion vector of a sub-macroblock corresponding to the other field (sub-macroblock positioned inside the object) may be used as the motion vector of the corresponding macroblock in the frame. In this case, the motion vectors of respective fields need not be averaged, and the motion vector to-be-referred to is not affected by the motion vector of the sub-macroblock outside the object having an insignificant value. In particular, in a case in which the output of the frame MV converter  110   b  is shared by the frame MV predictor  104   a  included in the color motion vector coding unit  1171  and an MV predictor  204   a  included in the shape motion vector coding unit  1271 , signal processing for averaging the motion vectors can be reduced and prediction precision is improved. 
     The shape coding section of the first embodiment differs from the prior art shape coding section  1200  shown in FIG. 9 in the construction of the shape motion vector coding unit  1271 . To be specific, the shape coding section of the first embodiment is adapted to convert the interlaced shape signal included in the interlaced image signal into a progressive shape signal and perform coding to the progressive shape signal. In brief, the shape coding section is adapted to perform the frame-by-frame motion compensation coding process in the inter-frame coding process. 
     The shape motion vector coding unit  1271  includes an MV predictor  204   a  which generates the prediction value of the shape motion vector of the target macroblock with reference to the frame-basis color motion vector stored in the frame MV memory  102   a  included in the color motion vector coding unit  1171  and the frame-basis color motion vector output from the frame MV converter  110   b , which has replaced the MV predictor  204  included in the shape motion vector coding unit  1270  shown in FIG.  13 . The MV predictor  204   a  is adapted to convert a 0.5 pixel-precision color motion vector sent from the frame MV memory  102   a  into one pixel-precision color motion vector and perform a prediction process by the use of the resulting one-pixel precision. This is because the color coding section performs the motion compensation process by the use of the 0.5 pixel-precision and the shape coding section performs the motion compensation process by the use of the one-pixel precision. 
     When the MV predictor  204   a  generates the prediction value of the shape motion vector MVs 0  of the target macroblock MBs 0  in FIG.  12 ( d ), reference macroblocks in the image space (shape image space) obtained from the shape signal, macroblocks RMbs 1 -RMBs 3  adjacent to the target macroblock MBs 0  are selected. As the motion vectors to be referred to, motion vectors Mv 1 -MVs 3  of the macroblock RMBs 1 -RMBs 3  are used. On the other hand, in the image space (color image space) obtained from the color signal, the reference macroblocks adjacent to the macroblock MB 0  corresponding to the target macroblock MBs 0  are not always the macroblocks which have been subjected to the frame-by-frame motion compensation process. Therefore, when the reference macroblock is the macroblock which has been subjected to the frame-by-frame motion compensation process, the motion vectors stored in the frame MV memory  102   a  are referred to, while the reference macroblock is the macroblock which has been subjected to the field-by-field motion compensation process, the frame-basis motion vector resulting from converting the field-basis motion vectors stored in the field MV memory  102   b  into the frame-basis motion vector by the use of the frame MV converter  110   b.    
     The MV predictor  204   a  decides whether or not each of the motion vectors MVt 1 , MVt 2 , and MVt 3  of the reference blocks is the coded motion vector, that is, each of the reference macroblocks is the inter-macroblock which has been inter-frame coded, according to a motion vector effectiveness signal Mmot output from an MV memory  103 . 
     As for the macroblocks RMBs 1 -RMBs 3  adjacent to the target macroblock MBs 0  in the shape image space, corresponding to the intra-macroblocks or the macroblocks outside the object, the corresponding motion vectors are not referred to in the process for generating the prediction value of the motion vector. Likewise, as for the blocks RB 1 -RB 3  adjacent to the target macroblock MB 0  in the color image space corresponding to the intra-macroblocks or the macroblocks outside the object, the corresponding motion vectors are not referred to in the process for generating the prediction value of the motion vector, either. 
     Subsequently, operation and effects will be described. 
     The operation of the image coding apparatus of the first embodiment differs from that of the prior art image coding apparatus  1000  only in the operation of the motion vector coding unit  1071  and the operation of the motion vector coding unit  1071  differs from the prior art motion vector coding unit in the operation of the shape motion vector coding unit  1071 . Therefore, the color motion vector coding unit  1171  is identical to the prior art color motion vector coding unit  1170 . 
     The MV predictor  204  included in the prior art shape motion vector coding unit  1270  shown in FIG. 13 refers to the motion vectors MVt 1 , MVt 2 , and MVt 3  stored in the MV memory  102  as the color vectors when it generates the prediction value of the shape motion vector of the target macroblock. 
     On the other hand, in the color coding section for the interlaced color signal, since switching between the frame-by-frame motion vector compensation coding process and the field-by-field motion compensation coding process is adaptively performed, two types of motion vectors (field-basis and frame-basis motion vectors) are generated. 
     In this case, the shape motion vector to be referred to by the MV predictor  204  is the frame-basis motion vector, and therefore the MV predictor  204  cannot refer to the field-basis color motion vector. 
     The MV predictor  204   a  included in the shape motion vector coding unit  1271  of the first embodiment, generates the prediction value of the shape motion vector of the target macroblock in the shape image space with reference to the motion vectors stored in the frame MV memory  102   a  included in the color motion vector coding unit  1171  when the reference macroblock in the color image space has been subjected to the frame-by-frame motion compensation process. On the other hand, when the reference macroblock has been subjected to the field-by-field motion compensation process, the MV predictor  204   a  refers to the frame-basis motion vector obtained by converting the field-basis motion vectors stored in the field MV memory  102   b  in the color motion vector coding unit  1171  by the use of the frame MV converter  110   b.    
     Therefore, the MV predictor  104   a  included in the shape motion vector coding unit  1271  always refers to the frame-basis motion vector, regardless of whether the reference macroblock is the macroblock which has been subjected to the field-by-field motion compensation process or the frame-by-frame motion compensation process. 
     In this manner, the motion vector coding unit  1171  included in the image coding apparatus which performs the motion compensation process for the interlaced image signal generates the prediction value of the shape motion vector of the target macroblock in the shape image space with reference to the color motion vector of the macroblock in the color image space, like the motion vector coding unit included in the image coding apparatus which performs the motion compensation coding process for the non-interlaced image signal. 
     Thus, in accordance with the first embodiment, the image coding apparatus which adaptively performs switching between the frame-by-frame motion compensation coding process and the field-by-field motion compensation coding process when performing the motion compensation coding process for the interlaced image signal including the color signal and the shape signal corresponding to the object, includes the MV predictor  204   a  which generates the prediction value of the shape motion vector with reference to the color motion vectors, i.e., the output of the frame MV converter  110   b  which converts the field-basis color motion vectors into the frame-basis motion vector when the reference color motion vector corresponds to the motion vector which has been subjected to the field-by-field motion compensation coding process. Therefore, in the process for predicting the shape motion vector performed by the shape coding section which always performs the frame-by-frame motion compensation coding process, the color motion vectors of the interlaced color signal can be referred to. 
     For the above reason, the construction of the shape motion vector coding unit is made identical to that of the color motion vector coding unit. To be specific, without the need for the motion vector coding means for the frame-basis motion vectors and the field-basis motion vectors and by altering the construction of the MV predictor included in the prior art shape motion vector coding unit, the simplified circuit construction of the shape motion vector coding unit which generates the prediction value of the shape motion vector with reference to the color motion vector can be realized, for the image coding apparatus which performs the motion compensation coding process for the interlaced image signal corresponding to the object. 
     In the first embodiment, the description has been given without distinguishing between the case where the reference macroblock in the color image space has one motion vector and the case where it has four motion vectors, because the motion vectors to-be-referred to are the same in either case. 
     To be more detailed, when the reference macroblock has one motion vector, it is assumed that the four motion vectors of the four blocks composing the macroblock are equivalent. Hence, in either case, for the target macroblock MBs 0  (see FIG.  12 ( d )) in the shape image space, motion vectors MVt 1 -MVt 3  of the blocks RB 1 -RB 3  adjacent to the target macroblock MB 0  (see FIG.  12 ( c )) in the color image space are used. 
     Embodiment 2 
     FIG. 2 is a block diagram showing a motion vector coding unit included in an image decoding apparatus according to a second embodiment of the present invention. 
     The image decoding apparatus of the second embodiment is an image decoding apparatus which decodes the coded signal obtained by coding the interlaced image signal according to MPEG4. The image decoding apparatus of the second embodiment includes a motion vector decoding unit  2041  for the interlaced image signal which has replaced the color signal motion vector decoding unit  2140  and the shape signal motion vector coding unit  2240  included in the prior art image decoding apparatus  2000  shown in FIG.  10 . The image decoding apparatus comprises a color decoding section which decodes the interlaced color signal, a shape decoding section which decodes the interlaced shape signal, and a transparency decoding section which decodes the interlaced transparency signal. The transparency decoding section is almost identical to that of the color decoding section and is irrelevant to the present invention. Therefore, this will not be discussed herein. 
     The motion vector decoding unit  2041  includes a color motion vector decoding unit  2141  included in the color decoding section and a shape motion vector decoding unit  2241  included in the shape decoding section. 
     The construction of color decoding section is almost identical to that of the color decoding section  2100  shown in FIG. 10, and in a motion compensation decoding process for the coded signal obtained by coding the interlaced color signal, switching between the frame-by-frame motion compensation process and the field-by-field motion compensation process is adaptively performed. The construction of the color motion vector decoding unit  2141  is identical to that of the prior art color motion vector decoding unit  2140   a  shown in FIG.  16 . 
     The field converter  310   a  and the frame converter  310   b  have not been discussed in the prior art description. In this embodiment, a field MV converter  310   a  included in the color motion vector decoding unit  2141  is adapted to receive a frame-basis color motion vector (frame CMV) of a macroblock which has been subjected to the frame-by-frame motion compensation process from a frame MV memory  304   a  and output two motion vectors identical to the frame CMV as color motion vectors (field CMV) of respective fields composing the frame. A frame MV converter  310   b  included in the color motion vector coding unit  2141  is adapted to receive two field-basis color motion vectors (field CMV 1  and field CMV 2 ) of a macroblock which has been subjected to the field-by-field motion compensation process from a field MV memory  304   b  and output a motion vector resulting from averaging the field CMV 1  and the field CMV 2  as the color motion vector (frame CMV) of a frame composed of these fields. 
     While in the frame MV converter  310   b , the average of the motion vectors of the fields is converted into the frame-basis motion vector, when a sub-macroblock corresponding to one field is positioned outside the object, a motion vector of a sub-macroblock corresponding to the other field (sub-macroblock positioned inside the object) may be used as the motion vector of the corresponding macroblock in the frame. In this case, the motion vectors of respective fields need not be averaged, and the motion vector to-be-referred to is not affected by the motion vector of the sub-macroblock outside the object having an insignificant value. In particular, in a case where the output of the frame MV converter  310   b  is shared by the frame MV predictor  305   a  included in the color motion vector decoding unit  2141  and the MV predictor  405   a  included in the shape motion vector decoding unit  2241 , signal processing for averaging the motion vectors can be reduced and prediction precision is improved. 
     The shape decoding section of the second embodiment differs from the prior art shape decoding section  2200  in FIG. 10 in the construction of the shape motion vector decoding unit  2200 . To be specific, the shape decoding section of the second embodiment is adapted to decode the coded shape signal obtained by coding the progressive signal by the use of the shape coding section of the first embodiment. In brief, the shape decoding section is adapted to perform the frame-by-frame motion compensation decoding process in the inter-frame decoding process. 
     The shape motion vector decoding unit  2241  includes the MV predictor  405   a  which adaptively selects one of frame-basis color motion vector stored in the frame MV memory  304   a  and the frame-basis color motion vector output from the frame MV converter  310   b  and generates the prediction value of the shape motion vector of the target macroblock with reference to the selected color motion vector, which has replaced the MV predictor  405  included in the prior art shape motion vector decoding unit  2240  shown in FIG.  14 . 
     When the MV predictor  405   a  generates the prediction value of the shape motion vector MVs 0  of the target macroblock MBs 0  in FIG.  12 ( d ), reference macroblocks in the image space (shape image space) obtained from the shape signal, i.e., macroblocks RMBs 1 -RMBs 3  adjacent to the target macroblock MBs 0  are selected. As the reference motion vectors to be referred to, motion vectors MVs 1 -MVs 3  of the macroblock RMBs 1 -RMBs 3  are used. On the other hand, in the image space (color image space) obtained from the color signal, the reference macroblocks adjacent to the macroblock MB 0  corresponding to the target macroblock MBs 0  are not always the macroblock which have been subjected to the frame-by-frame motion compensation process. Therefore, when the reference macroblock is the macroblock which has been subjected to the frame-by-frame motion compensation process, the motion vectors stored in the frame MV memory  304   a  are referred to, while the reference macroblock is the macroblock which has been subjected to the field-by-field motion compensation process, the frame-basis motion vector resulting from converting the field-basis motion vectors stored in the field MV memory  304   b  into the frame-basis motion vector by the frame MV converter  310   b  is referred to. 
     It is decided whether or not each of the motion vectors MVt 1 , MVt 2 , and MVt 3  of the reference blocks in the color image space is the coded motion vector, that is, each of the reference macroblocks is the inter-macroblock which has been inter-frame coded, according to a motion vector effectiveness signal Mmot output from the MV memory  402 . 
     As for the macroblocks RMBs 1 -RMBs 3  adjacent to the target macroblock MBs 0  in the shape image space corresponding to the intra-macroblocks or the macroblocks outside the object, the corresponding motion vectors are not referred to in the process for generating the prediction value of the motion vector. Likewise, as for the blocks RB 1 -RB 3  adjacent to the target macroblock MB 0  in the color image space corresponding to the intra-macroblocks or the macroblocks outside the object, the corresponding motion vectors are not referred to in the process for generating the prediction value of the motion vector, either, subsequently, operation of effects will be described. 
     The operation of the image decoding apparatus differs from that of the prior art image decoding apparatus  2000  only in the operation of the motion vector decoding unit  2041  and the operation of the motion vector coding unit  2041  differs from the prior art motion vector decoding unit in the operation of the shape motion vector decoding unit  2241 . Therefore, the color motion vector decoding unit  2141  is identical to the prior art color motion vector decoding unit  2140 . 
     The MV predictor  405  included in the prior art shape motion vector decoding unit  2240  shown in FIG. 14 refers to the motion vectors MVt 1 , MVt 2 , and MVt 3  stored in the MV memory  304  as the color motion vectors when it generates the prediction value of the shape motion vector of the target macroblock. 
     On the other hand, since in the color decoding section for the interlaced color signal, switching between the frame-by-frame motion vector compensation decoding process and the field-by-field motion compensation decoding process is adaptively performed, two types of motion vectors (field-basis and frame-basis motion vectors) are generated. 
     In this case, the shape motion vector to be referred to by the MV predictor  405  is the frame-basis motion vector, and therefore the MV predictor  405  cannot refer to the field-basis color motion vector. 
     The MV predictor  405   a  included in the shape motion vector decoding unit  2241  of the second embodiment, generates the prediction value of the shape motion vector of the target macroblock in the shape image space with reference to the motion vectors stored in the frame MV memory  304   a  included in the color motion vector decoding unit  2141  when the reference macroblock in the color image space has been subjected to the frame-by-frame motion compensation process. On the other hand, when the reference macroblock has been subjected to the field-by-field motion compensation process, the MV predictor  405   a  refers to the frame-basis motion vector obtained by converting the field-basis motion vectors stored in the field MV memory  304   b  in the color motion vector decoding unit  2141  by the use of the frame MV converter  310   b.    
     Therefore, the MV predictor  405   a  included in the shape motion vector decoding unit  2241  always refers to the frame-basis motion vector, regardless of whether the reference macroblock is the macroblock which has been subjected to the field-by-field motion compensation process or the frame-by-frame motion compensation process. 
     In this manner, the motion vector decoding unit  2041  included in the image decoding apparatus which performs the motion compensation decoding process for the interlaced image signal, generates the prediction value of the shape motion vector of the target macroblock in the shape image space with reference to the color motion vector of the macroblock in the color image space, like the prior art motion vector decoding unit included in the image decoding apparatus which performs the motion compensation decoding process for the non-interlaced image signal. 
     Thus, in accordance with the second embodiment, the image decoding apparatus which adaptively performs switching between the frame-by-frame motion compensation decoding process and the field-by-field motion compensation decoding process, includes the MV predictor  405   a  which generates the prediction value of the shape motion vector with reference to the color motion vectors, i.e., the output of the frame MV converter  310   b  which converts the field-basis color motion vectors into the frame-basis motion vector when the reference color motion vector corresponds to the motion vector which has been subjected to the field-by-field motion compensation decoding process Therefore, in the process for predicting the shape motion vector performed by the shape decoding section which always performs the frame-by-frame motion compensation coding process, the color motion vectors of the interlaced color signal are referred to. 
     For the above reason, the construction of the shape motion vector decoding unit is made identical to that of the color motion vector decoding unit. To be specific, without the need for the motion vector decoding means for the frame-basis motion vectors and the field-basis motion vectors and by altering the construction of the MV predictor included in the prior art shape motion vector decoding unit, the simplified circuit construction of the shape motion vector decoding unit which generates the shape motion vector with reference to the color motion vector in the process for decoding the coded signal of the shape motion vector, can be realized, for the image decoding apparatus which performs the motion compensation decoding process for the interlaced image signal corresponding to the object. 
     In the second embodiment, the description has been given without distinguishing between the case where the reference macroblock in the color image space has one motion vector and the case where it has four motion vectors, because the motion vectors to-be-referred to are the same in either case. 
     To be more detailed, when the reference macroblock has one motion vector, it is assumed that the four motion vectors of the four blocks composing the macroblock are equivalent. Hence, in either case, for the target macroblock MBs 0  (see FIG.  12 ( d )) in the shape image space, motion vectors MVt 1 -MVt 3  of the blocks RB 1 -RB 3  adjacent to the target macroblock MB 0  (see FIG.  12 ( c )) in the color image space are used. 
     Embodiment 3 
     FIG. 3 is a block diagram showing a motion vector coding unit included in an image coding apparatus according to a third embodiment of the present invention. 
     A motion vector coding unit  1072  of the third embodiment comprises a color motion vector coding unit  1172  included in the color coding section and a shape motion vector coding unit  1272  included in the shape coding section. 
     The construction of the color motion vector coding unit  1172  is identical to that of the color motion vector coding unit  1171  of the first embodiment. The shape motion vector coding unit  1272  includes an MV predictor  204   b  which generates the prediction value of the shape motion vector of the target macroblock with reference to the output of the MV memory  102   a  (frame-basis motion vector) included in the color motion vector coding unit  1172 , which has replaced the MV predictor  204   a  included in the shape motion vector coding unit  1271  of the first embodiment, and the other construction is identical to that of the shape motion vector coding unit  1271  of the first embodiment. 
     When the MV predictor  204   b  generates the prediction value of the shape motion vector MVs 0  of the target macroblock MBs 0  shown in FIG.  12 ( d ), reference macroblocks in the image space (shape image space) obtained from the shape signal, i.e., macroblocks RMBs 1 -RMBs 3  adjacent to the target macroblock MBs 0  are selected. As the motion vectors to-be-referred, the motion vectors MVs 1 -MVs 3  of the macroblock RMBs 1 -RMBs 3  are used. On the other hand, in the image space (color image space) obtained from the color signal, the reference macroblocks adjacent to the macroblock MB 0  corresponding to the target macroblock MBs 0  not always the macroblocks which have been subjected to the frame-by-frame motion compensation process. Therefore, when the reference macroblock is the macroblock which has bee subjected to the frame-by-frame motion compensation process, the motion vectors stored in the frame MV memory  102   a  are referred to, while when the reference macroblock is the macroblock which has been subjected to the field-by-field motion compensation process, the color motion vector is not referred to. The other construction of the MV predictor  204   b  is almost identical to that of the MV predictor  204   a  of the first embodiment. 
     Subsequently, operation and effects will be described. 
     Unlike the motion vector coding unit  1071  of the first embodiment, in the motion vector coding unit  1272 , the output of the frame MV converter  110   b  included in the color motion vector coding unit  1172  is not sent to the shape motion vector coding unit  1272 . 
     In the shape motion vector coding unit  1272 , the MV predictor  204   b  generates the prediction value of the shape motion vector of the target macroblock in the shape image space, with reference to the corresponding motion vectors stored in the frame MV memory  102   a , when the reference macroblock in the color image space has been subjected to the frame-by-frame motion compensation process. On the other hand, when the reference macroblock has been subjected to the field-by-field motion compensation process, the MV predictor  204   b  does not refer to the output (frame-basis motion vector) of the frame MV converter  110   b  included in the color motion compensation coding unit  1171 , unlike in the MV predictor  204   a  of the first embodiment. 
     Since the motion vector coding unit  1072  of the third embodiment does not refer to the field-basis color motion vector, precision in predicting the prediction value of the shape motion vector of the target macroblock might be reduced in contrast with the motion vector coding unit  1071  (see FIG.  1 ). 
     When the target macroblock has been subjected to the frame-by-frame motion compensation process and the reference macroblock has been subjected to the field-by-field motion compensation process, the output of the frame MV converter  110   b  included in the color motion vector coding unit  1172  is referred to by the frame MV predictor  104   a , and therefore the motion vector coding unit of the first embodiment is more appropriate. 
     In this case, the frame MV predictor  104   a  needs to refer to the field-basis motion vector, and hence, the frame MV converter  110   b  converts the field-basis motion vectors into the frame-basis motion vector. So, the amount of data to be processed by the frame Mv converter  110   b  is the same regardless of whether the output (frame-basis motion vector)of the frame MV converter  110   b  is sent to the frame MV predictor  104   a  and the MV predictor  204   b  or to the frame MV predictor  104   a . From the view point of the precision in predicting the prediction value of the shape motion vector, the motion vector coding unit of the first is more suitable. 
     When the target macroblock and the reference macroblock in the color image space have been subjected to the field-by-field motion compensation, the color motion vector coding unit  1172  refers to the output of the field MV memory  102   b . Therefore, using the construction of the motion vector coding unit  1071  of the first embodiment which refers to the color motion vector of the reference macroblock which has been subjected to the field-by-field shape motion compensation process when the shape motion vector is predicted, the frame MV converter  110   b  needs to convert the field-basis color motion vectors to the frame-basis color motion vector only for the prediction process for the shape motion vector. 
     On the other hand, in the motion vector coding unit  1072  of the third embodiment, only the output of the frame memory  102   a  included in the color coding section  1172  is supplied to the MV predictor  204   b  included in the shape motion vector coding unit  1272 . 
     In such construction, when the target macroblock and the reference macroblock in the color image space have been subjected to the field-by-field motion compensation process, in the shape motion vector coding unit  1272  predicts the shape motion vector without reference to the color motion vector of the reference macroblock, and thereby the frame MV converter  110   b  included in the color motion vector coding unit  1172  can omit the process for converting the field-basis color motion vectors into the frame-basis color motion vector. 
     Embodiment 4 
     FIG. 4 is a block diagram showing a motion vector decoding unit included in an image decoding apparatus according to a fourth embodiment of the present invention. 
     The motion vector decoding unit  2042  of the fourth embodiment is adapted to the motion vector coding unit  1072  of the third embodiment shown in FIG.  3 . The motion vector decoding unit  2042  includes a color motion vector decoding unit  2142  which decodes the coded signal Emvt of the color motion vector sent from the color motion vector coding unit  1172  and outputs the resulting decoded signal Dmvt and a shape motion vector decoding unit  2242  which decodes the coded signal Emvs of the shape motion vector sent from the shape motion vector coding unit  1272  included in the motion vector coding unit  1072  and outputs the resulting decoded shape signal Dmvs. 
     The construction of the color motion vector decoding unit  2142  is identical to that of the color motion vector decoding unit  2141  of the second embodiment. The shape motion vector decoding unit  2242  includes an MV predictor  405   b  which generates the prediction value of the shape motion vector of the target macroblock with reference to the output of a frame MV memory  304   a  (frame-basis motion vector) included in the color motion vector decoding unit  2142 , which has replaced the MV predictor  405   a  included in the shape motion vector decoding unit  2242  of the second embodiment. The other construction is identical to that of the shape motion vector decoding unit  2141 . 
     When the MV predictor  405   a  generates the prediction value of the shape motion vector MVs 0  of the target macroblock MBs 0  shown in FIG.  12 ( d ), reference macroblocks in the image space (shape image space) obtained from the shape signal, i.e., macroblocks RMBs 1 -RMBs 3  adjacent to the target macroblock MBs are selected. As the motion vectors to be referred to, motion vectors Mv 1 -MVs 3  of the macroblock RMBs 1 -RMBs 3  are used. On the other hand, in the image space (color image space) obtained from the color signal, the reference macroblocks adjacent to the macroblock MB 0  corresponding to the target macroblock MBs 0  are not always the macroblock which have been subjected to the frame-by-frame motion compensation coding process. Therefore, when the reference macroblock is the macroblock which has bee subjected to the frame-by-frame motion compensation process, the motion vectors stored in the frame MV memory  304   a  are referred to, while the reference macroblock is the macroblock which has been subjected to the field-by-field motion compensation process, the color motion vector is not referred to. The other construction of the MV predictor  405   b  is identical to that of the MV predictor  405   a  of the second embodiment. 
     Subsequently, operation and effects will be described. 
     In the motion vector coding unit  2042  of the fourth embodiment, unlike the motion vector decoding unit  2041  of the second embodiment, the output of the frame MV converter  310   b  included in the color motion vector decoding unit  2142  (frame-basis motion vector) is not sent to the shape motion vector decoding unit  2242 . 
     In the shape motion vector decoding unit  2242 , the MV predictor  405   b  generates the prediction value of the shape motion vector of the target macroblock in the shape image space with reference to the corresponding motion vector stored in the frame MV memory  304   a  when the reference macroblock in the color image space has been subjected to the frame-by-frame motion compensation process. On the other hand, when the reference macroblock has been subjected to the field-by-field motion compensation process, the MV predictor  405   b  does not refer to the output (frame-basis motion vector) of the frame MV converter  310   b  included in the color motion compensation decoding unit  2142 . 
     Since the motion vector decoding unit  2042  of the fourth embodiment does not refer to the field-basis color motion vector, precision in predicting the prediction value of the shape motion vector of the target macroblock might be reduced in contrast with the motion vector decoding unit  2041  (see FIG.  2 ). 
     When the target macroblock has been subjected to the frame-by-frame motion compensation process and the reference macroblock has been subjected to the field-by-field motion compensation process, the output of the frame MV converter  310   b  included in the color motion vector decoding unit  2142  is referred to by the frame MV predictor  305   a  included in the color motion vector decoding unit  2142 , the motion vector decoding unit of the second embodiment is more appropriate. 
     In this case, the frame Mv predictor  305   a  included in the color motion vector coding unit  2142  needs to refer to the field-basis motion vector, and hence, the frame MV converter  310   b  converts the field-basis motion vectors into the frame-basis motion vector. So, the amount of data to be processed by the frame MV converter  310   b  is the same regardless of whether the output (frame-basis motion vector)of the frame MV converter  310   b  is output to the frame MV predictor  305   a  and the MV predictor  405   b  or to the frame MV predictor  305   a . From the view point of the precision in predicting the prediction value of the shape motion vector, the motion vector decoding unit of the second embodiment is more suitable. 
     When the target macroblock and the reference macroblock in the color image space have been subjected to the field-by-field motion compensation, the color motion vector decoding unit  2142  refers to the output of the field MV memory  304   b . Therefore, using the construction of the motion vector decoding unit  2041  of the second embodiment which refers to the color motion vector of the reference macroblock which has been subjected to the field-by-field shape motion compensation process when the shape motion vector is predicted, the frame MV converter  310   b  needs to convert the field-basis color motion vectors to the frame-basis color motion vector only for the prediction process for the shape motion vector. 
     On the other hand, in the motion vector decoding unit  2042  of the fourth embodiment, only the output of the frame MV memory  304   a  included in the color decoding section  2142  is supplied to the MV predictor  405   b  included in the shape motion vector decoding unit  2242 . 
     In such construction, when the target macroblock and the reference macroblock in the color image space have been subjected to the field-by-field motion compensation process, the shape motion vector decoding unit  2242  predicts the shape motion vector without reference to the motion vector of the reference macroblock, and thereby the frame MV converter  310   b  included in the color motion vector decoding unit  2142  can omit the process for converting the field-basis color motion vectors into the frame-basis color motion vector. 
     Embodiment 5 
     FIG. 5 is a block diagram showing a motion vector coding unit included in an image coding apparatus according to a fifth embodiment of the present invention. 
     A motion vector coding unit  1073  of the fifth embodiment comprises a color motion vector coding unit  1173  included in the color coding section and a shape motion vector coding unit  1273  included in the shape coding section, like the motion vector coding unit  1073  of the third embodiment. 
     The construction to the color motion vector coding unit  1173  is identical to that of the color motion vector coding unit  1172  of the third embodiment. The shape motion vector coding unit  1273  includes a switch  220  provided between a frame MV memory  102   a  included in the color motion vector coding unit  1173  and an MV predictor  204   b  for performing control according to a non-interlacing decision signal Nit externally supplied in order to decide whether or not the output of the frame MV memory  102   a  (frame-basis motion vector) is to be supplied to the MV predictor  204   b . The other construction of the shape motion vector coding unit  1273  of the fifth embodiment is identical to that of the shape motion vector coding unit  1272  of the third embodiment. The non-interlacing decision signal Nit indicates whether an image signal input to the image coding apparatus is the interlaced signal or the non-interlaced signal. 
     Subsequently, operation and effects will be described. 
     When the image signal is the non-interlaced signal, the switch  220  enters into the ON-state according to the non-interlacing decision signal Nit input to an input terminal  9   a . Thereby, an MV predictor  204   b  can refer to the frame-basis motion vector stored in the frame MV memory  102   a  included in the color motion vector coding unit  1173 . 
     In other words, when the non-interlaced image signal is input, the motion vector coding unit  1073  of the fifth embodiment operates as in the case of the prior art motion vector coding unit shown in FIG. 13, and the same coding efficiency is realized. 
     On the other hand, when the image signal is the interlaced image signal, the switch  220  enters into the OFF-state according to the non-interlacing decision signal Nit. Thereby, the MV predictor  204   b  cannot refer to the color motion vector in the prediction process. 
     As a result, in this fifth embodiment, in the process for coding the interlaced image signal, the MV predictor  204   b  need not decide whether the motion vector of the reference macroblock is the field-basis motion vector or the frame-basis motion vector, and therefore can reduce amount of signal processing for this decision process, although efficiency in coding the shape motion vector is reduced. 
     Embodiment 6 
     FIG. 6 is a block diagram showing a motion vector decoding unit included in an image decoding apparatus according to a sixth embodiment of the present invention. 
     A motion vector decoding unit  2043  of the sixth embodiment is adapted to the motion vector coding unit  1073  of the fifth embodiment shown in FIG.  5 . The motion vector decoding unit  2043  includes a color motion vector decoding unit  2143  which decodes the coded signal Emvt of the color motion vector output from the color motion vector coding unit  1173  included in the motion vector coding unit  1073  and outputs the resulting decoded signal Dmvt, and a shape motion vector decoding unit  2243  which decodes the coded signal Emvs of the shape motion vector output from the shape motion vector coding unit  1273  included in the motion vector coding unit  1073 . 
     The construction of the color motion vector decoding unit  2143  is identical to that of the color motion vector decoding unit  2142  of the fourth embodiment. The shape motion vector decoding unit  2243  includes a switch  420  provided between the frame MV memory  304   a  included in the color motion vector decoding unit  2143  and an MV predictor  405   b  for performing control according to a non-interlacing decision signal Nit externally supplied in order to decide whether or not the output of the frame memory MV memory  304  (frame-basis motion vector) i&amp; to be supplied to the MV predictor  405   b . The other construction of the shape motion vector decoding unit  2243  of the sixth embodiment is identical to that of the shape motion vector decoding unit  2242  of the fourth embodiment. 
     The non-interlacing decision signal Nit indicates whether or not the coded image signal input to the image decoding apparatus is the non-interlaced coded image signal. 
     Subsequently, operation and effects will be described. 
     When the non-interlaced coded image signal is input to the image decoding apparatus, according to the non-interlacing decision signal Nit input to an input terminal  9   b , the switch  420  enters into the ON-state. Thereby, the MV predictor  405   b  can refer to the frame-basis motion vector stored in the frame MV memory  304   a  included in the color motion vector decoding unit  2143 . 
     When the non-interlaced image signal is input, the motion vector decoding unit  2043  of the sixth embodiment operates as in the case of the prior art motion vector decoding unit shown in FIG. 14, and the same decoding efficiency is realized. 
     On the other hand, when the image signal is the interlaced image signal, the switch  420  enters into the OFF-state according to the non-interlacing decision signal Nit. Thereby, the MV predictor  405   b  cannot refer to the color motion vector in the prediction process. 
     As a result, in this sixth embodiment, in the process for decoding the interlaced image signal, the MV predictor  405   b  need not decide whether the motion vector of the reference macroblock is the field-basis motion vector or the frame-basis motion vector, and therefore can reduce amount of signal processing for this decision process, although efficiency in decoding the shape motion vector is reduced. 
     Furthermore, when a coding or decoding program for realizing construction of the motion vector coding unit or the motion vector decoding unit according to any of the above-described embodiments by software is recorded in a data storage medium such as a floppy disk, the image processing can be easily implemented in an independent computer system. 
     FIGS.  7 ( a )- 7 ( c ) are diagrams for explaining the case where the motion vector coding unit of the first, third, or fifth embodiment or the motion vector decoding unit of the second, fourth, or sixth embodiment is executed by a computer system, using a floppy disk which contains the image coding or decoding program. 
     FIG.  7 ( a ) shows a front view of a floppy disk FD, a cross-sectional view thereof, and a floppy disk body D. FIG.  7 ( b ) shows an example of a physical format of the floppy disk body D. 
     The floppy disk FD has the configuration in which a floppy disk case FC contains the floppy disk body D. On the surface of the floppy disk body D, a plurality of tracks Tr are formed concentrically from the outer circumference of the disk toward the inner circumference. Each track Tr is divided into  16  sectors (Se) in the angular direction. Therefore, in the floppy disk FD having the above-mentioned program, data of the program are recorded in the assigned sectors on the floppy disk body D. 
     FIG.  7 ( c ) illustrates the structure for recording the program in the floppy disk FD and performing the image processing by software using the program stored in the floppy disk FD. 
     To be specific, when the program is recorded in the floppy disk FD, data of the program are written in the floppy disk FD from the computer system Cs through the floppy disk drive FDD. When the above-described image coding apparatus or image decoding apparatus is constructed in the computer system Cs by the program recorded in the floppy disk FD, the program is read from the floppy disk FD by the floppy disk drive FDD and then loaded to the computer system Cs. 
     While in the above description a floppy disk is employed as a data storage medium, an optical disk may be employed. Also in this case, coding or decoding by software can be performed in like manner as the case of using the floppy disk. Further, the data storage medium is not restricted to these disks. Any medium may be employed as long as it can contain the program, for example, an IC card, ROM cassette, etc. Also in the case of using these data storage media, image processing by software can be performed in like manner as the case of using the floppy disk.