Abstract:
A method for supplementing a digital image with picture elements, by which prediction signals having small errors can be generated for a digital image in which objects move greatly through a process which does not cause a long delay time and does not need a large quantity of calculation. In the method, the image is divided into areas. The insignificant sampled values of the areas containing the boundary of the shape of an object are transformed with a function of significant picture element values near insignificant picture element values and used to supplement the digital image. A digital image encoder and a digital image decoder both using the method are also disclosed.

Description:
THIS APPLICATION IS A U.S. NATIONAL PHASE APPLICATION OF PCT INTERNATIONAL APPLICATION PCT/JP97/00117. 
     DESCRIPTION 
     1. Field of the Invention 
     The present invention relates to a method of padding a digital picture having an arbitrary shape, and an encoder and a decoder of digital picture using the same method. 
     2. Background Art 
     It is necessary to compress (encode) a digital picture for promoting the efficiency of its storage and transmission. Several methods of encoding are available as prior arts such as “discrete cosine transform” (DCT) including JPEG and MPEG, and other wave-form encoding methods such as “subband”, “wavelet”, “fractal” and the like. Further, in order to remove a redundant signal between pictures, a prediction method between pictures is employed, and then the differential signal is encoded by wave-form encoding method. 
     According to the recent trend, the object constituting a picture are individually encoded and transmitted, for improving the coding efficiency as well as allowing reproduction of the individual objects which constitute a picture. On a reproducing side, each object is decoded, and the reproduced objects are composited into the picture for displaying. Per-object base encoding method allows the user to combine objects arbitrarily, whereby a motion picture can be re-edited with ease. Further, depending on the congestion of the communication channel, performance of a reproducing apparatus or a user&#39;s taste, even a less important object is saved from being reproduced, a motion picture can be still identified. 
     In order to encode a picture having an arbitrary shape (i.e., an object), an appropriate transformation method adapted to the shape is employed, such as the “shape adaptive discrete cosine transform”, or an insignificant region of the picture is padded by a predetermined method and then a conventional cosine transform (8×8) is provided, where the insignificant region is an outside of the display region of the object, and contains no pixel data for displaying an object, in other words, the region consists of insignificant sample values only. On the other hand, insignificant sample values can be found at the object boundary of a prediction region (e.g., a block consisting of 16×16 pixels) which is obtained through a motion compensation of a reference picture reproduced in the past for removing a redundant signal between pictures. This type of prediction region is firstly padded, then a the difference between the subject region and the predict region is obtained, and then, transformed and encoded. The reason why the prediction region is padded is to suppress a differential signal. 
     When the efficiency of encoding/decoding a digital picture is considered, how to pad the insignificant pixels is an important subject, and this influences a decoded picture quality and transmitting data quantity. 
     The prior art discussed above discloses the following steps: An overall picture is referenced and padded first, to prevent a prediction region from including insignificant sample values, then the prediction region is obtained by a motion compensation or other methods. How to pad the overall picture is, repeating a significant sample value on an object boundary and replacing an insignificant sample values therewith. When a sample is padded by scanning both horizontal and vertical directions, an average of both the padded values are taken. This conventional method pads the whole picture, and therefore providing a prediction region with less errors for a picture having a great motion. 
     However, when the whole image of a reproduced reference picture is referenced and padded, the reference picture must be entirely decoded, before padding can be started. When repetitive padding is applied, the amount of calculation increases in proportion to the picture size. In other words, this padding method requires a large amount of processing and a long delay time, and sometimes results in very large amount of calculation, for reproducing a picture. 
     In order to avoid calculation proportional to the picture size, a reproduced boundary region should be padded on per-region basis. This method can solve the delay time and volumes of calculation. However, since this method pads only the boundary region, the significant regions are limited within the internal region bounded by the boundary regions, and hence limiting the effect of padding. Therefore, this method cannot produce a prediction signal having less errors for a motion picture with a great motion. 
     Since the method of padding the overall picture results in increasing data amount, only a small advantage can be expected. In other words, an insignificant pixel has no pixel values to be encoded, and when significant pixels are encoded together with an insignificant pixel, coding efficiency is lowered. For example, when the significant pixels are all in black, the coding efficiency is lowered if insignificant pixels are in white, on the other hand, the coding efficiency is promoted if the insignificant pixels are in black. As such, a value of the insignificant pixel does not influence a quality of a reproduced picture, but influences the coding efficiency, therefore, how to deal with the insignificant pixel value should have been discussed with care. 
     DISCLOSURE OF THE INVENTION 
     The present invention aims to, firstly, provide a padding method, through which a prediction signal with less errors can be produced for a motion picture having a great motion, accompanying a short delay time and a small volume of calculation. 
     In order to achieve the above goal, according to the present invention, in a digital picture data including picture information which indicates an object, a picture is resolved into a plurality of regions adjoining with each other, and insignificant sample value of a region containing the boundary of the object shape is padded by the values obtained from transforming the significant pixel values near to the insignificant pixel values. 
     The simplest functional transformation is that an insignificant pixel value is replaced with a significant pixel value adjoining thereto, and this replacement is just repeated. The combination of this repetitive replacement method and the above method can produce the more effective padding. 
     Further, there is a method of enlarging a padding region to an appropriate extent. This method extends the region to be padded to an insignificant regions consisting of insignificant pixel values only, where the insignificant regions are near to the regions containing an object boundary. In addition to padding these insignificant regions, this method also pads the regions containing the object boundary using values obtained by applying a functional transformation to the significant pixel values of the region. This method enables processing involving larger motion compensation. 
     The present invention aims to, secondly, apply the above method of padding a digital picture to the methods of encoding/decoding digital picture and the apparatus thereof, whereby a picture compression process producing the better picture quality with a small amount of processing data can be realized. 
     In order to achieve the above goal, a picture encoder comprising the following elements is prepared: In a digital picture data including picture information which indicates an object of the input signal, where the input signal comprises (1) a signal indicating a pixel value and (2) a significant signal indicating whether a pixel value of each pixel is significant or not, the picture encoder comprises, 
     (a) predicted picture generation means for producing a predicted picture signal corresponding to the input signal by using a decoded picture signal, 
     (b) pixel value generation means for resolving the picture into a plurality of regions adjoining to each other, padding the insignificant sample value of the region containing a boundary of the object shape with a functional-transformed significant pixel values located near to the above insignificant pixel value, 
     (c) subtraction means for subtracting the output of the predicted picture generation means from an output of the pixel value generation means, 
     (d) encoding means for encoding the output of the subtraction means, 
     (e) decoding means for decoding the output of the encoding means, 
     (f) adding means for adding an output of the decoding means and the output of the predicted picture generation means, and 
     (g) memory means for storing the output of the adding means temporarily for further use in the predicted picture generation means, 
     wherein the output of the encoding means is an output of this picture encoder. 
     The corresponding digital picture decoder comprising the following elements is also prepared: 
     (a′) decoding means for decoding the input signal, 
     (b′) predicted picture generation means for producing a predicted picture signal corresponding to the input signal by using a decoded picture signal, 
     (c′) pixel value generation means for producing a pixel value from significant pixel value in the predicted picture signal by using a predetermined function, replacing insignificant pixel value of the predicted picture signal with the produced picture value, and outputting the replaced pixel value, 
     (d′) adding means for adding an output of the decoding means and an output of the pixel value generation means, and 
     (e′) memory means for storing temporarily an output of the adding means for further use in the predicted picture generation means, 
     wherein the output of the decoding means is an output of this picture decoder. 
     An insignificant region adjoining to the boundary of object shape and consisting of insignificant sample values only, is padded, whereby processing region is appropriately enlarged without increasing data volume remarkably, and as a result, the accuracy of processes including a motion compensation is promoted. 
     To be more specific about the padding method of a digital picture according to the present invention, the method comprising the following steps is prepared: 
     a first padding process for scanning a picture sample having an arbitrary shape consisting of significant and insignificant sample values along a first direction, and in the first direction, producing a first padded picture by replacing the insignificant sample values with the significant sample values selected through a predetermined method, 
     a second padding process for scanning each sample of the first padded picture consisting of significant and insignificant sample values along a second direction, and in the second direction, replacing the insignificant sample values of the first padded picture with the significant sample values selected through a predetermined method or the sample values padded in the first padding process. 
     To be more specific about the padding method of a digital picture according to the present invention, another method comprising the following steps is prepared: 
     resolving a digital picture having an arbitrary shape into a plurality of regions, 
     processing the regions according to a predetermined order, 
     padding the insignificant region adjoining to a boundary region at the shape boundary and consisting of insignificant sample values only, with a predetermined padding values. 
     When the subject region is not an insignificant region, in particular, if a previous region adjoining to a subject region is an insignificant region at the predetermined order, the previous region is padded with a padding value found through a predetermined method. 
     When the subject region is an insignificant region, in particular, if a previous region adjoining to a subject region is not an insignificant region at the predetermined order, the subject region is padded with a padding value found through a predetermined method. 
     A picture encoder employing the method of padding a digital picture according to the present invention comprises the following elements: 
     input means for receiving a digital picture data having an arbitrary shape, 
     process means for resolving the digital picture into a plurality of regions adjoining to each other, 
     a first adding device for receiving a data of a subject region and a data of a prediction region, and producing a data of a differential region, 
     an encoding device for receiving the data of the differential region, and compressing thereof into a data of a compressed differential region through a predetermined method, 
     a decoding device for receiving the data of the compresssed differential region, and decoding thereof into a data of an expanded differential region, 
     a second adding device for receiving the data of the expanded differential region, adding the data of the prediction region thereto, and producing a data of a reproduced region, 
     a first padding device for receiving the data of the reproduced region and padding the insignificant sample values included in the reproduced region through the previously described padding method, 
     a frame memory for storing the data of the reproduced region of which insignificant sample value has been padded. 
     Instead of or in addition to the first padding device, a second padding device is employed for padding insignificant sample values included in the prediction region. 
     A picture decoder employing the method of padding a digital picture according to the present invention comprises the following elements: 
     input means for receiving a compressed coded data, 
     a data analyzing device for analyzing the compressed coded data, and outputting a compressed differential signal, 
     a decoding device for decoding the compressed differential signal into an expanded differential signal, 
     an adding device for adding the expanded differential signal and a prediction signal, producing a reproduced signal and outputting thereof, 
     a first padding device for padding an insignificant sample values included in the reproduced signal through the previously described method, 
     a frame memory for storing a picture data padded by the first padding device as the prediction signal. 
     Instead of or in addition to the first padding device, a second padding device is employed for padding insignificant sample values included in the prediction region. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic diagram depicting a padding method of a digital picture in a first exemplary embodiment of the present invention. 
     FIG. 2 is a schematic diagram depicting a first modification of the padding method of the digital picture in the first exemplary embodiment of the present invention. 
     FIG. 3 is a schematic diagram depicting a second modification of the padding method of the digital picture in the first exemplary embodiment of the present invention. 
     FIG. 4 is a schematic diagram depicting a third modification of the padding method of the digital picture in the first exemplary embodiment of the present invention. 
     FIG. 5 is a schematic diagram depicting a padding method of a digital picture in a second exemplary embodiment of the present invention. 
     FIG. 6 is a schematic diagram depicting a padding method of a digital picture in a third exemplary embodiment of the present invention. 
     FIG. 7 is a schematic diagram depicting a first modification of the padding method of the digital picture in the third exemplary embodiment of the present invention. 
     FIG. 8 is a schematic diagram depicting a padding method of a digital picture in a fourth exemplary embodiment of the present invention. 
     FIG. 9 is a schematic diagram depicting a padding method of a digital picture in a fifth exemplary embodiment of the present invention. 
     FIG. 10 is a schematic diagram depicting a padding method of a digital picture in a sixth exemplary embodiment of the present invention. 
     FIG. 11 is a schematic diagram depicting a padding method of a digital picture in a seventh exemplary embodiment of the present invention. 
     FIG. 12 is a schematic diagram depicting a padding method of a digital picture in a eighth exemplary embodiment of the present invention. 
     FIG. 13 is a schematic diagram depicting a padding method of a digital picture in a ninth exemplary embodiment of the present invention. 
     FIG. 14 is a schematic diagram depicting a first modification of the padding method of the digital picture in the seventh exemplary embodiment of the present invention. 
     FIG. 15 is a schematic diagram depicting a padding method of a digital picture in a ninth exemplary embodiment of the present invention. 
     FIG. 16 is a schematic diagram depicting a first modification of the padding method of the digital picture in the ninth exemplary embodiment of the present invention. 
     FIG. 17 is a schematic diagram depicting a padding method of a digital picture in a tenth exemplary embodiment of the present invention. 
     FIG. 18 is a schematic diagram depicting a first modification of the padding method of the digital picture in the tenth exemplary embodiment of the present invention. 
     FIG. 19 is a flow chart depicting a padding method of a digital picture in a 11 th  exemplary embodiment of the present invention. 
     FIG. 20 is a schematic diagram depicting an embodiment of a method of padding a region, which is employed in the padding method of the digital picture in the 11 th  exemplary embodiment of the present invention, where (A) shows an example; a padding value is an average of significant pixel values arranged along the horizontal direction, (B) shows an example; a padding value is repeated significant pixel values arranged along the horizontal direction, and (C) shows another example; a padding value is repeated significant pixel values arranged along the horizontal direction. 
     FIG. 21 is a schematic diagram depicting an embodiment of a method of padding a region, which is employed in the padding method of the digital  10  picture in the 12 th  exemplary embodiment of the present invention, where (A) shows an example; a padding value is an average of significant pixel values arranged along the vertical direction, (B) shows an example; a padding value is repeated significant pixel values arranged along the vertical direction, and (C) shows another example; a padding value is repeated significant pixel values arranged along the vertical direction. 
     FIG. 22 is a flow chart depicting a padding method of a digital picture in a 13 th  exemplary embodiment of the present invention. 
     FIG. 23 is a flow chart depicting a second modification of the padding method of the digital picture in a 14 th  exemplary embodiment of the present invention. 
     FIG. 24 is a schematic diagram of a first example of the picture padded through the padding method of the digital picture in the 14 th  exemplary embodiment of the present invention. 
     FIG. 25 is a schematic diagram of a second example of the picture padded through the padding method of the digital picture in the 14 th  exemplary embodiment of the present invention. 
     FIG. 26 is a schematic diagram of a third example of the picture padded through the padding method of the digital picture in the 14 th  exemplary embodiment of the present invention. 
     FIG. 27 is a block diagram depicting a digital picture encoder utilized in a 15 th  exemplary embodiment of the present invention. 
     FIG. 28 is a block diagram depicting a modification of the digital picture encoder utilized in the 15 th  exemplary embodiment of the present invention. 
     FIG. 29 is a block diagram depicting a digital picture decoder utilized in a 16 th  exemplary embodiment of the present invention. 
     FIG. 30 is a block diagram depicting a digital picture encoder utilized in a 17 th  exemplary embodiment of the present invention. 
     FIG. 31 is a block diagram depicting a digital picture decoder utilized in a 17 th  exemplary embodiment of the present invention. 
     FIG. 32 is a block diagram depicting a digital picture decoder utilized in a 18 th  exemplary embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention is detailed hereinafter by referring to exemplary embodiments. 
     Exemplary Embodiment 1 
     FIG. 1 is a schematic diagram depicting a padding method of a digital picture in a first exemplary embodiment of the present invention. A picture  501  is a subject picture to be padded. Each lattice in the picture  501  represents a pixel i.e., a sample from the picture. Pixels  502 - 507  are significant samples, and other samples are insignificant. 
     In this embodiment, a shape signal of the picture is referred to for determining whether a sample is significant or insignificant. When the shape signal is “0”, the sample is insignificant, and when the shape signal is “1”, the sample is significant. 
     When a picture  508  is produced from the picture  501 , each insignificant sample is padded as described below: 
     First, scan each line of the picture  501 . In this scanning process, when a significant sample is detected, a value thereof is substituted to an insignificant sample, e.g., when the first line is scanned, padding process is not done because of no significant sample, and when the second line is scanned, samples  509 ,  510  and  511  are insignificant, while a sample  502  is significant, thus the insignificant samples are padded with a value “a” of the sample  502 . In other words, the value of sample  502  is repeatedly padded to the adjoining insignificant samples  511 ,  510  and  509  sequentially. In the same manner, a value “b” of sample  503  is repeatedly padded to samples  512 ,  513  and  514 . 
     The third line is padded as same as the second line, and the fourth line is not padded because of no significant sample. In the picture  508  thus padded, the second and third lines have significant values. 
     Next, based on the picture  508 , the remaining insignificant samples are padded. As shown in picture  519 , scan the picture in vertical direction, and pad insignificant samples  520  and  528  respectively with the samples  509  and  515  which have been padded in the picture  508 . As such, samples  521 - 527  and  529 - 535  are padded in the same manner. 
     Through the above steps, the insignificant samples can be padded in a simple manner while the continuity between the samples is maintained, and therefore, improves the calculation efficiency including compression of pictures, while a picture quality is maintained. 
     In this embodiment, padding is performed through scanning along the horizontal and vertical directions which are perpendicular to each other; however, the scanning along a slanted line produces also the same effect. Further, a method of vertical scanning first, followed by horizontal scanning also produces the same effect. As long as the continuity of the samples is maintained, methods other than padding an insignificant sample with the nearest significant sample are applicable. 
     FIG. 2 is a schematic diagram depicting a first modification of the padding method of the digital picture in the first exemplary embodiment of the present invention. In the picture  508 , when a horizontal scanning is performed, mirroring can be done with respect to the boundary as a center between the insignificant and significant samples. For example, samples  511  and  502  are the boundary in a mirror, and a value of sample  502  is substituted into a value of sample  511 , then a value of sample  503  is substitute into a sample  510 . As such, the picture  501  is sequentially padded along the arrow mark, to the picture  508 , and then to the picture  519 , until all insignificant samples are padded. 
     FIG. 3 is a schematic diagram depicting a second modification of the padding method of the digital picture in the first exemplary embodiment of the present invention. This method is applied when an insignificant sample is located between significant samples. A case of horizontal scanning is detailed here, however, the details can be applied in the case of scanning other directions: Samples  612  and  613  are padded with a value of a sample  602 . Another method is that samples  611  and  614  can be padded with a value of sample  607 . The first method is that samples are scanned from left to right by extending a significant sample as it is for padding. The second method is that samples are scanned from right to left by extending the significant sample as it is for padding. The third method is that an insignificant sample is padded with its nearest sample along the scanning direction. Samples  615  and  618  are padded by this method. Lastly, an insignificant sample is padded with an average value of significant samples on both sides of the insignificant sample. 
     Samples  616  and  617  are padded by this method. 
     FIG. 4 is a schematic diagram depicting a third modification of the padding method of the digital picture in the first exemplary embodiment of the present invention. When a picture indicates an oval object, i.e., significant samples gather so that they shape into an oval, and this picture is basically padded by the method used in FIG.  1 . 
     A picture  701  comprises collected significant samples  702 . First, as shown in a picture  703 , insignificant samples are padded by horizontal scanning, next, as shown in a picture  704 , insignificant samples are padded by using significant samples or the samples padded in the picture  703  through vertical scanning. On the other hand, as shown in a picture  705 , insignificant samples are padded by vertical scanning first, and then by horizontal scanning. An average of the pictures  704  and  706  thus padded is taken, whereby a picture  707  is produced. This padding method can maintain sequence between the significant samples and the padded samples even in a more complex picture, and thus can deal with calculations efficiently while maintaining a picture quality. 
     Exemplary Embodiment 2 
     FIG. 5 is a schematic diagram depicting a padding method of a digital picture in a second exemplary embodiment of the present invention. 
     A picture  801  comprises collected significant samples  802 . First, scan the picture  801  horizontally, and substitute significant sample values into the nearest insignificant samples to produce a picture  803 . At the same time, scan the picture  801  vertically, and substitute significant samples into the nearest insignificant samples to produce a picture  804 . 
     An average of the pictures  803  and  804  is taken to produce a picture  806 . An average of the collected significant samples  802  would result in the same value, thus the averaging is not needed. 
     Since there are some samples values double padded in the picture  803  and  804 , an average of both the padded values is taken. If there is only one padded value available, this value becomes the padded value of the picture  806 . In the padding process of the pictures  803  and  804 , a sample having no padding value remains as an insignificant sample as it is. This insignificant sample is then to be padded with a value of the nearest significant sample or padded sample. When more than one padding values are available, an average of these values, or one of them is used for padding. All samples are finally padded as shown in a picture  811 . 
     This embodiment shows an another padding method to maintain continuity between the collected significant samples and insignificant samples both forming a complex shape, like the exemplary embodiment 1. 
     Exemplary Embodiment 3 
     FIG. 6 is a schematic diagram depicting a padding method of a digital picture in a third exemplary embodiment of the present invention. 
     A picture  901  comprises collected significant samples  902 . In this embodiment, a region  904  surrounding the collected significant samples  902  is determined and an insignificant sample is padded within the region  904 . The same padding method detailed above is utilized also in this embodiment. 
     A remaining region  905  is padded through a simple method by referring to the padded region  904 , thus all insignificant samples are padded (Ref. to FIG. 906.) 
     The region  904  is preferably rectangular; however, it may be another shape. The region  904  may be the smallest rectangular which includes the collected significant samples  902 , or a rectangular after extending the smallest rectangular by “k” samples. The value “k” is determined so that a size of the rectangular can satisfy a predetermined condition, e.g., “k” is determined so that the size of the rectangular can be a multiple of 16. 
     FIG. 7 is a schematic diagram depicting one modification of the padding method of the digital picture in the third exemplary embodiment of the present invention, and a picture  910  comprises collected significant samples  911 ,  912  and  913 . The picture  910  is resolved into respective regions  915 ,  916  and  917  which include the above collected significant samples, and then the respective regions are padded through the method previously described. 
     Exemplary Embodiment 4 
     FIG. 8 is a schematic diagram depicting a padding method of a digital picture in a fourth exemplary embodiment of the present invention. 
     A picture  920  is resolved into blocks each of which consists of M×N samples, and then are padded. Preferably M=N=8 or 16, or another arbitrary value is acceptable, or the picture can be resolved into triangles or another shape. Blocks  921  through  929  include partially significant samples, and insignificant samples thereof are padded through the method previously described by referring to the values of the significant samples. 
     When blocks  930  and  931 , which do not contain significant samples, are padded, a predetermined value (preferably “128”) is used for padding, or the nearest sample value is referred for padding. The block  930  is taken as an example; the block  930  is nearest to a block  929  among the blocks having significant samples. This is obtained by finding a distance between the coordinates points in the upper left corners of respective blocks. Then an average of significant samples in the block  929  is taken to be used for padding. 
     In the case of the block  931 , the nearest block which has significant samples is a block  922 , therefore, an average of the significant samples can be taken for padding; however, samples  934 ,  935 ,  936  and  937  in boundary can be repeated for padding. 
     As such, padding block by block in the predetermined procedure can realize more efficient calculation process. 
     Various exemplary embodiments are available as follows when the method of padding a digital picture according to the present invention is applied to a picture encoder and decoder. 
     Exemplary Embodiment 5 
     FIG. 9 is a schematic diagram depicting a digital picture encoder in a fifth exemplary embodiment of the present invention. FIG. 9 lists the following elements: an input terminal  201 , a first adder  202 , an encoder  203 , a discrete cosine transformer (DCT)  204 , a quantizer  205 , an output terminal  206 , a decoder  207 , an inverse quantizer  208 , an inverse discrete cosine transformer  209 , a second adder  210 , variable length encoder (VLC)  211 , a frame memory  213 , a motion estimator  214 , a motion compensator  215 , a first padder  240 , and a second padder  241 . 
     An operation of the digital picture encoder comprising the above elements is detailed hereinafter. First, input a picture having an arbitrary shape into the input terminal  201 . Second, resolve the picture into a plurality of regions adjoining each other. In this embodiment, the picture is resolved into blocks each of which consists of 8×8, or 16×16 samples; however, an any other shapes can be acceptable. Then, input subject blocks to be encoded into the motion estimator  214  via a line  225 . At the same time, input a previously reproduced picture (hereinafter called a reference picture) stored in a frame memory  213  into the motion estimator  214 , and then, output a motion displacement information (hereinafter called a motion vector) which gives the prediction signal having the least error with respect to the subject block through the block-matching method or other methods. Third, send this motion vector to the motion compensator  215 , where a prediction block is produced from the reference picture. The motion vector is sent to the VLC  211  via a line  228 , and is converted into a variable length signal. 
     The subject block is sent to the first padder  240 , where the block is padded through the method previously mentioned to produce a padding subject block. A prediction block is sent to the second padder  241 , where the block is padded through the method previously mentioned to produce a padding prediction block. 
     The padding subject block and padding prediction block are sent to the first adder  202 , where a difference between the two blocks is found to produce a differential block, which is compressed by the encoder  203 , namely by the DCT  204  and quantizer  205 , in this exemplary embodiment. The quantized data is sent to the VLC  211 , where the data is converted into a variable length code, which is fed together with other side information including motion vectors into the output terminal  206 . 
     On the other hand, the compressed data is sent to the decoder  207 , where the data is expanded, namely, the compressed data undergoes the inverse quantizer  208  and is expanded into a data in spatial domain by IDCT  209 . The expanded data of the differential block is added to a padding prediction block data which is sent via line  227  to produce a reproduced block. The data of the reproduced block is stored in the frame memory  213 . To indicate whether a sample value is significant or insignificant, a corresponding shape signal, encoded and subsequently decoded, is used as reference, although this is not shown in the drawings. 
     As such, a subject block and a prediction block are padded, whereby a large predicted error, which is caused by a shift of an edge part because of a motion compensation, can be suppressed. 
     This is not shown in the drawings; however, the padder  246  can be placed before the motion compensator  215 . In this embodiment, DCT is adopted; however, a shape adaptive DCT, subband or wavelet can be adopted instead. 
     Exemplary Embodiment 6 
     FIG. 10 is a schematic diagram depicting a digital picture encoder in a sixth exemplary embodiment of the present invention. The sixth exemplary embodiment has basically the same operation as that of the fifth exemplary embodiment. The different point is at the first adder  240 , a value for padding the prediction block is used for padding the subject block. This value is transmitted from the second padder  241  via a line  243  to the first padder  240 . Sharing the padding value as such makes almost all the differential values “0” (zero), whereby the prediction error is further suppressed. 
     Exemplary Embodiment 7 
     FIG. 11 is a schematic diagram depicting a digital picture decoder in a seventh exemplary embodiment of the present invention. FIG. 11 lists the following elements: input terminal  301 , data analyzer (parser)  302 , inverse quantizer  304 , IDCT  305 , adder  306 , output terminal  307 , frame memory  309 , motion compensator  310  and a padder  330 . 
     An operation of the digital picture decoder comprising the above elements is detailed hereinafter. First, input a compressed data into the input terminal  301 , then analyze the data by the data analyzer  302 , second, output the data of the compressed differential block to the decoder  303  via a line  312 , third, output a motion vector to the motion compensator  310  via a line  318 . In the decoder  303 , expand the compressed differential block to restore thereof to a expanded differential block, namely, in this embodiment, the compressed differential block undergoes the inverse quantizer  304  and IDCT  305 , where a signal in the frequency domain is transformed into a signal in the spatial domain. Then, input the motion vector via a line  318  into the motion compensator  310 , where an address for accessing the frame memory  309  is produced based on the motion vector, and a prediction block is produced using the picture to be stored in the frame memory  309 . Then, transmit the prediction block into the padder  330 , where insignificant samples are padded through the method previously detailed, and thereby producing a padding prediction block. Next, input the padding prediction block and the expanded differential block into the adder  306  to add both the block, thereby producing a reproduced block. Finally, output the reproduced block to the output terminal  307 , and at the same time, store the reproduced block into the frame memory  309 . 
     The above embodiment describes that the prediction block undergone the motion compensation is padded; however, the block can be padded during the motion compensation, which includes overlapped motion compensation. To indicate whether a sample value is significant or insignificant, a decoded shape signal should be referred, although this is not shown in the drawings. FIG. 14 is a schematic diagram depicting a first modification of the padding method of the digital picture in the seventh exemplary embodiment of the present invention, and has basically the same operation shown in FIG.  11 . In this embodiment, the padder  332  is placed before the motion compensator  310 . 
     Exemplary Embodiment 8 
     FIG. 12 is a schematic diagram depicting a digital picture encoder in an eighth exemplary embodiment of the present invention. The basic operation is the same as shown in FIG.  9 . The padder  212  is placed before the frame memory, whereby a reproduced block tapped off from the adder  210  can be advantageously padded immediately. Further the padder  244  is placed before DCT  204 . The padder  244  pads the blocks so that DCT coefficients becomes smaller. Regarding the differential block, in particular, insignificant regions of the subject blocks are padded with “0” (zero). 
     FIG. 13 is a schematic diagram depicting a padding method of a digital picture in a ninth exemplary embodiment of the present invention. The padder  246  is placed after the motion compensator  215 , which is an additional element to those in FIG.  12 . After the motion compensation, the predicted signal is further padded in order to give an effectiveness of suppressing the prediction errors. This is not shown in the drawings, however, the padder  246  can be placed before the motion compensator  215 . 
     Exemplary Embodiment 9 
     FIG. 15 is a schematic diagram depicting a digital picture decoder in a ninth exemplary embodiment of the present invention. This decoder corresponds to the decoder depicted in FIG.  12 . The operation of this decoder is basically the same as that in FIG.  14 . In this embodiment, a padder  308  is placed before the frame memory  309 , whereby a reproduced block can be padded immediately and then stored in the frame memory  309 . 
     FIG. 16 is a schematic diagram depicting a first modification of the decoder of the digital picture in the ninth exemplary embodiment of the present invention. This decoder corresponds to that in FIG.  13 . The operation of the decoder is basically the same as that in FIG.  15 . Only the different point is that a padder  330  is placed after the motion compensator  310  in order to pad the predicted block. 
     Exemplary Embodiment 10 
     FIG. 17 is a schematic diagram depicting a padding method employed in an encoder/decoder of a digital picture in a tenth exemplary embodiment of the present invention. The operation of the padder  330  is described hereinafter using FIG. 11 as an example. In FIG. 17, a subject block comprises collected significant samples  943  and collected insignificant samples  944 . A portion hatched by oblique lines represents significant regions. A predicted block  941  is obtained through a motion compensation, and comprises collected significant samples and collected insignificant samples. 
     In the decoder shown in FIG. 11, a predicted block  941  is padded and then sent to the adder  306 . In the padder  330 , the entire insignificant region (of the predicted block)  946  can be padded; however, it preferable to pad the insignificant region of the predicted block covered by the significant region of the subject block because of the less calculation volumes. By referring to the shape of the subject block  940 , both the significant and insignificant regions are determined (region  947  of the block  942 ), and then only the region  947  is padded by referring to itself. 
     FIG. 18 is a schematic diagram depicting a modification of the padding method employed in a digital picture encoder/decoder in the tenth exemplary embodiment of the present invention. Assume that no significant samples exist in a subject block of padding, and the padder  308  shown in FIG. 15 is used as an example. Assume that a block  962  of FIG. 18 is the subject block of padding, and since no significant samples exist in this block, the block cannot be padded within the block by referring to itself 
     In order to overcome the above problem, find an adjacent block comprising at least one significant sample, and pad the subject block by referring to the adjacent block. The padder in FIG. 15; however, reproduces the block  962  in advance of the block  964 , thus it is impossible to pad the block by referring to the block  964 . Then, search the reproduced blocks  966 ,  965 ,  961  and  963  sequentially for a first block which contains significant samples, and pad the block by referring to the found block. 
     In the case that the predicted block undergone the motion compensation does not have a significant sample, a subject block is padded in the same manner, i.e., through referring to the reproduced blocks having a significant sample and being adjacent to the subject block. A method of calculating a padding value can be an averaging method or a repetitive padding method. 
     The above embodiments prove that the picture encoder and decoder of the present invention can encode insignificant pixels, which do not influence a picture quality, by making the pixels such values as increasing the coding efficiency, whereby the coding efficiency is promoted, thus the encoder and decoder of the present invention have a great advantage in practical uses. 
     Exemplary Embodiment 11 
     FIG. 19 is a flow chart depicting a padding method of a digital picture in an 11 th  exemplary embodiment of the present invention. First, input a picture having an arbitrary shape, second resolve the picture into regions adjacent with each other, third, scan each region according to a predetermined order, and finally, process each region one by one according to the flow chart shown in FIG.  19 . In this embodiment, start scanning from the upper left and follow the same order as the raster scanning. The scanned region can be a triangle, rectangle or square. In this embodiment, the picture is resolved into squares each of which consisting of N×N samples, where N=8 or 16. The square of N×N samples is called a block hereinafter. 
     On Step  12 , determine whether a subject block is entirely outside an object picture having an arbitrary shape) or not. When the subject block is entirely outside the object, every sample of the subject block is not significant sample. In this embodiment, to determine whether a sample value is significant or not, the shape signal of the respective picture is referred. When the shape signal is “0”, the sample value is insignificant. When the shape signal is “1”, the sample value is significant. 
     When the subject block is not entirely outside the object, advance to Step  14 . Then determine whether previous blocks adjacent to the subject block are entirely outside the object or not, where the previous block is the block already processed according to the scanning order. When the adjacent previous blocks are entirely outside the object, on Step  16 , a padding value is calculated according to a predetermined method. On Step  18 , the sample values of the previous blocks adjacent to the subject block is substituted with the padding value so that the sample values are padded. 
     On Step  12 , when the subject block is entirely outside the object, advance to Step  20 . Then determine whether the previous blocks adjacent to the subject block is entirely outside the object or not. When the previous blocks are not entirely outside the object, a padding value is calculated according to the predetermined method on Step  22 , and the sample values of the subject block are substituted with the padding value on Step  24  so that the sample values are padded. When the adjacent previous blocks are padded on Step  18 ,the previous blocks can be taken as not to be entirely outside of the object on Step  20 . Repeat this process until the last block is processed (Steps  26  and  28 .) 
     Exemplary Embodiment 12 
     FIGS. 20 and 21 are schematic diagram depicting calculation methods of padding values. FIG. 20 shows a case where a present block is adjacent to a previous block in a horizontal direction. In FIG.  20 (A), a block  132  is a present block and a block  130  is a previous block. Each lattice represents a sample (pixel) of the picture. Assume that a block  130  is entirely outside an object, and take an average of the values of significant samples,  134 ,  136 ,  138 ,  140 ,  142  and  144 , then substitute the average value for each sample (lattice) in the previous block for padding. In FIG.  20 (B), pad each sample Gattice) of the previous block  146 , which is entirely outside the object, by repeating values of significant samples  150 ,  152 ,  154 ,  156  of the present block  148 . In other words, each lattice on the first, second, third and fourth lines of the previous block  146  is substituted with the values of samples  150 ,  152 ,  154 , and  156 . In FIG.  20 (C), the present block  160  is entirely outside the object and the previous block  158  is not outside the object. In this case, each lattice of the present block  160  is padded by repeating values of significant samples  162 ,  164 ,  166  and  168  of the previous block  158 . 
     FIG. 21 depicts the case where the present block is adjacent to the previous block in a vertical direction. In FIG.  21 (A), a block  172  is the present block and a block  170  is the previous block. Each lattice represents a sample (pixel) of the picture. Assume that a block  170  is entirely outside the object, and take an average of the values of significant samples  174 ,  176 ,  178 ,  180 ,  182  and  184  which are contained in the present block  172 , then substitute the average value for each sample (lattice) in the previous block  170 ,for padding. In FIG.  21 (B), pad each sample (lattice) of the previous block  186 , which is entirely outside the object, by repeating values of significant samples  190 ,  192 ,  194 ,  196 . In other words, each lattice on the first, second, third and fourth rows of the previous block  186  is substituted with the values of samples  196 ,  194 ,  192 , and  190 . In FIG.  20 (C), the present block  160  is entirely outside the object and the previous block  158  is not outside the object. In this case, each lattice of the present block  198  is padded by repeating values of significant samples  1100 ,  1102 ,  1104 ,  1106  of the previous block  199 . This embodiment details a block of 4×4 for making the long story short, but the same description can be applied to a block of N×N (N: arbitrary integer.) 
     Exemplary Embodiment 13 
     In FIG. 22, Step  13  is added to the flow chart shown in FIG.  19 . In other words, when a present block is not entirely outside an object, the region contained in the present block and outside the object is padded through Step  13  and thereafter. The present block  132  of FIG.  20 (A) is an example of a block containing regions outside the object. Samples  134 ,  136 ,  138 ,  140   142  and  144  are significant and within the object. The other samples (the lattices not painted) are insignificant and outside the object. 
     A method of padding these insignificant samples is to substitute the average of significant samples therefor. In this embodiment, the samples  134 ,  136  and  144  at boundary are repeated in the horizontal and vertical directions for padding. When two padding values are available, an average thereof is used for padding. Due to the padding of the present block through Step  13 , all the samples of the present block are substituted with a unique value, therefore, the previous block can be padded on Step  18  by repeating the values of significant samples of the present block existing at the boundary between the present and previous blocks, as shown in FIG.  20 (B) or FIG.  21 (B). An average of the significant samples can be used instead of repeating the sample values. 
     Exemplary Embodiment 14 
     FIG. 23 is a flow chart depicting the processes where the previous block adjacent to the present block in horizontal direction is utilized on Step  15 ,  19  and  21  shown in FIG.  22 . FIG. 24 shows a picture  108  which is an example padded through the process shown in FIG. 23. A star shape  110  is a significant object, and the other part consists of insignificant samples. The picture  108  is resolved into blocks of 7×7. A block having the same texture as the block  1114  is padded through Step  19  or Step  24  shown in FIG.  23 . 
     The padding method of this embodiment is described by referring to FIGS. 23 and 24. First, the block  1112  is discussed. Since the present block  1112  is not entirely outside the object on Step  12 , the present block is padded through Step  13 . On Step  15 , the previous block adjacent to the present block is not entirely outside the object, thus no padding is provided. 
     Next, the block  1114  is discussed. Since the present block  1114  is entirely outside the object, the process is advanced to Step  21 , where the previous block adjacent to in the horizontal direction is not entirely outside the object, thus the present block  1114  is padded by referring thereto on Step  24 . 
     Finally, the block  1116  is discussed. Since the present block  1116  is entirely outside the object on Step  12 , the process is advanced to Step  21 , where the previous block  1115  is not entirely outside the object, thus the present block  1116  is padded by referring thereto on Step  24 . 
     When the block  1117  is processed, the present block  1117  is not entirely outside the object on Step  12 , thus the block is padded on Step  13 . On Step  15 , the previous block  116  adjacent to in horizontal direction is entirely outside the object, the previous block is padded on Step  19 . In other words, the block  1116  is padded twice. When a plurality of padding values are available, an average of these values are taken, or one of these values can be selected for padding. The picture  108  is thus padded through expanding thereof in the horizontal direction. 
     When the horizontal direction is changed to vertical direction in the processes on Steps  15 ,  19  and  21 , a picture undergone the padding through vertical expansion as shown in FIG. 25 is obtained. When both blocks adjacent to in horizontal and vertical directions are processed in combination, a picture which is padded through extension in both horizontal and vertical directions as shown in FIG. 26 can be obtained. In this case, when a sample is padded twice or more, an average of all the padding values or a part of them are taken. When a plurality of padding candidates are available, the nearest candidate in the process order can be used. 
     A picture encoder and decoder which employ the padding method according to the present invention is described hereinafter. 
     Exemplary Embodiment 15 
     FIG. 27 depicts a digital picture encoder used in the 15 th  exemplary embodiment. FIG. 27 lists the following elements: input terminal  201 , first adder  202 , encoder  203 , discrete cosine transformer (DCT)  204 , quantizer  205 , output terminal  206 , decoder  207 , inverse quantizer  208 , inverse DCT  209 , second adder  210 , variable length encoder (VLC)  211 , padder  212 , frame memory  213 , motion estimator  214  and motion compensator  215 . 
     An operation of the digital picture encoder comprising the above elements is described hereinafter. First, input a picture having an arbitrary shape into the input terminal  201 , then resolve the picture into a plurality of regions adjacent with each other. In this embodiment, the block is resolved in to 8×8 blocks or 16×16 blocks; however, the blocks can be resolved into arbitrary shapes. 
     FIG. 24 should be referred. Input a subject block of padding into the motion estimator  214  via a line  225 . At the same time, input a previously produced picture (called “reference picture”) stored in the frame memory  213  to the motion estimator. 
     And then, output a motion displacement information (hereinafter called a motion vector) which gives the prediction signal having the least error with respect to the subject block through the block-matching method or other method. 
     Send this motion vector to the motion compensator  215 , where a predicted block is produced from the reference picture. Send this motion vector also to the VLC  211  via a line  228 , where the vector is converted into a variable length code. Then, send the subject block and predicted block to the first adder  202 , where a differential block is produced by using the difference therebetween. Next, compress the differential block in the encoder  203 . In this embodiment, the differential block is compressed in the DCT  204  and the quantizer  205 . 
     On the other hand, send the compressed data to the decoder  207  and expand it. In this embodiment, inversely quantize the compressed data in the inverse quantizer  208 , and then expand thereof into the data in spatial do main in the IDCT  209 . Ad d the predicted block sent via a line  227  to the expanded differential block to produce a re produced block. Then, input the reproduced block to the padder  212 , where insignificant samples of the reproduced block are substituted for padding through the padding method described in the 11 th  exemplary embodiment. Then, store the padded reproduced block in the frame memory  213 . Refer to the shape signal already encoded or decoded when a sample value should be indicated as significant or insignificant (this is not described in the drawings though.) 
     The padded picture to be stored in the frame memory  213  is, e.g., shown in FIG. 24,  25  or  26 . Send the padded picture via a line  224  to the motion estimator  214  and the motion compensator  215 . In this embodiment, an active area of the motion estimator and motion compensator is limited within the padded region (the painted regions in FIG. 24,  25  and  26 ), in other words, samples outsides the padded region are not accessed. 
     FIG. 28 depicts the picture encoder having a recorder  229  coupled to the picture encoder shown in FIG.  27 . The data converted to a variable length code by the VLC  211  is stored into a magnetic medium (tape or disc) or an optical disc via the recorder  229 . 
     As such, the region adjacent to the object boundary is padded, whereby the active area of the motion estimation and motion compensation can be enlarged. Thus, the predicted block with less remaining difference can be obtained for the picture having a great motion. Further, the padding method according to the present invention can suppress the delay time and calculation volumes. 
     The discrete cosine transform is employed in this embodiment; however, the shape adaptive discrete cosine transform, subband, or wavelet can also produce the same effect. 
     Exemplary Embodiment 16 
     FIG. 29 depicts a digital picture encoder used in the 16 th  exemplary embodiment. FIG. 29 lists the following elements: input terminal  301 , data analyzer  302 , decoder  303 , inverse quantizer  304 , IDCT (inverse discrete cosine transformer)  305 , adder  306 , output terminal  307 , padder  308 , frame memory  309  and padder  310 . 
     An operation of the digital picture decoder comprising the above elements is described hereinafter. First, input a compressed data to the input terminal  301 , then analyze the data in the data analyzer  302 . Output the data of the compressed differential block to the decoder  303  via a line  312 . Next, output a motion vector to the motion compensator  310  via a line  318 . In the decoder  303 , expand the compressed remaining block and restore it to a expanded differential block. In this embodiment, the compressed differential block undergoes the inverse quantizer  304  and IDCT  305  to be transformed from a signal in frequency domian into a signal in a spatial domain. Then input the motion vector to the motion compensator  310  via a line  318 . 
     In the motion compensator  310 , produce an address based on the motion vector in order to access the frame memory  309 , and also produce a predicted block using a picture stored in the frame memory  309 . Then, input the produced predicted block and the expanded differential block to the adder  306  to produce a reproduced block. Output the reproduced block to the output terminal  307 , and at the same time, input thereof to the padder  308 . Finally, pad the reproduced block through the padding method detailed in the 11 th  exemplary embodiment, and store the padded block in the frame memory  309 . 
     Exemplary Embodiment 17 
     FIG. 30 depicts a digital picture encoder used in the 17 th  exemplary embodiment. The basic structure is the same as shown in FIG.  27 . An initializer  230  is used instead of the padder  212 . Before a picture is stored in the frame memory  213 , the frame memory  213  picture is initialized with a predetermined initialization value by the initializer  230 . The reproduced block tapped off from the second padder  210  is stored in the frame memory  213 . The initialization value can be a fixed value, or an average value of significant samples of reproduced picture in the past. 
     FIG. 31 depicts the picture encoder having the recorder  229  coupled to the picture encoder shown in FIG.  30 . The data converted to a variable length code by the VLC  211  is stored into a magnetic medium (tape or disc) or an optical disc via the recorder  229 . 
     Exemplary Embodiment 18 
     FIG. 32 depicts a digital picture decoder used in the 18 th  exemplary embodiment. It has basically the same structure as that in FIG. 29, and employs an initializer  320  instead of the padder  308 . Before a picture is stored in a frame memory  309 , the frame memory is initialized with a predetermined initialization value by the initializer  320 . The reproduced block tapped off from a padder  306  is stored in the frame memory  309 . The initialization value can be a fixed value, or an average value of significant samples of reproduced picture in the past. 
     Industrial Applicability 
     The present invention provides a simple padding method, through which a small region undergone a motion compensation or a small reproduced region are padded, whereby calculation volumes can be substantially reduced. Since a subject region of padding is a closed small region, it takes a shorter delay time than when padding is performed across the entire picture. Further, not only a boundary region but also a region adjacent thereto, which comprises insignificant samples only is padded, and a motion is estimated as well as motion is compensated using the padded regions, whereby a predicted signal with less difference can be obtained. These factors contribute to the higher efficiency of encoding/decoding a picture having an arbitrary shape.