Abstract:
A moving picture coding apparatus divides each frame of a moving picture into parts and assigns the parts to different coding units, which compressively code their respective parts. The coding process includes motion compensation with respect to a reference frame. Each coding unit has its own reference frame memory. To generate reference frame data, each coding unit receives, decodes, and decompresses the coded data generated by at least one other coding unit, as well as decompressing the data it has coded itself. Consequently, only ordinary coded data have to be passed between different coding units, which saves bandwidth and eliminates the need for special coding hardware and development and testing tools.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a picture coding method and apparatus that code a moving picture by dividing each frame of the picture into parts and assigning the parts to different computing resources. 
     2. Description of the Related Art 
     With the increasing definition (resolution) of display apparatus in recent years has come the need to code high-definition moving pictures. 
     Moving picture coding has always been a computationally intensive task. The coding of moving pictures with the resolution of high-definition television (HDTV), for example, has required specialized hardware. 
     The performance of computing devices is also increasing, however, and this has led to proposed systems that code moving pictures in real time by assigning different parts of each frame to different computing resources. Japanese Patent Application Publication No. 9-294262, for example, describes a system that harnesses multiple hardware coders to operate concurrently on the separate parts. Japanese Patent No. 3621598 describes a similar multiprocessor system in which the multiple processors execute software to decode the different parts of each frame concurrently. 
     The coders in Japanese Patent Application Publication No. 9-294262 communicate with one another to transfer coding information so that in performing motion compensation, they can cross the boundaries between the different parts. The motion compensation areas of the different coders accordingly overlap. This overlapping scheme prevents the boundaries between different parts from becoming visible when the picture is decoded, but the disclosed scheme has the disadvantage of requiring a protocol analyzer and other tools to determine whether the coders are communicating correctly. Moreover, because of the special nature of the coding information being communicated, standard protocol analyzers or modified versions thereof cannot be used; it is necessary to develop a completely new protocol analyzer, which adds to the development cost and product cost of the coding apparatus. 
     A further problem is that the overlapping parts of each frame must be transmitted to both coders involved in the overlap, which uses up extra bandwidth on the data paths within the coding apparatus. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to improve the efficiency of information transfer when a picture is divided into parts for coding by separate coding units in such a way as to prevent degradation of the picture at the boundaries between the parts. 
     The invented picture coding method codes a moving picture made up of a series of frames. Each frame is divided into parts. The different parts of the frame are assigned to different coding units and coded by the coding units, and the resulting coded parts are combined to obtain compressively coded data for the frame. 
     Each coding unit has a reference frame memory that stores reference picture data representing a preceding frame. Portions of the reference picture data resembling portions of the assigned part of the current frame are selected to carry out motion compensation. The coding unit compresses its assigned part to obtain first compressed data, codes the first compressed data, and outputs the result as first coded data. 
     In addition, each coding unit receives second coded data that have been compressively coded by one or more other coding units. A decoder decodes the second coded data to obtain second compressed data. The first and second compressed data are both decompressed to obtain new reference picture data, which are stored in the reference frame memory for use in compressing and decompressing the picture data in the next or a subsequent frame. 
     This method enables coded data to be passed from one coding unit to another by the same communication protocol as used to send the coded data to the combiner that combines the data. The coded data can be passed between different coding units efficiently not only because the data are compressed but also because the data can be coded and decompressed by the same methods that each coding unit uses to code and decompress its own data. Moreover, in development, debugging, and troubleshooting, the flow of coded data between coding units can be analyzed by the same tools and methods as used to analyze other coded data flows. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the attached drawings: 
         FIG. 1  is a block diagram of a moving picture coding apparatus according to a first embodiment of the invention; 
         FIG. 2  is a functional block diagram of a conventional coding unit; 
         FIG. 3  is a functional block diagram of a coding unit in the first embodiment; 
         FIG. 4  illustrates the slicing of a picture into two parts in the first embodiment; 
         FIG. 5  illustrates the division of the moving picture into macroblocks in the input frame memory in coding unit in  FIG. 3 ; 
         FIG. 6  illustrates the switching of storage positions in the reference frame memory in the first embodiment; 
         FIG. 7  is a block diagram of a moving picture coding apparatus according to a second embodiment of the invention; 
         FIG. 8  illustrates the slicing of a picture into three parts in the second embodiment; and 
         FIG. 9  illustrates the switching of storage positions in the reference frame memory in the second embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Embodiments of the invention will now be described with reference to the attached drawings, in which like elements are indicated by like reference characters. 
     First Embodiment 
     Referring to  FIG. 1 , the moving picture coding system or apparatus  100  in the first embodiment comprises an input picture partitioner  101 , a pair of coding units  102 ,  103 , and a compressed stream combiner  104 . These constituent elements may be separate computing devices linked by a network, or separate processing units in a single computing device interconnected by a bus. The network configuration will be assumed in the description below. 
     The input picture partitioner  101  receives an input picture, divides it into parts by following a predetermined procedure, outputs one part to the picture input port  105  of coding unit  102 , and outputs the other part to the picture input port of coding unit  103 . If necessary, the input picture partitioner  101  may attach information to each part to specify the location of the part in the picture. 
     In dividing the input picture, the input picture partitioner  101  may apply any of various methods. For example, it may apply a rule that dynamically determines the size ratio of the two areas, and attach information specifying the size ratio to each part. Alternatively, the way in which the input picture is divided and the coding unit to which each part is assigned may be predetermined, and the input picture partitioner  101  may simply execute the fixed division and assignment procedure. 
     The coding units  102  and  103  receive respective parts of the picture from the input picture partitioner  101  through their picture input ports  105 , compressively code their respective parts, and output the compressively coded parts to the compressed stream combiner  104  from their compressed stream output ports  106 . Coding unit  102  also outputs its compressively coded part to a compressed stream input port  107  of coding unit  103 ; coding unit  103  also outputs its compressively coded part to a compressed stream input port of coding unit  102 . The functions of the coding units  102 ,  103  in the first embodiment will be described in more detail below, but briefly, each of the coding units  102 ,  103  decompresses both the part it coded itself and the part it receives from the other coding unit through its compressed stream input port  107 , and uses both decompressed parts as reference data in compressively coding its own part of the next picture. 
     The compressed stream combiner  104  receives the coded partial data streams from coding unit  102  and coding unit  103 , combines them, and outputs a single compressively coded data stream. 
     Each coding unit  102 ,  103  is generally similar to the picture coders-specified in the H.264 and MPEG-4 standards. As shown in  FIG. 2 , this general type of picture coder includes an input frame memory  1 , a motion estimator (ME)  2 , a motion compensator (MC)  3 , a subtractor  4 , a transform processor (T)  5 , a quantizer (Q)  6 , a dequantizer (Q-)  7 , an inverse transform processor (T-)  8 , an adder  9 , a reference frame memory  10 , a variable length coder (VLC)  11 , a picture input port  12 , and a compressed stream output port  13 . 
     To this conventional coder configuration the first embodiment adds a variable length decoder and a pair of switches, giving both coding units  102 ,  103  the novel structure shown in  FIG. 3 . The coding unit shown in  FIG. 3  is designated by reference numeral  300 , representing either of the coding units  102  and  103 . Many of the internal components of the coding unit  300  are the same as in  FIG. 2 , but they are renumbered so as to use the reference numerals in  FIG. 1  for the picture input port  105 , compressed stream output port  106 , and compressed stream input port  107 . The components not shown in  FIG. 1  are the subtractor  301 , transform processor  302 , quantizer  303 , variable length coder  304 , dequantizer  305 , inverse transform processor  306 , adder  307 , reference frame memory  308 , motion compensator  309 , switches  310  and  311 , motion estimator  312 , input frame memory  313 , and variable length decoder (VLC-)  314 . 
     The input frame memory  313  receives the part of the picture to be coded from the input picture partitioner  101  via the picture input port  105 . The input frame memory  313  stores the received part, divides the stored part into macroblocks, and outputs each macroblock to the motion estimator  312  and subtractor  301 . A macroblock is a block measuring sixteen pixels vertically and sixteen pixels horizontally, as specified in the H.264 and MPEG-4 standards. 
     When the subtractor  301  receives each macroblock from the input frame memory  313 , it also receives corresponding predicted picture data from the motion compensator  309 . The subtractor  301  takes the difference between the pixel value of each pixel in the macroblock and the corresponding pixel value in the predicted picture data received from the motion compensator  309 , and outputs the difference to the transform processor  302 . The difference represents the unpredicted component of the pixel value, that is, the difference between the actual pixel value and the predicted value. The data output from the subtractor  301  will be referred to as prediction difference picture data. 
     Upon receiving prediction difference picture data from the subtractor  301 , the transform processor  302  transforms the received data to the spatial frequency domain by using a method such as a discrete cosine transform (DCT). This transform replaces the pixel data with frequency coefficient data. Because nearby pixels are highly correlated in typical pictures, the frequency coefficients with large values tend to cluster in the lower spatial frequencies. The transform processor  302  outputs the transformed coefficients to the quantizer  303 . 
     The quantizer  303  receives the transformed coefficients from the transform processor  302  and quantizes them, thereby compressing the coefficient data by replacing substantially continuous data with data that change in discrete steps. Known quantization methods may be employed. In the first embodiment, the quantized coefficients are represented as multiples of a basic quantization step size, small frequency coefficients becoming zero. The quantizer  303  outputs the quantized transform coefficients to the variable length coder  304  and, through switch  310 , to the dequantizer  305 . 
     The dequantizer  305  receives quantized coefficients from the quantizer  303  or variable length decoder  314  through switch  310  and expands them back to frequency coefficients (e.g., DCT coefficients) by performing a process substantially inverse to the quantization process performed by the quantizer  303 : for example, by multiplying the quantized coefficients by the quantization step size. The dequantizer  305  outputs the dequantized coefficients to the inverse transform processor  306 . 
     Upon receiving the dequantized coefficients from the dequantizer  305 , the inverse transform processor  306  transforms them by performing a transform inverse to the transform performed by the transform processor  302  to obtain decompressed difference picture data in pixel space. The inverse transform processor  306  outputs the decompressed difference picture data to the adder  307 . 
     When the adder  307  receives the decompressed difference picture data from the inverse transform processor  306 , it also receives the corresponding predicted picture data from the motion compensator  309 . The adder  307  adds the value of each pixel in the decompressed difference picture data to the value of the corresponding pixel in the predicted picture data to obtain reference picture data for use in predicting the next picture. The adder  307  outputs the reference picture data to the reference frame memory  308 . 
     The reference frame memory  308  stores the reference picture data received from the adder  307 . 
     When the motion estimator  312  receives a macroblock from the input frame memory  313 , it uses the macroblock and the reference picture data stored in the reference frame memory  308  to search for a motion vector. The motion estimator  312  may use various known search methods: for example, it may search for a sixteen-by-sixteen block of pixels in the reference data such that the mean difference between corresponding pixel values in the received macroblock and the sixteen-by-sixteen reference data block satisfies a predetermined smallness criterion. The motion estimator  312  outputs the spatial coordinates of the sixteen-by-sixteen reference block relative to the received macroblock as motion vector information. The motion vector information is passed to the variable length coder  304  and, through switch  311 , to the motion compensator  309 . 
     When the motion compensator  309  receives motion vector information from switch  311 , it uses the motion vector information to retrieve reference picture data from the reference frame memory  308  and thereby creates predicted picture data. The motion vector information may originate in the motion estimator  312  in either the same or a different coding unit. Motion vectors from a different coding unit are received at the compressed stream input port  107 , decoded by the variable length decoder  314 , and supplied to the motion compensator  309  through switch  311 . The motion compensator  309  outputs the predicted picture data to the subtractor  301 . 
     The variable length coder  304  receives the quantized coefficients from the quantizer  303  and the motion vector information from the motion estimator  312 , and assigns variable-length codes thereto. The variable length coder  304  may employ any of various known variable-length coding methods, such as Huffman coding. The variable length coder  304  outputs compressively coded picture data through the compressed stream output port  106 . 
     The variable length decoder  314  receives the compressively coded data stream from another coding unit and decodes it to obtain compressed coefficient information (quantized coefficients) and motion vector information. The variable length decoder  314  outputs the coefficient information (the quantized coefficients) to the dequantizer  305  through switch  310  and the motion vector information to the motion compensator  309  through switch  311 . 
     Switch  310  selects either the input from the quantizer  303  or the input from the variable length decoder  314 , and outputs the selected input to the dequantizer  305 . 
     Switch  311  selects either the input from the motion estimator  312  or the input from the variable length decoder  314 , and outputs the selected input to the motion compensator  309 . 
     The compressed stream input port  107  receives the compressively coded data stream from other coding units (in the first embodiment there is only one other coding unit), and outputs the received compressively coded data stream to the variable length decoder  314 . The compressed stream input port  107  also supplies storage location switching control information  315  to the reference frame memory  308  to control the switching of storage locations in the reference frame memory  308 . 
     Next, the coding operation of the moving picture coding apparatus  100  in the first embodiment will be described with reference to  FIGS. 1 ,  3 ,  4 , and  5 . 
     First, the general coding process carried out by the moving picture coding apparatus  100  will be described with reference to  FIG. 1 . 
     The input picture partitioner  101  receives an input picture, divides it into parts, sends one part to coding unit  102 , and sends the other part to coding unit  103 . Each part is accompanied (if necessary) by information specifying the location of the part. In the following description, each part is made up of a number of uniform horizontal slices.  FIG. 4  illustrates the division of a picture into two substantially equal parts P 1  and P 2 . 
     The coding units  102  and  103  receive their respective parts of the picture, compressively code their parts, and output respective compressively coded data streams to each other and to the compressed stream combiner  104 . More specifically, coding unit  102  compressively codes its part and outputs a compressively coded data stream from its compressed stream output port  106  to the compressed stream combiner  104  and to the compressed stream input port  107  of coding unit  103 . Similarly, coding unit  103  compressively codes its part and outputs a compressively coded data stream from its compressed stream output port  106  to the compressed stream combiner  104  and to the compressed stream input port  107  of coding unit  102 . 
     The coding units  102 ,  103  use the compressively coded picture data they receive from each other as reference data in the compressive coding process, as described below. More precisely, the coding units  102  and  103  decode and decompress the received compressively coded data streams, and use the decompressed picture data as reference picture data. 
     The compressed stream combiner  104  combines the compressively coded data streams output by the coding units  102 ,  103  into a single compressively coded data stream. 
     Next, the operation of the coding units  102  and  103  will be described by describing the operation of the representative coding unit  300  in  FIG. 3 . First, the basic operation will be described; then particular features characterizing the first embodiment will be described. 
     The picture input port  105  receives one of the parts into which the input picture partitioner  101  has divided the picture and outputs the received part to the input frame memory  313 . 
     The input frame memory  313  stores the received part of the picture, and divides each slice of the part into macroblocks. The input frame memory  313  outputs the stored macroblocks one by one. Upon completing the output all of the macroblocks in the stored part, the input frame memory  313  begins receiving its part of the next picture. 
       FIG. 5  illustrates the division into macroblocks. P n  indicates the part of the picture stored in the input frame memory  313 . P MB  (the area indicated by hatching) indicates a macroblock, measuring sixteen pixels vertically and horizontally. Each macroblock is output to the motion estimator  312  and the subtractor  301 . 
     Upon receiving each macroblock from the input frame memory  313 , the subtractor  301  obtains prediction difference picture data by taking differences between pixel values in the macroblock and pixel values in predicted picture data supplied from the motion compensator  309 , and outputs the prediction difference picture data to the transform processor  302 . The prediction difference picture data represent differences from a more or less similar block, specified by a motion vector, in the preceding frame. 
     Upon receiving the prediction difference picture data from the subtractor  301 , the transform processor  302  transforms the prediction difference picture data to the spatial frequency domain by a method such as the discrete cosine transform, generating coefficient data which are output to the quantizer  303 . 
     The quantizer  303  quantizes the transformed coefficients and outputs the quantized coefficients, represented as multiples of the basic quantization step size, to the variable length coder  304  and switch  310 . Switch  310  selects the input from the quantizer  303  and outputs the quantized coefficients to the dequantizer  305 . 
     The dequantizer  305  expands the quantized coefficients back to frequency coefficients (e.g., DCT coefficients) by, for example, multiplying them by the quantization step size, thereby performing a process substantially inverse to the quantization process performed by the quantizer  303 . The quantizer  303  outputs the dequantized coefficients to the inverse transform processor  306 . 
     The inverse transform processor  306  transforms the dequantized coefficients by performing a transform inverse to the transform performed by the transform processor  302  to obtain decompressed difference picture data in pixel space. The inverse transform processor  306  outputs the decompressed difference picture data to the adder  307 . 
     In synchronization with the output of the decompressed difference picture data, the motion compensator  309  supplies corresponding predicted picture data to the adder  307 . The adder  307  adds the value of each pixel in the decompressed difference picture data to the value of the corresponding pixel in the predicted picture data to obtain picture data for use as reference data for predicting the next picture. The reference frame memory  308  stores the reference picture data received from the adder  307 . 
     As noted above, the motion estimator  312  also receives each macroblock output from the input frame memory  313 . The motion estimator  312  compares each received macroblock with the reference picture data stored in the reference frame memory  308 , finds the position of a block of reference pixels having values close to the values of corresponding pixels in the received macroblock, and outputs the position as motion vector information to the variable length coder  304  and switch  311 . Switch  311  selects the input from the motion estimator  312  and outputs the motion vector information to the motion compensator  309 . 
     Upon receiving motion vector information from switch  311 , the motion compensator  309  uses the motion vector information and the reference picture data stored in the reference frame memory  308  to create predicted picture data, and outputs the predicted picture data to the subtractor  301  and adder  307 . 
     The variable length coder  304  assigns variable-length codes to the quantized coefficients received from the quantizer  303  and the motion vector information received from the motion estimator  312 , and outputs the resulting compressively coded data stream through the compressed stream output port  106 . 
     The general operation of coding a moving picture carried out by the coding unit  300  described above conforms to the MPEG-4 and H.264 standards. 
     Characterizing features of the operation of the coding unit  300  in the first embodiment will now be described below with reference to  FIGS. 1 ,  3 , and  6 . More specifically, the compressive coding process, in which coding unit  102  and coding unit  103  decode and decompress each other&#39;s compressively coded data streams and store the decompressed picture data as reference picture data, will be described. 
     When the compressed stream input port  107  receives a stream of compressively coded data, switch  310  selects the input from the variable length decoder  314  and outputs the selected input to the dequantizer  305 , while switch  311  selects the input from the variable length decoder  314  and outputs the selected input to the motion compensator  309 . 
     The reference frame memory  308  receives storage location switching control information  315 , which includes identifying information (ID) identifying the location of the decompressed picture data, and controls the switching of storage locations in the reference frame memory  308  accordingly. The switching of storage locations in the reference frame memory  308  will be described below. 
     The variable length decoder  314  receives the compressively coded data stream from the other coding unit and decodes it to obtain the original quantized coefficients and motion vector information. 
     The variable length decoder  314  outputs the original quantized coefficients to the dequantizer  305  through switch  310 . The dequantizer  305  expands the quantized coefficients back to frequency (e.g., DCT) coefficients, and outputs the dequantized coefficients to the inverse transform processor  306 . The inverse transform processor  306  transforms the dequantized coefficients received from the dequantizer  305  to obtain decompressed difference picture data in pixel space, and outputs the decompressed difference picture data to the adder  307 . 
     The variable length decoder  314  outputs the original motion vector information to the motion compensator  309  through switch  311 . The motion compensator  309  uses the motion vector information and the reference picture data stored in the reference frame memory  308  to create predicted picture data, and outputs the predicted picture data to the adder  307 . 
     The adder  307  adds the value of each pixel in the decompressed difference picture data obtained from the inverse transform processor  306  to the value of the corresponding pixel in the predicted picture data obtained from the motion compensator  309  to generate picture data for use as reference picture data for predicting the next picture, and outputs the new reference picture data to be stored in the reference frame memory  308 . 
     The storage location of the new reference picture data in the reference frame memory  308  is determined according to the identifying information (ID) included in the storage location switching control information  315 . The switching of storage locations of reference picture data in the reference frame memory  308  will be described with reference to  FIG. 6 . 
     When the coding unit  300  (i.e., coding unit  102  or  103 ) generates a compressively coded data stream, it also generates identifying information (ID) identifying the location of the part of the input picture encoded in the compressively coded data stream, and outputs the identifying information together with the compressively coded data stream. 
     When the coding unit  300  receives a compressively coded data stream, including the above identifying information, through its compressed stream input port  107 , the identifying information (ID) is extracted in the compressed stream input port  107  and placed in the storage location switching control information  315  sent to the reference frame memory  308 . In this way the coding unit  300  learns the location of the reference picture data decompressed from the received compressively coded data stream in relation to the input picture as a whole. The storage location switching control information  315  may also include information indicating the coding unit from which the compressively coded data stream was received. 
     In  FIG. 6 , the decompressed picture data obtained from the compressed data that the coding unit  300  has compressed itself (first decompressed data) are stored in the location indicated by hatching. A switching processor  401  in the frame memory  308  selects this storage location and stores the decompressed picture data obtained from the adder  307  in it. When the coding unit  300  receives a compressively coded data stream and storage location switching control information  315  from the other coding unit via the compressed stream input port  107 , the switching processor  401  selects a storage location (adjacent to the hatched storage location in  FIG. 6 ) assigned to store data originating in the other coding unit, and stores the picture data (second decompressed data) from the adder  307  in the selected location. 
     The switching processor  401  determines the storage location of picture data in the reference frame memory  308  according to the identifying information (ID) included in the storage location switching control information  315  received from the other coding unit. This enables the reference data from both coding units to be stored so as to form a continuous reference picture in the reference frame memory  308 . Both coding units  102  and  103  can therefore use the entire preceding frame as reference data in the compressive coding process. 
     The switching processor  401  may perform further switching functions within each of the two memory areas shown in  FIG. 6 , enabling each memory area to hold both decompressed picture data for the current frame and decompressed picture for the preceding frame simultaneously. A well-known double buffering scheme may be used, for example, to assure that regardless of the order in which the data are decompressed, all necessary data from the preceding frame will still be available for reference. 
     If the length of motion vectors is limited, storage schemes that permit partial overwriting of data for the preceding frame may also be employed, depending on the decompression sequence. 
     As described above, according to the first embodiment, since each coding unit receives the compressively coded picture data output from the other coding unit, in searching for motion vectors, it can search not only in its own assigned part of the preceding frame but also in the part assigned to the other coding unit. This feature of the invention prevents degradation of the picture at the boundaries between the parts. 
     According to the first embodiment, coded data are passed from one coding unit to another by having each coding unit read the coded data stream the other coding unit sends to the combiner. This form of data exchange requires no extra communication bandwidth, no new coding methods or facilities, no new decompression methods or facilities, and no new protocol analyzer, and is therefore inexpensive in terms of both product hardware and product development effort. 
     Second Embodiment 
     Referring to  FIG. 7 , the moving picture coding apparatus  600  in the second embodiment comprises an input picture partitioner  601 , coding units  102 ,  103 , and  602  and a compressed stream combiner  603 . 
     The differences between the first embodiment and the second embodiment are that the input picture partitioner  601  divides the input picture into three parts, the moving picture coding apparatus  600  comprises three coding units  102 ,  103 ,  602 , and the compressed stream combiner  603  combines three coded parts received from these three coding units. 
     Structures and operations related to these differing features of the second embodiment will be described below. Descriptions of the structure and operation of constituent elements also found in the first embodiment will be omitted. 
     The input picture partitioner  601  has a single picture data input port and three picture data output ports. Like the input picture partitioner  101  in the first embodiment, the input picture partitioner  601  has the function of dividing the received input picture, but it differs from the input picture partitioner  101  by dividing the input picture into three parts. The input picture partitioner  601  outputs one part to the coding unit  102 , outputs another part to the coding unit  103 , and outputs the remaining part to the coding unit  602 . 
       FIG. 8  illustrates the division of a picture into three substantially equal parts P 1 , P 2 , and P 3 . As in the first embodiment, when the picture is divided into parts, each coding unit may receive identifying information identifying the location of its part of the picture. 
     Coding unit  102 , coding unit  103 , and coding unit  602  decode and decompress each other&#39;s compressively coded data streams, and use the decompressed picture data as reference picture data in the compressive coding process. 
     The compressed stream output port  106  of coding unit  102  is connected to the compressed stream input port  107  of coding unit  103  and the compressed stream input port  107  of coding unit  602 , the compressed stream output port  106  of coding unit  103  is connected to the compressed stream input port  107  of coding unit  102  and the compressed stream input port  107  of coding unit  602 , and the compressed stream output port  106  of coding unit  602  is connected to the compressed stream input port  107  of coding unit  102  and the compressed stream input port  107  of coding unit  103 . Each of the coding units  102 ,  103 ,  602  accordingly passes its compressively coded data stream to both of the other two coding units. 
     The compressed stream output port  106  of coding unit  102 , the compressed stream output port  106  of coding unit  103 , and the compressed stream output port  106  of coding unit  602  are also connected to the compressed stream combiner  603 . Operating like the compressed stream combiner  104  in  FIG. 1 , the compressed stream combiner  603  combines the coded parts from the coding units  102 ,  103 ,  602  and outputs a compressively coded data stream. 
     Because the basic internal structure of each of coding units  102 ,  103 ,  602  is the same as shown in  FIG. 3 , the features of the second embodiment will be described with reference to  FIG. 3 . 
     The coding unit  300  in the second embodiment includes a different reference frame memory  308  from the first embodiment. More specifically, the switching of storage positions in the reference frame memory  308  differs from the first embodiment, as illustrated in  FIG. 9 . 
     In  FIG. 9 , the coding unit  300  is assumed to code the middle part of the input picture. The coding unit  300  stores the reference picture data that it has coded and decompressed itself (first decompressed data) in the middle one of the three blocks in the reference frame memory  308 . The upper and lower blocks in the reference frame memory  308  are used to store the reference picture data derived from the compressively coded data streams received from the other coding units (second decompressed data and third decompressed data). 
     Decompressed picture data are received from the adder  307  and stored by a switching processor  801  in the reference frame memory  308 . When the switching processor  801  stores decompressed picture data that the coding unit  300  has coded itself, the switching processor  801  selects a location in the reference frame memory  308  corresponding to the position of the part of the input picture assigned to the coding unit  300 . 
     As in the first embodiment, when the switching processor  801  receives the reference picture data from another coding unit, the switching processor  801  identifies the location assigned to the other coding unit according to identifying information included in the storage location switching control information  315  from the another coding unit: if the upper adjacent location is assigned to the another coding unit, the switching processor  801  selects the upper adjacent location to store the decompressed picture data from the adder  307 ; if the lower adjacent location is assigned to the another coding unit, the switching processor  801  selects the lower adjacent location to store the decompressed picture data from the adder  307 . 
     The switching processor  801  may perform further switching functions within each of the three memory areas shown in  FIG. 9 , enabling each memory area to store both decompressed picture data for the current frame and decompressed picture for the preceding frame simultaneously, as discussed in the first embodiment. 
     The second embodiment has the same effects as described in the first embodiment regarding efficient communication between coding units and reduced costs. 
     In addition, because the number of coding units is increased from two to three, if the performance of a single coding unit is the same as in the first embodiment, the second embodiment enables higher-definition moving pictures to be coded in real time. 
     The invention is not limited to the two coding units shown in the first embodiment or the three coding units shown in the second embodiment. There may be four or more coding units. 
     When there are more than two coding units, it is not always necessary for each coding unit to receive the compressively coded data streams from all other coding units. Most coding schemes periodically disable the motion estimator  312 , motion compensator  309 , and subtractor  301  in  FIG. 3  to generate intra-coded frames or macroblocks that can be decompressed without using the reference frame memory. If intra-coded frames or macroblocks are inserted at sufficiently frequent intervals and if the lengths of motion vectors are limited, it may be sufficient for each coding unit to receive only the compressively coded data streams from the coding units that code adjacent parts of the input picture. 
     In the second embodiment, for example, if the input picture is always divided as shown in  FIG. 8  with coding unit  102  coding part P 1 , coding unit  103  coding part P 2 , and coding unit  602  coding part P 3 , then coding units  102  and  602  may only have to receive the compressively coded data stream output from coding unit  103 . 
     The same applies if a coding scheme is adopted in which only intra-coded frames are stored in the reference frame memory. 
     If a coding unit stores only reference picture data for its own assigned part and adjacent parts of the input picture, it may not be possible to decompress all macroblocks in the adjacent parts, but this is permissible as long as the coding unit can always decompress enough macroblocks to generate the reference data it needs for compressing its own assigned part, including at least the macroblocks immediately adjacent to its own assigned part. 
     The invention is also applicable to coding schemes that employ bidirectional prediction, in which case the reference frame memory  308  stores reference picture data for two frames, or parts thereof, for use in coding frames located temporally between the two stored frames. 
     Although the method of dividing the input picture into parts is not specified in the first and second embodiments, various methods are available. For example, if the performance of coding units differs greatly, larger parts of the input picture may be assigned to coding units with higher performance, so that all parts of the input picture take substantially equally long to code. 
     Dynamic methods of dividing a picture into parts may also be used, so that the result may vary from one input picture to another. The position of the part in the input picture represented by the compressively coded stream data output by a particular coding unit may accordingly vary with every input picture. In this case the input picture partitioner  101  must attach location information to each output part of the input picture, and each coding unit must incorporate the location information into the identifying information (ID) it outputs in the storage location switching control information  315 . The location information and ID may be, for example, a starting macroblock number. When the parts are made up of uniform horizontal slices, the location information and ID may be starting slice IDs that have a predetermined correspondence to macroblock numbers. Additional information such as the position of the last macroblock or slice in the part, or the number of macroblocks or slices in the part, may also be used. 
     The present invention is applicable to any system including a plurality of computing resources that code different parts of a picture concurrently. The computing resources may be different computers, different integrated circuit chips, or different parts of a single integrated circuit chip. 
     The present invention can also be practiced in picture-coding software that runs on a computing device or system of any of the types described above. 
     Those skilled in the art will recognize that further variations are possible within the scope of the invention, which is defined in the appended claims.