Abstract:
The method is for coding and for decoding a video data stream, only one picture which is required for the reconstruction of interpolated pictures being stored in completely decompressed form during the coding or during the decoding of the video data stream. That part of a second basic picture (G 2 ) which is required in decompressed form for the construction or reconstruction of an interpolated picture is temporarily decompressed in each case. A further possibility envisages storing a first basic picture (G 1 ) and the second basic picture (G 2 ) in compressed form and in each case temporarily decompressing only those regions which are required for the construction or reconstruction of an interpolated picture.

Description:
BACKGROUND OF THE INVENTION 
     Method for decoding and coding a compressed video data stream with a reduced memory requirement 
     The present standards for compressing video data, MPEG 1  and MPEG 2 , operate in accordance with the principles of prediction and transformation. A distinction is made between three differently coded types of pictures. So-called I pictures (intra-coded pictures) are only transform-coded. The transform used is the discrete cosine transform (DCT). P pictures are predicted from the respectively preceding I picture or P picture. The difference between the predicted picture and the actual picture is DCT-transformed. The third type of pictures are the so-called B pictures, which are bidirectionally predicted from the respectively preceding I picture or P picture and from the respectively succeeding I picture or P picture. The difference between the predicted picture and the actual picture is again DCT-transformed. 
     An MPEG decoder must therefore make available the data of two pictures (I picture and P picture) for the reconstruction of the B pictures. Accordingly, the memory requirement for pictures is high. 
     The fundamental mode of operation of the compression methods MPEG 1  and MPEG 2  has been disclosed to the person skilled in the art by various publications (D. J. Le Gall, The MPEG Video Compression Algorithm, Signal Processing: Image Communication 4, pp. 129-140, 1992; International Standard ISO/IEC 11172-2: Coding of Moving Pictures and Associated Audio, ISO/MPEG, 1993 and Draft International Standard ISO/IEC 13818-2, Generic Coding of Moving Pictures and Associated Audio, 25.3.1994, 1994). 
     In order to improve the quality of the prediction in those picture areas in which moving objects occur, use is made of so-called motion-compensated prediction. The principles of the motion estimation required for this purpose and their application for the motion-compensated prediction have been disclosed to the person skilled in the art, for example by (M. Bierling, Displacement estimation by hierarchical block matching, 3rd SPIE Symp. on Visual Communications, Cambridge, Mass., November 1988, 1988) and (Draft International Standard ISO/ICE 13818-2, Generic Coding of Moving Pictures and Associated Audio, 25.3.1994, 1994). 
     A distinction is made between three differently coded types of pictures. So-called I pictures are transmitted without any chronological prediction, but rather are subjected only to intra-picture coding, preferably DCT coding with subsequent quantization of the coding transform coefficients. In the context of this patent application, “intra-picture coding” is to be understood quite generally as any method which is suitable for treating local correlations in video data. The so-called P pictures are predicted with the aid of the DPCM loop from chronologically preceding I pictures or P pictures (forward prediction). The difference between the predicted picture and the actual picture is subjected to intra-picture coding, preferably to transformation using a DCT with subsequent quantization of the coding transform coefficients. So-called B pictures, which are also designated as interpolated pictures in the context of the present patent application, are chronologically situated between an I picture and a P picture or between two P pictures. B pictures are determined by means of (bidirectional) motion-compensated interpolation from a chronologically preceding I picture or P picture and from a chronologically succeeding I picture or P picture. In this case, the expressions (chronologically) “succeeding” and “preceding” do not refer to the order in which these pictures are transmitted in the video data stream of the compressed pictures, but rather they refer to the order in which these pictures are recorded/reproduced. In the same way as P pictures, B pictures, too, are coded in the form of quantized coding transform coefficients of a difference picture. 
     In the case of currently known implementations, the reconstruction of a B picture by means of motion-compensated interpolation from a chronologically preceding I picture or P picture and from a chronologically succeeding I picture or P picture necessitates the provision of the two reference pictures (which are also occasionally designated as support pictures in the literature) in fully decoded form. 
     Therefore, two fully decoded reference pictures (I pictures or P pictures) have to be stored in a frame store in the case of the methods belonging to the prior art for carrying out motion-compensated interpolation. 
     The re-interlacing during the video output requires further storage capacity. The overall required memory is a decisive cost factor in the hardware used for decoding and encoding. A reduction in the storage capacity required is therefore desirable. 
     SUMMARY OF THE INVENTION 
     The invention is based on the problem of specifying a method for decoding compressed video data with a reduced memory requirement. 
     In general terms the present invention is a method for the iterative decoding of compressed video data streams which have a sequence of basic pictures and interpolated pictures. A first basic picture is stored in a reference picture memory. A second basic picture is stored in compressed form in a buffer memory. The following steps are provided for a reconstruction of at least a first interpolated picture, which steps are executed iteratively in each case for the reconstruction of an interpolated picture: only a part of the second basic picture which is required for the reconstruction of a part of the first interpolated picture is decompressed in each case, the decompressed part of the second basic picture is stored in a second search area memory during a time in which the part of the first interpolated picture is reconstructed, and the part of the interpolated picture is reconstructed using the first basic picture and the decompressed part of the second basic picture. 
     Advantageous developments of the present invention are as follows. 
     A no longer required part of the decompressed part of the second basic picture is overwritten in the second search area memory by a following decompressed part, of the second basic picture, which is required for the reconstruction of a following part of the first interpolated picture. 
     An end part, which is no longer required for the reconstruction of a last part of a preceding interpolated picture, of the decompressed part of the second basic picture is overwritten in the second search area memory by a beginning part of the decompressed part, of the second basic picture, which is required for the reconstruction of a first part of a subsequent interpolated picture. 
     The first basic picture is stored in completely decompressed form in the reference picture memory. 
     In the event of a further basic picture being received, the second basic picture is completely decompressed. The completely decompressed second basic picture is stored in the reference picture memory. The further basic picture is stored in compressed form in the buffer memory. 
     One physical memory is used to realize the reference picture memory and the second search area memory. 
     The first basic picture is stored in compressed form in the reference picture memory. The following additional steps are provided for the reconstruction of the at least first interpolated picture, which steps are executed iteratively in each case for the reconstruction of an interpolated picture: only a part of the first basic picture which is required for the reconstruction of a part of the first interpolated picture is decompressed in each case, the decompressed part of the first basic picture is stored in a first search area memory during a time in which the part of the first interpolated picture is reconstructed, the part of the first interpolated picture is reconstructed using the decompressed part of the first basic picture and the decompressed part of the second basic picture, the decompressed part of the first basic picture is compressed, the compressed part of the first basic picture is stored in the reference picture memory, the decompressed part of the second basic picture is compressed, and the compressed part of the second basic picture is stored in the buffer memory. 
     A no longer required part of the decompressed part of the first basic picture is overwritten in the first search area memory by a following decompressed part, of the first basic picture, which is required for the reconstruction of a following part of the first interpolated picture. 
     An end part, which is no longer required for the reconstruction of a last part of a preceding interpolated picture, of the decompressed part of the first basic picture is overwritten in the first search area memory by a beginning part of the decompressed part, of the first basic picture, which is required for the reconstruction of a first part of a subsequent interpolated picture. 
     In the event of a further basic picture being received, the first basic picture is completely decompressed. The second basic picture is then stored in compressed form in the reference picture memory, and the further basic picture is stored in compressed form in the buffer memory. 
     The present invention is also a method for the iterative coding of compressed video data streams which have a sequence of basic pictures and interpolated pictures. A first basic picture is stored in a reference picture memory. An uncompressed second basic picture is compressed. The compressed second basic picture is stored in an output buffer. The following steps are provided for a construction of at least a first interpolated picture, which steps are executed iteratively in each case for the construction of an interpolated picture: only a part of the compressed second basic picture which is required for the construction of a part of the first interpolated picture is decompressed in each case, the decompressed part of the second basic picture is store din a second search area memory during a time in which the part of the first interpolated picture is constructed, and the part of the first interpolated picture is constructed using the first basic picture and the decompressed part of the second basic picture. 
     Advantageous developments of this embodiment of the present invention are as follows. 
     A no longer required part of the decompressed part of the second basic picture is overwritten by a following decompressed part, of the second basic picture, which is required for the construction of a following part of the first interpolated picture. 
     An end part, which is no longer required for the construction of a last part of a preceding interpolated picture, of the decompressed part of the second basic picture is overwritten in the second search area memory by a beginning part, of the second basic picture, which is required for the construction of a first part of a subsequent interpolated picture. 
     The first basic picture is stored in uncompressed form in the reference picture memory. When the first basic picture is transmitted to a receiver unit, the second basic picture is completely decompressed. The decompressed second basic picture is stored in the reference picture memory, and a further basic picture is stored in compressed form in the output buffer. 
     The first basic picture is compressed. The compressed first basic picture is stored in the reference picture memory. The following additional steps are provided for the construction of the at least first interpolated picture, which steps are executed iteratively in each case for the construction of an interpolated picture: only a part of the compressed first basic picture which is required for the construction of a part of the first interpolated picture is decompressed in each case, the decompressed part of the first basic picture is stored in a first search area memory during a time in which the part of the first interpolated picture is constructed, and the part of the interpolated picture is constructed using the uncompressed first basic picture and the decompressed part of t he second basic picture. 
     A no longer required part of the decompressed part of the first basic picture is overwritten by a following decompressed part, of the first basic picture, which is required for the construction of a following part of the first interpolated picture. 
     An end part, which is no longer required for the construction of a last part of a preceding interpolated picture, of the decompressed part of the first basic picture is overwritten in the first search area memory by a beginning part, of the first basic picture, which is required for the construction of a first part of a subsequent interpolated picture. 
     When the first basic picture is transmitted to a receiver unit, a further basic picture is stored in compressed form in the first output buffer. 
     The invention is in no way restricted to the area of transform coding, and is in no way whatsoever restricted to the area of block-by-block DCT coding of DCT decoding. Since no preconditions have to be made concerning. the type of intra-picture coding, the invention can be applied in connection with virtually all known methods, or methods to be developed in future, of intra-picture coding, for example also in connection with so-called quadtree coding or in connection with methods based on object segments. The decisive prerequisite for the applicability of the invention is the (bidirectional) motion-compensated interpolation. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The features of the present invention which are believed to be novel, are set forth with particularity in the appended claims. The invention, together with further objects and advantages, may best be understood by reference to the following description taken in conjunction with the accompanying drawings, in the several Figures of which like reference numerals identify like elements, and in which: 
     FIG. 1 shows a block diagram which describes the method of the present invention; 
     FIG. 2 shows a sketch illustrating those decoded areas of the reference pictures which are required for the reconstruction of a macroblock line of a B picture; 
     FIG. 3 shows a block diagram outlining the method of a further embodiment of the present invention. 
     FIGS. 4 a ,  4   b  show a sketch outlining the decoded areas required for the reconstruction of a macroblock line of a B picture when using the method depicted in FIG.  3 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The invention will be explained further with reference to FIGS. 1 to  4   a, b.    
     In the further context of this patent application, both the I pictures and the P pictures are referred to as basic pictures G. The B pictures are referred to as interpolated pictures B. 
     FIG. 1 outlines the method in the form of a block diagram. 
     A coded data stream, which is composed of a sequence of basic pictures G and interpolated pictures B in coded form, is first of all loaded into an input buffer EP. When using the MPEG 2  method, the input buffer EP has, for example, a minimum size of 1.75 Mbits, where one Mbit is 1024 * 1024 bits, that is to say 1048576 bits. 
     The data are read from the input buffer EP at a variable rate controlled by the decoder and are fed to a header evaluation arrangement HA. The header evaluation arrangement HA determines what type of picture the received picture is. 
     A first received basic picture G 1  is decoded in a first decoding path DP 0  and is stored in a reference picture memory RS. 
     The memory requirement is reduced if a second basic picture G 2  is stored in compressed form in a buffer memory PS. Since, however, at least that part of the second basic picture G 2  which is necessary for reconstruction is required in uncompressed form for the reconstruction of an interpolated picture B, it is necessary in each case, as described below, to decompress a part of the second basic picture G 2  stored in compressed form. To ensure that this component of the decompressed data of the second basic picture G 2  is kept small, use is made of the fact that upper limits for the search area are prescribed in the MPEG 1  and MPEG 2  standards for video coding and video decoding. The search area refers to that area of the second basic picture G 2  which is required together with the first basic picture G 1  to reconstruct a macroblock line of the interpolated picture B (cf. FIG.  2 ). 
     The idea is that the part of the second basic picture G 2  which is decoded in a second decoding path DP 1  and is stored in decompressed form (DG 2 )in a second search area memory SB 2  is always only that part of the said second basic picture G 2  to which motion vectors of exactly that part of the interpolated picture B which is to be reconstructed can refer. The method can also be used for coding, which is explained below. 
     This means that, for example using a motion estimation BM of the first basic picture G 1 , the corresponding part of the second basic picture G 2  is decompressed and is stored in the second search area memory SB 2 . 
     The first basic picture G 1  is therefore stored in fully decompressed form in a reference picture memory RS. In a further embodiment, it is also possible, as described below, to store the first basic picture G 1  in compressed form. 
     The second basic picture G 2  is stored in compressed form in the buffer memory PS. The buffer memory PS must be designed for the “worst case”, that is to say its storage capacity must correspond at least to the standardized “video buffering verifier” buffer (VBV buffer). In the case of MPEG 2  for digital high-definition television (HDTV), the size of the VBV buffer is approximately 9.4 Mbits (1 Mbit=1024×1024 bits). 
     It is now necessary to ensure the availability in each case of the search area, required at a specific point in time, from the second basic picture G 2  in decompressed form DG 2 . For the decoding of a macroblock line of a first interpolated picture B 1 , two a macroblock lines of the second basic picture G 2  in decompressed form DG 2  must be present in each case for this purpose. A macroblock line consists of horizontally consecutive macroblocks, and two a macroblock lines form the total search area required (cf. FIG.  2 ). 
     The search area must in each case be shifted in a macroblock line-by-macroblock line manner with the B picture macroblock lines to be decoded. The decoding of the search area must therefore take place at the same rate as the decoding of the first interpolated picture B 1 . That part DG 2  of the second basic picture G 2  which is decompressed in each case in the second decoding path DP 1  is stored in the second search area memory SB 2  in each case at least during the time in which that part of the first interpolated picture B 1  for which the respectively decompressed part DG 2  of the second basic picture G 2  is required is reconstructed. 
     Using the first basic picture G 1  stored in the reference picture memory RS, a part DG 2  of the second basic picture G 2  is decompressed, which part is stored in the second search area memory SB 2 . 
     The first basic picture G 1  is output at the same time. The decoding and output of the first interpolated picture B 1  via the first decoding path DP 0  begins immediately after the output of the first basic picture G 1 . 
     Decoding takes place in the manner described above. That search area of the second basic picture G 2  which is required for the decoding of the first macroblock line is present in the second search area memory SB 2  after it has been decompressed via the second decoding path DP 1  and stored in the second search area memory SB 2 . 
     During the decoding of the interpolated picture B 1 , the contents of the second search area memory SB 2  must be constantly updated in a macroblock line-by-macroblock line manner via the second decoding path DP 1 . When updating the second search area memory SB 2 , which contains the search area from the second basic picture G 2 , access is made for this purpose to the reference picture memory RS in which the first basic picture G 1  is stored. 
     In order to save further memory space and reduce the required computing power, the first a macroblock lines of the second basic picture G 2  in decompressed form DG 2  can be written to the second search area memory SB 2  during the decoding of the last a macroblock lines of the interpolated picture B 1 . These first a macroblock lines of the second basic picture G 2  in decompressed form DG 2  are required if a second interpolated picture is to be reconstructed from the first basic picture G 1  and from the second basic picture G 2 . As a result, the search areas are again available for decoding the next interpolated picture. 
     The decoding of subsequent interpolated pictures B 1 , where i designates an index for any desired number of interpolated pictures, which are reconstructed on the basis of the first basic picture G 1  and of the second basic picture G 2 , can be carried out cyclically by the method described above. 
     As described above, therefore, a no longer required part of the decompressed part DG 2  of the second basic picture G 2  can in each case be overwritten in the second search area memory SB 2  by a following decompressed part, of the second basic picture G 2 , which is required for the reconstruction of a following part, that is to say a following macroblock line, of the interpolated picture B 1 . As described above, a last part of an end part, which is no longer required for the reconstruction of a preceding interpolated picture, of the decompressed part DG 2  of the second basic picture G 2  can also be overwritten in the second search area memory SB 2  by a beginning part of the decompressed part DG 2 , of the second basic picture G 2 , which is required for the reconstruction of a first part of a subsequent interpolated picture B 2 . 
     If a further basic picture G 3  is received, then the second basic picture G 2  is completely decompressed and stored in the reference picture memory RS. Since the first basic picture G 1  is required for the complete decompression of the second basic picture G 2 , the said first basic picture G 1  must likewise remain stored in the reference picture memory RS until the second basic picture G 2  has been decompressed. 
     However, in order to reduce further the required memory space, it is possible successively to overwrite that part of the first basic picture G 1  which is no longer required for the decompression of the second basic picture G 2  by already decompressed parts of the second basic picture G 2 . 
     The further basic picture G 3  is stored in the buffer memory PS. A new “cycle” of the method described above can thus begin. 
     In all of the methods described above, steps for transmitting data can be omitted, and the required data movement can thus be reduced, if the reference picture memory RS and the second search area memory SB 2  are realized as one physical memory, since in this case it is not necessary to copy the data from the second search area memory SB 2  to the reference picture memory RS. 
     In order to reduce the required memory space even further it is also possible to store the first basic picture G 1  in compressed form in the reference picture memory RS. In this case, the reference picture memory RS can be dimensioned to be smaller than in the method in which the first basic picture G 1  is stored in fully decompressed form in the reference picture memory RS. 
     In this case, that part of the first basic picture G 1  which is required in each case for the reconstruction of the part of the interpolated picture B 1  is in each case decompressed via a third decoding path DP 2  and stored in a first search area memory SB 1  (cf. FIG.  3 ). Consequently, a respective part of the first interpolated picture B 1  is reconstructed using the respectively decompressed part DG 1  of the first basic picture G 1  which is situated in the first search area memory SB 1  and using the decompressed part DG 2  of the second basic picture G 2  which is situated in the second search area memory SB 2 . 
     A no longer required part of the decompressed part DG 1  of the first basic picture G 1  is compressed again in an intra-coding path ICP. A no longer required part of the decompressed part DG 2  of the second basic picture G 2  is likewise compressed again in the intra-coding path ICP. 
     In this context, “no longer required” means that the part is no longer required for the reconstruction of the current part of the first interpolated picture B 1 . 
     The procedure when reconstructing a plurality of interpolated pictures B 1  on the basis of the first basic picture G 1  and of the second basic picture G 2  corresponds to the procedure described above. 
     The procedure for when a further basic picture G 3  is received has also been described above and is correspondingly applied in connection with the method when both the first basic picture G 1  and the second basic picture G 2  are stored in compressed form. 
     In the intra-coding path ICP, the coding transform which has been used to code, that is to say compress, the entire video data stream is used for the respective part to be processed. 
     FIG. 4 a  illustrates, for the method employing storage of the first basic picture G 1  and the second basic picture G 2  in compressed form, the search area required in each case for the reconstruction of the first interpolated picture B 1 . 
     If the second basic picture G 2  is a P picture, the required search area from the preceding reference picture, that is to say from the first basic picture Gi, must additionally be present in decoded form. Further a macroblock lines plus one macroblock line of the first basic picture G 1  suffice for this purpose, since the search areas for the reconstruction of the interpolated picture B 1  and for the decoding of the second basic picture G 2  overlap (cf. FIG. 4 b ). The search area for the decoding of the second basic picture G 2  overlaps with the search area required for the reconstruction of the interpolated picture B 1  by exactly a macroblock lines. 
     In this case, too, it is possible to reduce the memory requirement further in that, for example, the no longer required part of the decompressed part DG 1  of the first basic picture G 1  is overwritten in the first search area memory SB 1  by a following decompressed part, of the first basic picture G 1 , which is required for the reconstruction of a following part of the first interpolated picture B 1 . Likewise, the no longer required part of the decompressed part DG 2  of the second basic picture G 2  can also be overwritten in the second search area memory SB 2  by a following decompressed part, of the second basic picture G 2 , which is required for the reconstruction of the following part of the first interpolated picture B 1 . 
     Correspondingly, if a plurality of interpolated pictures B 1  are reconstructed using the first basic picture G 1  and the second basic picture G 2 , a respective end part, which is no longer required for the reconstruction of a last part of a preceding interpolated picture, of the decompressed part DG 1  of the first basic picture G 1  can also be overwritten in the first search area memory SB 1  by a beginning part of the decompressed part DG 1 , of the first basic picture G 1 , which is required for the reconstruction of a first part of a subsequent interpolated picture. 
     Likewise, an end part, which is no longer required for the reconstruction of the last part of the preceding interpolated picture, of the decompressed part DG 2  of the second basic picture G 2  can also be over-written in the second search area memory SB 2  by a beginning part of the decompressed part DG 2 , of the second basic picture G 2 , which is required for the reconstruction of a first part of the subsequent interpolated picture. 
     In the event of a further basic picture G 3  being received, the first basic picture G 1  is stored in completely decompressed form and the second basic picture G 2  in compressed form in the reference picture memory RS, and the further basic picture G 3  is stored in compressed form in the buffer memory PS. 
     Since the video data stream is composed of a sequence of basic pictures and interpolated pictures, this method is carried out cyclically for all the pictures of the entire video data stream. 
     The above-described principles for the decoding of compressed video data streams can be correspondingly applied to a method for the iterative coding of compressed video data streams which likewise have a sequence of basic pictures G and interpolated pictures B. 
     During the coding of the video data stream, a first basic picture G 1  is stored in the reference picture memory RS. The second basic picture G 2  is stored in compressed form in an output buffer. In a manner corresponding to the procedure described above, for the construction of a respective interpolated picture B 1 , only a part of the compressed second basic picture G 2  which is required for the construction of the part of the first interpolated picture B 1  is decompressed DG 2  and stored in a second search area memory SB 2  during the time in which the part of the first interpolated picture B 1  is constructed. 
     The respective part, that is to say the macroblock line, of the first interpolated picture B 1  is constructed using the first basic picture G 1  and the decompressed part of the second basic picture G 2  which is situated in the second search area memory SB 2 . Options for reducing the required memory space, for example by overwriting no longer required parts of the decompressed part DG 2  of the second basic picture G 2  in the second search area memory SB 2 , have been described above and can likewise be employed in the coding method. The same applies to the construction of a plurality of interpolated pictures B 1  using the first basic picture G 1  and the second basic picture G 2 . 
     If the first basic picture G 1  is also held in compressed form in the reference picture memory RS, that part DG 1  of the first basic picture G 1  which is required for the construction of the part of the interpolated picture B 1  must also be decompressed in each case. That part of the first basic picture G 1  which is required for the construction of the interpolated picture B 1  is stored in a first search area memory SB 1  during the time in which the interpolated picture B 1  is constructed. Consequently, the part of the first interpolated picture B 1  can be constructed using the decompressed part DG 1  of the first basic picture G 1  and the decompressed part DG 2  of the second basic picture G 2 . 
     As described above, the required search area must be shifted in a macroblock line-by-macroblock line manner with the macroblock lines to be coded of the first interpolated picture B 1  which is constructed. 
     When the first basic picture G 1  is transmitted to a receiver unit, the second basic picture G 2  is stored in the reference picture memory RS, and a further basic picture G 3  is stored in compressed form in the output buffer A. 
     The method described above is carried out cyclically for the entire video data stream, since the entire video data screen also has a plurality of basic pictures and a plurality of interpolated pictures during coding. 
     The invention is not limited to the particular details of the method and apparatus depicted and other modifications and applications are contemplated. Certain other changes may be made in the above described method and apparatus without departing from the true spirit and scope of the invention herein involved. It is intended, therefore, that the subject matter in the above depiction shall be interpreted as illustrative and not in a limiting sense.