Abstract:
The discrete cosine transform of the prediction error is not necessarily calculated, rather the DCT of the original picture to be transmitted and the DCT of the temporally preceding reconstructed picture are calculated separately from one another. By comparing the amplitudes of the coefficients from the original signal with respect to the difference between the coefficients from the original signal and the reconstructed picture, a decision is made for each coefficient as to whether the coefficient of the original signal or the difference between the coefficients is transmitted.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   This application is based on and hereby claims priority to Application No. 100 45 247.7 filed on 13 Sep. 2000 in Germany, the contents of which are hereby incorporated by reference. 
   BACKGROUND OF THE INVENTION 
   The invention relates to methods and apparatuses for coding and decoding a digitized original picture and, respectively, a digitized coded picture. 
   Such methods and such apparatuses are disclosed in J. De Lameillieure, R. Schäfer, MPEG-2-Bildcodierung für das digitale Fernsehen (MPEG-2 picture coding for digital television), Fernseh-und Kino-Technik, Volume 48, No. 3/1994, pp. 99–107, 1994. 
   In the known, standardized MPEG2 method, for the compression of digital moving picture sequences, that is to say for the coding of digitized pictures, coding information is assigned to pixels which are usually grouped into picture blocks or into picture objects. 
   Coding information shall hereinafter be understood to be, for example, brightness information (luminance information) and/or color information (chrominance information), which are assigned to the pixels of a digitized picture. 
   The coding information, which is originally present in the space domain, is, according to MPEG2, subjected to a Discrete Cosine Transform (DCT) and the DCT coefficients generated in this way are quantized and subjected to entropy coding. 
   A reconstruction picture is determined from the quantized DCT coefficients by an inverse quantization and an Inverse Discrete Cosine Transform (IDCT). 
   In this connection, a motion estimation between the original picture and a temporally preceding reconstruction picture is carried out. 
   For the coding, i.e. for the compression of the video data to be transmitted, a changeover is usually made between an inter coding mode and an intra coding mode. 
   In the context of the inter coding mode, the prediction error which has been determined as a result of the motion estimation as a difference value between the coding information of the picture block to be coded and the coding information of the picture block—determined by the motion estimation—of the temporally preceding picture is subjected to a DCT, quantized and fed to an entropy coding. 
   In the context of the intra coding mode, without taking account of coding information of temporally preceding pictures, the coding information of the original picture to be coded is subjected to a DCT and the resulting DCT coefficients are quantized and subjected to entropy coding. 
   Depending on the available data rate, in this case a coarse or a fine quantization of the DCT coefficients is chosen, i.e. set. 
   The coarser the quantization of the DCT coefficients is chosen to be, the more DCT coefficients are quantized to the value “zero” and the lower the data rate required for transmitting the coded original picture. 
   Often, a lower data rate is required for transmitting the prediction error than for transmitting the DCT coefficients—coded in accordance with the intra coding mode—of the original picture to be coded. 
   However, if the motion estimation is inaccurate, which, in particular, can often be observed at the boundaries of moving objects or at block boundaries between two respective picture blocks, the data rate for the prediction error may, however, also be higher than the data rate required for coding the original picture. 
   For this reason, a changeover between the inter coding mode and the intra coding mode is often provided in known coding methods, which changeover is intended to ensure that the signal transmitted is always the one which, overall, requires the lower data rate for its transmission. 
   The signal variance usually serves as a criterion for the decision as to whether a coding is to be carried out in accordance with the inter coding mode or in accordance with the intra coding mode. The said signal variance is calculated from the original picture directly, that is to say in the space domain. As an alternative, it could also be calculated by summation of the DCT coefficients determined, that is to say in the frequency domain. The signal with the lower sum of the DCT coefficients has the lower signal variance. 
   Furthermore, it is known that the low-frequency signal components of a video signal are usually well predicted by the motion-compensating prediction. 
   Accordingly, the corresponding DCT coefficients of low-frequency signal components of the respective original picture to be coded have a smaller amplitude than the DCT coefficients of the original signal. On account of estimation errors in the context of the motion estimation, however, the high-frequency signal components are often predicted only with inadequate quality, with the result that the DCT coefficients of the prediction error, which are hereinafter referred to as prediction coefficients, in this case have a higher amplitude than the DCT coefficients of the original picture. 
   SUMMARY OF THE INVENTION 
   Consequently, one aspect of the invention is based on the problem of coding and decoding a digitized picture which requires a lower transmission rate for its transmission by comparison with known coding methods. 
   In a method for coding a digitized original picture with pixels which are assigned coding information, spectral coefficients of the coding information of the original picture are determined. Furthermore, spectral coefficients of a reconstruction picture of a temporally preceding picture are determined. For at least a portion of the spectral coefficients determined, coefficient differences are determined from spectral coefficients of the original picture and the corresponding spectral coefficients of the reconstruction picture. A prediction coefficient is in each case formed by forming the respective difference between a spectral coefficient of the original picture and the corresponding spectral coefficient of the reconstruction picture. The prediction coefficients determined are compared with corresponding spectral coefficients of the original picture. At least in part, the respective smaller coefficient is selected and coded. 
   The method and apparatus described herein can be understood in the fact that selectively at the abstraction level of the individual spectral coefficients of the original picture to be coded and of a reconstructed picture of a temporally preceding picture, in each case that spectral coefficient is selected and transmitted whose spectral coefficient has a smaller value, that is to say which has a lower spectral power density. 
   Consequently, a DCT of the prediction error is not determined, rather a spectral transform is calculated separately both on the original picture to be coded and on the reconstructed picture of a temporally preceding picture and, on the basis of a comparison of the amplitude of the coefficients of the original signal with respect to the difference between the coefficients from the original signal and the reconstructed picture, for each coefficient a separate decision can be taken as to which is selected, coded and transmitted in the context of the coding of the picture. In this case, the decision that has been made is additionally signaled to the receiver. 
   A considerable reduction of the required data rate for the coding and transmission of a digitized picture in a moving picture sequence is obtained in this way. 
   By way of example, brightness information and/or color information may be assigned to the pixels as coding information. 
   One refinement provides for only the prediction coefficients with the value zero to be selected and coded. 
   If a prediction coefficient has a value not equal to zero, then, in accordance with one development of the invention, the corresponding spectral coefficient, that is to say spectral coefficient situated at the corresponding location within the picture, of the original picture is selected and coded. 
   In accordance with this refinement, a very simple coding that can thus be carried out in a manner requiring very little computation time is achieved, with improved coding efficiency. 
   A further refinement provides for a comparison in the manner described above to be carried out for all the coefficients of the original picture to be coded, and for the respectively suitable coefficient to be selected and coded, a coefficient being suitable if the selection leads to a lower volume of data for the coefficient and the selection information. 
   In order to form spectral coefficients, in accordance with a further refinement, it is possible to use a discrete cosine transform, as a result of which DCT coefficients are formed. 
   In order to enable decoding of the coded picture in a simple manner, for each selection made a selection decision indication, for example in the form of a bit which in each case indicates whether a prediction coefficient or a spectral coefficient of the original picture was selected and coded, is stored and transmitted to a decoder for the decoding thereof. 
   The selection decision indications may be effected, for example, in a selection decision vector of the dimension of the transmitted coefficients. The selection decision vector is coded in a suitable manner and transmitted. 
   In a method for decoding a digitized coded picture with spectral coefficients, prediction coefficients and selection decision indications which in each case indicate whether the respective coefficient is a spectral coefficient of the original picture or of the prediction error picture, spectral coefficients are formed for a temporally preceding reconstruction picture. 
   The coded picture is decoded taking account of the spectral coefficients, prediction coefficients and selection decision indications of the coded picture and the spectral coefficients of the temporally preceding reconstruction picture. 
   Corresponding apparatuses for carrying out the methods described above each have at least one processor which is set up in such a way that the method steps presented above are carried out. 
   The method and apparatus can be realized both by a specific electrical circuit, that is to say in hardware, and by a computer program, that is to say in software. 
   The method and apparatus can generally be used in any block-based and/or object-based method for coding and decoding a digitized picture, in particular a sequence of digitized pictures, that is to say a moving picture sequence, in which use is made of motion-compensating prediction, in particular motion estimation and motion compensation. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other objects and advantages of the present invention will become more apparent and more readily appreciated from the following description of the preferred embodiments, taken in conjunction with the accompanying drawings of which: 
       FIG. 1  shows a block diagram illustrating an apparatus for coding a moving picture sequence in accordance with an exemplary embodiment of the invention; 
       FIG. 2  shows a block diagram illustrating a video communication system with a camera, a coder and a decoder in accordance with an exemplary embodiment of the invention; 
       FIG. 3  shows a block diagram illustrating a decoding apparatus for decoding a coded video data stream in accordance with an exemplary embodiment of the invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Reference will now be made in detail to the preferred embodiments of the present invention, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to like elements throughout. 
     FIG. 2  shows a video communication system  200  with a camera  201 , which records a scene by a sequence  203  of temporally successive pictures. 
   The scene  202  has an arbitrary number of different objects. 
   The recorded sequence  203  of pictures is fed from the camera  201  via a first communications link  204 , for example a cable or a radio link, to a coding apparatus  205 . 
   After the coding apparatus  205  has received the pictures  203  via an input/output interface  206 , the pictures are digitized by an analog-to-digital converter  207  of the coding apparatus  205  and the digitized pictures are stored in a memory  208 . 
   Furthermore, the coding apparatus  205  has a processor  209 , which is set up in such a way that the method steps described below can be carried out. 
   The logical components which are necessary in the context of the coding and decoding in accordance with the exemplary embodiment are illustrated in  FIG. 1  and  FIG. 3  and are explained in detail below. 
   In accordance with the exemplary embodiment, the functionalities of these individual components are stored as a program in the memory  208  of the coding apparatus or in a memory of the decoding apparatus described below and are executed by the processor  209  of the coding apparatus  205  or of the decoding apparatus, respectively, as explained in more detail below. 
   The individual components of the coding apparatus  205  are coupled to one another via a computer bus  210 . 
   The coding apparatus  205  is coupled to a decoding apparatus  212  via the input/output interface  206  and via a second communications link  211 , for example a radio link or a fixed network line, that is to say a cable. 
   Digitized, coded video pictures  213  are transmitted from the coding apparatus to the decoding apparatus  212  via the second communications link  211 . 
   The coded pictures  213 ,  214  are received by the decoding apparatus  212  via an input/output interface  215  and stored in a memory  216 . 
   Furthermore, the decoding apparatus  212  has a processor  217 , which is set up in such a way that the corresponding method steps for decoding the coded pictures and thus for reconstructing digitized pictures can be carried out. 
   The components of the decoding apparatus  212  are coupled to one another via a computer bus  218 . 
     FIG. 1  shows the logical structure of the coding apparatus  100 , which, in accordance with this exemplary embodiment, is implemented in the form of a computer program carried out by the processor  207 . 
   However, the components of the coding apparatus  100  can also be realized in each case as a specific electronic circuit on a separate computer card. 
   The pixels grouped into picture blocks are fed as a digitized picture  101  to the coding apparatus  100 . 
   In a transformation unit  102 , a Discrete Cosine Transform (DCT) is carried out on the picture blocks. 
   Furthermore, the pixels of the original picture  101  to be coded are fed to a unit for motion estimation and motion compensation  110 . 
   The DCT coefficients  106  formed by the transformation unit  102  are stored in a memory  103 . 
   Furthermore, the DCT coefficients of the original picture  101  are read from the memory  103  and fed as read-out DCT coefficients  107  to a subtraction unit  104 . 
   In the subtraction unit  104 , a read-out spectral coefficient  107  of the original picture  101  to be coded has subtracted from it in each case a corresponding spectral coefficient  109  of the temporally preceding picture, which spectral coefficient is formed by the unit for motion estimation and motion compensation  110  and corresponds to the corresponding picture position. 
   The difference, referred to below as difference coefficient  108 , determined by forming the difference between the two mutually corresponding spectral coefficients of the original picture  101  to be coded and of the temporally preceding picture is fed to a quantization unit  113  and quantized there. 
   Furthermore, the spectral coefficients  107  of the original picture  101  to be coded are likewise fed to the quantization unit  113  and quantized there. 
   The quantized coefficients  108  are fed to a decision unit  105 , for example a unit for forming the maximum  105  of the input quantities fed to the unit  105  for forming the maximum. 
   The respective larger value from the spectral coefficient  107  and the coefficient difference  108  is selected as selected coefficient  111  and coded. 
   As an alternative or in addition, in the decision unit  105 , the data rate for signaling can also be taken into account for selection purposes. 
   Furthermore, the respective selected quantized coefficient  115  and also an indication  112  of whether the quantized coefficient is a spectral coefficient  107  of the original picture or the difference coefficient  108  (referred to below as coefficient indication  112 ) are fed to a unit for entropy coding  116 , whereby a coded video data stream  121  is formed which is transmitted to the decoding apparatus  300 , which is described below. 
   In a feedback loop  114 , the selected and quantized coefficients  115  and also the coefficient indication  112  are fed to an inverse quantization unit  117 , where they are converted into inverse-quantized coefficients  118 . 
   The inverse-quantized coefficients  118  are transformed into inverse-coded coefficients  120  in an inverse transformation unit  119  by an Inverse Discrete Cosine Transform (IDCT). 
   The inverse-coded coefficients  120  are fed to the unit for motion estimation and motion compensation  110 , which effects a customary motion estimation and motion compensation. 
   Consequently, in accordance with this procedure, for each DCT coefficient to be transmitted, a check is made to determine whether the DCT coefficient of the original picture  101  or of the reconstructed picture, that is to say of the prediction error picture, is transmitted, depending on which coefficient is smaller, i.e. has a lower value. 
     FIG. 3  shows the decoding apparatus  300 , to which the coded video data stream  121  is fed. 
   In an inverse quantization unit  301 , inverse-quantized spectral coefficients  302  are formed, which are fed to an inverse transformation unit  303 , in which an inverse discrete cosine transform is carried out on the inverse-quantized coefficients  302  in order to form inverse-transformed coefficients  305 . 
   The inverse-transformed coefficients  305  and the indication of whether the coefficients are difference coefficients or spectral coefficients of the original picture are fed to an addition unit  304 . 
   Accordingly, either a reconstructed DCT coefficient  311 —described below—of the temporally preceding picture is added or the reconstructed coefficient  305  is simply stored as spectral coefficient  306  in a memory  307 . 
   In the unit for motion compensation  308 , for the case where the coefficient  305  is a difference coefficient  108 , motion compensation is carried out and the motion-compensated signal  309  is fed to a transformation unit  310  for carrying out a DCT transform. 
   The DCT coefficients  311  of the reconstructed picture that are formed are fed from the transformation unit  310  to the addition unit  304  and, for the case where a difference coefficient  108  is involved, the respective DCT coefficient  311  formed by the transformation unit  310  is added to the inverse-quantized coefficient  302 . 
   From the memory  307 , the reconstructed picture stored therein is read out as a reconstructed video signal  312  and displayed to the user of the decoding apparatus  300 , for example via a screen. 
   The invention has been described in detail with particular reference to preferred embodiments thereof and examples, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention.