Method and apparatus for coding and for decoding a picture sequence

Abstract: Motion-compensated hybrid codecs are used in data compression for moving picture sequences. By virtue of the regular insertion of intraframe-coded pictures, these compression methods enable access to any desired individual pictures in the entire bit stream or the playback of the bit stream from virtually any desired location. A disadvantage is the high bit outlay necessary for intraframe-coded pictures. In a feedback loop, a codec usually contains a simulation of the receiver-end decoder, whose coding errors can thus also be taken into account by the encoder. According to the invention, an attenuation element is inserted into this feedback loop. The coding and the receiver-end decoding of intraframe-coded pictures becomes superfluous. After the receiver-end decoding of a few pictures, beginning with a grey-scale picture as starting picture, a viewer does not perceive a difference between picture sequences which are coded in a known manner, and contain I pictures at relatively short intervals, and picture sequences which are coded according to the invention.

Description: The invention relates to a method and an apparatus for coding and decoding a picture sequence. Prior Art Predominantly motion-compensated hybrid codecs (encoder with simulated decoder contained therein) are used in data compression for moving picture sequences, such as e.g. in the MPEG1 or MPEG2 standard. By virtue of the regular insertion of intraframe-coded pictures (I frames), these compression methods enable access to any desired individual pictures in the entire bit stream or the playback of the bit stream from virtually any desired location. An intraframe-coded picture can be inherently individually decoded from the associated data and does not require any data from other pictures for the purpose of reconstruction. In contrast to this, interframe-coded pictures (P frames) cannot be inherently decoded but rather require in each case at least one reference picture for their reconstruction. This reference picture (anchor frame) must already have been decoded beforehand. M. FUJIMURA et al: "Application of Leaky Prediction to an Interframe Coding Scheme", Electronics & Communications in Japan, Part 1 - Communications, vol.80, No.4, April 1997, pages 20-28, XP000692880, discloses picture coding using DCT transformation, 'leaky' prediction, interframe coding and intraframe coding. EP-A-0405754 shows image subband coding, wherein a subpicture in a higher resolution band is predicted from the corresponding subpicture in the adjacent lower resolution band. The filtered subpictures are subsequently coded, transmitted and correspondingly decoded. There is a special initial picture in any sequence to be decoded. By virtue of the insertion of intraframe-coded pictures into a video bit stream, each picture of a picture sequence could be decoded, starting from such an intraframe-coded picture, without the video bit stream of the entire picture sequence having to be decoded. Each intraframe-coded picture could be decoded immediately and each interframe-coded picture could be decoded by decoding the chronologically nearest preceding intraframe-coded picture and subsequently decoding the picture sequence as far as the desired interframe-coded target picture. A disadvantage of such a method is the high bit outlay necessary for intraframe-coded pictures. The factor of the required bits for intraframe-coded pictures I to the required bits for singly forward-predicted interframe-coded pictures P is approximately 10:1 in the typical MPEG2 format (M=3; N=12). In this case, N is the interval separating one intraframe-coded picture from the next and M is the interval separating an I picture and the succeeding P picture - or vice versa - and the interval separating one P picture and the next P picture. Situated in between there may be B pictures, which may be bi-directionally predicted for example. Invention The invention is based on the object of specifying a method for picture sequence coding and decoding in which it is possible to dispense with such I pictures transmission, but decoding from virtually any desired location in a picture sequence can nevertheless take place. This object is achieved by means of the methods specified in Claims 1 and 4. The invention is based on the further object of specifying an apparatus for coding and for decoding a picture sequence with application of the method according to the invention. This object is achieved by means of the apparatuses specified in Claims 6 and 7. As mentioned above, a codec usually contains a simulation of the receiver-end decoder. Coding errors thereof can thus also be taken into account by the encoder. The decoder simulation is usually arranged in a feedback loop of the hybrid codec. By virtue of the inventive insertion of an attenuation element into this feedback loop, the insertion and coding and the receiver-end decoding of intraframe-coded I pictures becomes superfluous. The attenuating element effects a decrease in amplitude values of predicted coefficients. The inventive attenuation in the feedback path of the hybrid codec therefore initially effects an artificial, actually undesirable deterioration in the prediction and leads to an increase in the prediction error to the coded. If the attenuation inserted in the feedback path is chosen suitably in terms of its size, however, then it is surprisingly possible to dispense entirely with the use of intraframe-coded pictures, without thereby losing the property of being able to decode the video bit stream at virtually any desired location. The starting of the receiver-end decoding at any desired location in the bit stream or in a picture sequence occurs as follows: A grey-scale picture, preferably a grey-scale picture of average brightness, is used as the first prediction picture or as the reference picture. The first prediction error signal decoded in the receiver is combined with this grey-scale picture. The subsequent interframe-coded pictures are then decoded in a known manner. As a result of the insertion of the attenuation element in the encoder, the receiver-end reconstruction error generated in this way at the beginning is progressively reduced in the course of the decoding of the subsequent pictures since, on account of the attenuation element, errors which the encoder attempts to reduce have been artificially introduced in the encoder-end decoder function simulation as well. The effect of the invention is that after the receiver-end decoding of L pictures, a viewer cannot perceive a difference between picture sequences which are coded in a known manner and contain I pictures at relatively short intervals, and picture sequences which are coded according to the invention and contain no I pictures B pictures may be arranged between the P pictures. The effect of these B pictures is no error propagation, on the one hand, but also no reduction of visible reconstruction errors, on the other hand. The receiver-end video decoder can therefore display decoded pictures after the decoding of L pictures - or even a few pictures beforehand. The value of the parameter L depends on the setting of the attenuation D of an attenuation element in the feedback of the codec and also determines the resulting bit rate. When L is small, the attenuation D is large and the convergence of the receiver-end decoding error is faster, but the bit rate is increased. When L is large, the attenuation D is small and the convergence of the receiver-end decoding error is slower, but the bit rate is low. Drawings Exemplary embodiments of the invention are described with reference to the drawings, in which: Figure 1 shows a known encoder for video data; Figure 2 shows a decoder for video data; Figure 3 shows an inventive encoder for video data. Exemplary Embodiment The video data input signal IE of the encoder in Figure 1 contains macroblock data for encoding. In the case of intraframe video data, a subtractor SUB simply allows these to pass. They are processed in discrete cosine transform means DCT and quantising means Q and are fed to an entropy encoder ECOD, which outputs the encoder video data output signal OE. ECOD can, for example, carry out Huffman coding for the coefficients and add header information and motion vector data. In the case of interframe video data, predicted macroblock data PMD are subtracted from the input signal IE in the subtractor SUB and the difference data are fed to the entropy encoder ECOD via the discrete cosine transform means DCT and the quantising means Q. The output signal of Q is also processed in inverse quantising means Q E -1 , the output signal of which is fed via inverse discrete cosine transform means DCT E -1 to the combiner ADDE in the form of reconstructed macroblock difference data RMDD. The output signal of ADDE is buffer-stored in a frame store in motion estimation and compensation means FS_MC_E, which carry out motion compensation for reconstructed macroblock data and output macroblock data PMD predicted in this way to the subtracting input of SUB and to the other input of the combiner ADDE. The quantising means Q, the inverse quantising means Q E -1 and ECOD for example, are controlled by the occupancy level of the encoder buffer ENCB. Q E -1 , DCT E -1 , ADDE and FS_MC_E constitute a simulation of the receiver-end decoder, which is also described in connection with Figure 2. In Figure 2, the video data input signal ID is fed via entropy decoder means EDEC, inverse quantising means Q D -1 and inverse discrete cosine transform means DCT D -1 to a combiner ADDD, which outputs the video data output signal OD. EDEC can, for example, carry out Huffman decoding for the coefficients and decode and/or evaluate header information and motion vector data. Q E -1 and Q D -1 , DCT E -1 and DCT D -1 and ECOD have a function which is the corresponding inverse of the function of Q, DCT and ECOD. The output signal of ADDD is buffer-stored in a frame store in motion compensation means FS_MC_D. FS_MC_D effects a motion compensation for reconstructed macroblock data. In known decoders, the macroblock data PMD predicted in FS_MC_D are passed to the second input of the combiner ADDD only in the case of interframe-decoded macroblock data. In the case of intraframe-decoded macroblock data, the combiner ADDD simply passes on the output signal from DCT D -1 . In decoders according to the invention, which receive no intraframe-coded I pictures, the macroblock data PMD predicted in FS_MC_D are always passed to the second input of the combiner ADDD. Therefore, such a decoder according to the invention can in this respect be constructed more simply than known decoders. Detection as to whether intraframe-coded or interframe-coded video data are present can be obviated. A changeover for ADDD between the two modes can likewise be obviated. At the beginning of a picture sequence decoding, FS_MC_D or another unit only has to generate grey-scale picture data for a decoding starting picture for the second input of ADDE or in ADDD itself. In terms of digital image processing, however, this only means interrogating or passing on a fixedly set numerical value, that is to say a minimal outlay. Then, for example not until after a defined number of decoded pictures, output OD can output an output picture which is visible to a viewer. Until then, the picture may be blanked or a grey-scale value, preferably the starting grey-scale value mentioned above, may be displayed. The starting grey-scale value may also be chosen adaptively, as a result of which convergence of the prediction errors can be achieved more rapidly. For example, if the DC difference coefficients of the macroblock data in the case of the current picture at the decoding starting instant have predominantly larger positive values, the starting grey-scale value can be chosen to be rather darker. If the DC difference coefficients of the macroblock data in the case of the current picture at the decoding starting instant have predominantly larger negative values, the starting grey-scale value can be chosen to be rather lighter. If the DC difference coefficients of the macroblock data in the case of the current picture at the decoding starting instant have predominantly small values, then an average brightness can rather be chosen for the starting grey-scale value. The video data input signal IE of the encoder according to the invention in Figure 3 contains macroblock data for encoding. In an embodiment of the encoder according to the invention, intraframe video data are never generated, rather only the attenuation according to the invention is used. In this embodiment, predicted and attenuated macroblock data PMDD are subtracted from the input signal IE in the subtractor SUB and the difference data are fed to the entropy encoder ECOD via the discrete cosine transform means DCT and the quantising means Q. The output signal Q is also processed in inverse quantising means Q E -1 , whose output signal is fed via inverse discrete cosine transform means DCT E -1 to the combiner ADDE in the form of reconstructed macroblock difference data RMDD. The output signal of ADDE is buffer-stored in a frame store in motion-estimation and compensation means FC_MC_E, which carry out motion compensation for reconstructed macroblock data and output macroblock data PMD predicted in this way to the other input of the combiner ADDE and, via an attenuation unit DU, to the subtracting input of SUB. The quantising means Q, the inverse quantising means Q E -1 and ECOD, for example, are controlled by the occupancy level of the encoder buffer ENCB. Q E -1 , DCT E -1 , ADDE and FS_MC_E once again constitute a simulation of the receiver-end decoder. The attenuation unit DU reduces incoming pixel values for example by multiplication by a factor D. Small values of D lie in the range from 0.1 to 0.3, for example, and large values of D lie in the range from 0.5 to 0.9, for example. D preferably lies in the range from 0.4 to 0.8. DU can also subtract only a constant value or additionally a constant value from all the pixel values, but the pixel values cannot become smaller than the black level or, in the case of chrominance signals, a value corresponding to the black level. A limiter can be used to ensure that the pixel values do not reach an impermissible value. D may differ when coding luminance and chrominance signals. The invention can be used for example in the transmission of digital television signals or in the transmission of digital video signals in networks such as the Internet or in a videophone or in the course of recording or in the course of mastering optical or magnetic storage media, e.g. DVD and in the course of playback thereof.

Claims: Method for coding a picture sequence (IE), wherein transformed (DCT) and quantised (Q) video data coefficients formed from difference values relating to block-format pixel values of the picture sequence are entropy-encoded (ECOD), with the transformed video data coefficients being subjected to inverse quantisation (Q E -1 ) and inverse transformation (DCT E -1 ), and being used in predicted form (FS_MC_E, ADDE) for the formation of the difference values relating to said pixel values, and wherein the predicted pixel values (PMD) are attenuated (DU) in terms of their amplitude prior to said formation (SUB) of the difference values, characterised in that the encoded picture sequence contains neither intraframe-coded pictures nor intraframe-coded pixel blocks. Method according to claim 1, wherein said attenuation (DU) is effected by multiplication of said predicted pixel values by a predetermined factor. Method according to claims 1 or 2, wherein said attenuation (DU) is effected by reduction of the absolute values of said predicted pixel values by a predetermined factor. Method for decoding a picture sequence containing transformed and interframe-encoded video data coefficients (ID) formed from difference values relating to block-format pixel values of said picture sequence, wherein said encoded video data coefficients are entropy-decoded (EDEC), inversely quantised (Q D -1 ) and inversely transformed (DCT D -1 ) to provide pixel difference values that are combined (ADDD) with predicted (FS_MC_D) pixel values derived from previous pixel difference values, wherein said difference values on which said transformed and encoded video data coefficients are based are derived from predicted pixel values that were attenuated in terms of their amplitude, characterised in that said picture sequence contains neither intraframe-coded pictures nor intraframe-coded pixel blocks and that upon starting a decoding for the first picture, said inversely transformed (DCT D -1 ) pixel difference values are combined (ADDD) with pixel values of a decoder-generated grey picture. Method according to claim 4, wherein said block-format pixel values form macroblocks containing DC difference coefficients and said grey picture pixel values of the decoding start picture are determined adaptively in particular such that, if the DC difference coefficients of the macroblock data of the decoding start picture have predominantly larger positive values, the corresponding starting grey-scale pixel values are chosen to be darker, whereas, if the DC difference coefficients of the macroblock data of the decoding start picture have predominantly larger negative values, the corresponding starting grey-scale pixel values are chosen to be brighter. Apparatus for coding a picture sequence (IE), provided with: means (SUB) for forming pixel difference values, to which first input block-format pixel data of said picture sequence are fed; means (DCT, Q) for transforming and quantising said pixel difference values to provide video data coefficients; an entropy-encoder (ECOD) for said video data coefficients to provide encoded video data coefficients (OE); means (Q E -1 , DCT E -1 ) for inversely quantising and inversely transforming said video data coefficients to provide reconstructed pixel difference values; means (FS_MC_E, ADDE) for forming predicted pixel values (PMD) from said reconstructed pixel difference values; an attenuation unit (DU) for attenuating said predicted pixel values (PMD) before they are fed to a second input of said means (SUB) for forming pixel difference values, characterised in that in the encoded video data coefficients (OE) output from said entropy-encoder (ECOD) neither intraframe-coded pictures nor intraframe-coded pixel blocks are contained. Apparatus for decoding encoded video data coefficients (IE) of a picture sequence, provided with: an entropy-decoder (EDEC) for decoding said video data coefficients to provide decoded video data coefficients; means (Q D -1 , DCT D -1 ) for inversely quantising and inversely transforming said decoded video data coefficients to provide pixel difference values that were attenuated in terms of their amplitude in a corresponding encoder; combination means (ADDD) and means (FS_MC_D) for forming predicted pixel values derived from previous pixel difference values, the output signal of which means for forming predicted pixel values is combined in said combination means with said pixel difference values to provide block-format pixel data of said picture sequence, characterised in that said picture sequence contains neither intraframe-coded pictures nor intraframe-coded pixel blocks and that upon starting a decoding for the first picture, said inversely transformed (DCT D -1 ) pixel difference values are in said combination means (ADDD) combined with pixel values of a grey picture generated in said decoding apparatus. Recording medium, in particular an optical disc, containing or having recorded on it a sequence of digital video data (OE) that are encoded according to the method of claim 1, wherein, when the data of said storage medium is input into a decoding apparatus according to claim 7, said data cause carrying out a method according to claim 4.

Inventors: ADOLPH DIRK, BLAWAT MEINOLF, ADOLPH, DIRK, BLAWAT, MEINOLF
Applicants: THOMSON BRANDT GMBH, DEUTSCHE THOMSON-BRANDT GMBH
Application year: 1998
Main classification: H04N7
IPC codes: H04N7, G06T9