METHOD AND APPARATUS FOR MOTION ESTIMATION

Abstract: The present invention relates to a method of generating and transmitting digitally encoded signals in which portions of the code are predicted from earlier. The invention is applied to a digital video compression system in which temporal predictive coding is used in order to reduce the data rate of the transmitted signal. Predictions are made for the reconstruction of the current picture based on elements of pictures that have been coded in the past. A prediction is made for each reference block from each of five picture areas. A distortion value is derived for each prediction and a selection is made from the predictions based on the distortion values of the predictions. By biasing the selection between the different predictions towards that prediction which was last selected, predictions of approximately equal quality from completely different picture areas will be avoided. This avoids the situation where small advantages in prediction quality are outweighed by a larger codeword for the transmission of the prediction.

Description: METHOD AND APPARATUS FOR MOTION ESTIMATION The present invention relates to a method and apparatus for generating digitally encoded signals which include portions of code that have been predicted from earlier code in the signal and which include prediction parameters relating the predicted portions of code to the earlier code. The invention is of particular advantage in the generation and transmission of digital video signals. Digital video compression systems generally use temporal predictive coding in order to reduce the data rate of the transmitted signal. In the coding process, predictions are made for the reconstruction of the current picture based on elements of pictures that have been coded in the past. These picture elements, which may be referred to as 'blocks' are manipulated in a certain way (translation, scaling, rotation) before they form the basis of the prediction. In order to make it possible to reconstruct the sequence in the decoder, the parameters used in the manipulation of the blocks have to be transmitted, usually accompanied by a correction signal which minimises the effects of prediction errors. Due to the spatial correlation of adjacent picture elements, the manipulation parameters themselves are usually predictively coded in such a way that small changes require fewer bits for transmission than larger ones which are less common. The quality of the block predictions is assessed using a metric based on the difference between the predicted block and the block which is to be coded. This metric will be referred to as the distortion value hereinafter. As the processing power dedicated to making better predictions increases, larger areas are searched in the reference picture and small differences in the quality of the predictions become more significant. If predictions of approximately equal quality are made from completely different picture areas, SUBSTITUTE SHEET (RULE 26| however, there is a danger that a small advantage in prediction quality is outweighed by a larger code word for the transmission of the prediction parameter. The present invention is concerned with improving the selection of predictions. According to the present invention, there is now provided a method of generating a digitally encoded signal which includes portions of code that have been predicted from earlier code in the signal, the method comprising; making a plurality of candidate predictions for each portion of predicted code, deriving a distortion value representing the extent of distortion for each candidate prediction, selecting a prediction based on the distortion values of the candidate predictions, and biasing the selection between candidate predictions towards that prediction selected for the preceding predicted code portion. Further according to the present invention there is provided apparatus for generating a digitally encoded signal which includes portions of code that have been predicted from earlier code in the signal, the apparatus comprising; prediction means to undertake a plurality of candidate predictions for each portion of predicted code, means to derive a distortion value representing the extent of distortion for each candidate prediction, selection means to select a prediction based on the distortion values of the candidate predictions and, means to bias the selection between candidate predictions towards that prediction selected for the preceding predicted code portion. The invention will now be described, by way of example, with reference to the accompanying drawings in which: Figure 1 is an explanatory diagram to illustrate the problem addressed by the invention; Figure 2 shows a block diagram of apparatus embodying the invention; and Figure 3 is illustrative of the functioning of the apparatus shown in Figure 2. Referring now to Figure 1 , there are shown a series of picture areas X-.., (10), X n (11 ) and X n+1 (12). The picture area X-., (10) has been predicted by a translation MP-., of a block A (13) in the previously coded picture. For the prediction of the picture area X n (11 ) there are two candidate blocks, B (14) and C (15) with respective associated quality measures M B and M c of approximately equal value. In the case where M c is slightly better than M B , then since block C (15) is adjacent to block A (13), the previous manipulation parameter MP n ., can also be used for the prediction of the block X n (11 ) and of the parameter MP n . The prediction of MP n is therefore simple and likely to require only a small codeword. Furthermore, the block X n+1 (12) can be predicted using the same manipulation parameter yet again. In the second case M B is assumed to be slightly better than M c . In this case the block X n (11 ) and the manipulation parameter MP n would be predicted on the basis of the block B (14) which is no longer adjacent to the block A (13). The result is a large codeword for the prediction of MP n . A second codeword of similar size would also be required for MP n+1 . The combined effect of these two codewords is likely to outweigh the small coding advantages in the prediction of the block X n (11 ). The example described with reference to Figure 1 is illustrative of how the selection of prediction parameters based purely on a distortion metric will give less than the optimum performance. The invention is concerned to improve performance by taking the coding history of the prediction parameters into account in making a selection between possible predictions. Referring now to Figure 2, there is shown a motion estimation system including five motion estimation blocks A (20), B (21), C (22), D (23) and E (24), each working on the same reference block to be coded but with different search areas A to E. The reference block (30) and the search segments or areas A to E (31-35) are illustrated in Figure 3. The five search areas (31-35) are assumed to cover horizontally adjacent panels. The input to each motion estimator (20-24) includes a signal input for the reference block (30) and a signal input for the corresponding search area A (31 ), B (32), C (33), D (34) or E (35). The motion estimators (20-24) produce respective motion parameters MP A , MP B , MP C , MP D and MP E at their outputs. Pre-selector A (25), selects between the motion parameters MP A and MP B to select one of two motion parameters which is labeled MP AB at the output from the pre-selector A (25). Pre-selector B (26) selects between the motion parameters MP D and MP E to produce a motion parameter at the output which is labeled MP DE . An output selector (27) makes a selection from the motion parameters MP AB , MP C and MP DE . The manipulation parameters and the distortion values for the different segments or areas A to E (31-35) are calculated separately. Once a manipulation parameter of one search area has been selected, a small bias is introduced towards the same search area. This means that distortion values of a different search area have to be lower, by a certain threshold, in order for the manipulation parameter for that search area to be selected. A further bias is introduced towards small displacement parameters, i.e. the search area closest to the reference block to be coded. This ensures start up and recovery in situations where the prediction from each search area is of exactly equal quality, e.g. in the case of blank pictures. Thus, referring once again to Figure 2, once pre-selectors A (25) and B (26) and the output selector (27) have chosen a particular path, any other path will need a lower distortion value in order to be chosen. If the distortion value associated with MP AB or MP DE is equal to MP C , then MP C will be chosen by default so as to introduce a slight bias towards smaller displacement parameters, i.e. the search area closest to the reference block (30) to be coded as shown in Figure 3. The distortion value is available as a binary number of N digits where N has a value of 10 but may have other values. The bias is preferably introduced by adding a threshold to the distortion value of each of the search areas except the one that was selected for the last block to be encoded. Alternatively, the comparison may be made on a number of bits, say M bits, which is less than N. While the probability increases that, two search areas have the same distortion value when measured over the M bits (M<N), the same search area is chosen as for the previous block.

Claims: CLAIMS 1. A method of generating a digitally encoded signal which includes portions of code that have been predicted from earlier code in the signal, the method comprising: making a plurality of candidate predictions for each portion of predicted code, deriving a distortion value representing the extent of distortion for each candidate prediction, selecting a prediction based on the distortion values of the candidate predictions, and biasing the selection between candidate predictions towards that prediction selected for the preceding predicted code portion. 2. A method according to Claim 1 , which is applied to generating a digitally encoded signal representing picture information. 3. A method according to Claim 2, wherein the step of making the plurality of candidate predictions consists of making estimations of motion parameters corresponding to respective picture search areas. 4. A method according to Claim 3, wherein the step of biasing the selection between the Candidate predictions includes introducing a further bias in the selection of candidate predictions towards the estimation of the motion parameter representing the smallest displacement. A method according to Claim 1 , 2, 3 or 4, wherein the steps of biasing the selection between candidate predictions comprises adding a threshold to the distortion value of all the candidate predictions except the one selected for the preceding predicted code portion. 6. Apparatus for generating a digitally encoded signal which includes portions of code that have been predicted from earlier code in the signal, the apparatus comprising: prediction means to undertake a plurality of candidate predictions for each portion of predicted code, means to derive a distortion value representing the extent of distortion for each candidate prediction, selection means to select a prediction based on the distortion values of the candidate predictions, and biasing means to bias the selection between candidate predictions towards that prediction selected for the preceding predicted code portion. 7. Apparatus according to Claim 6, which is adapted to generate a digitally encoded signal representing picture information. 8. Apparatus according to Claim 7, wherein the prediction means comprise a plurality of motion estimators operable to generate the candidate predictions in the form of parameters corresponding to respective picture search areas. 9. Apparatus according to Claim 8, wherein the biasing means is adapted to introduce a further bias in the selection of the candidate predictions towards the estimation of the motion parameter representing the smallest displacement. 10. Apparatus according to Claim 6, 7, 8 or 9, wherein the biasing means is adapted to introduce a threshold to the distortion value of all the candidate predictions except the one selected for the preceding predicted code portion.

Inventors: BOCK ALOIS MARTIN, BOCK, ALOIS, MARTIN
Applicants: BOCK ALOIS MARTIN, NDS LTD, BOCK, ALOIS, MARTIN, NDS LIMITED
Application year: 1997
Main classification: H04N7
IPC codes: H04N7