MOTION ESTIMATION

Abstract: In a method of estimating motion vectors from sub-sampled video data (I), first vectors are estimated (ME, PM2) between an image with a first sub-sampling phase and an earlier image sub-sampled with a second sub-sampling phase, second vectors are estimated (ME, PM1) between an image with a second sub-sampling phase and an earlier image sub-sampled with a first sub-sampling phase, and the first and second vectors are combined (CD) to obtain output motion vectors (MV).

Description: Motion estimation. The invention relates to motion estimation on sub-sampled (interlaced) video data for e.g. scanning format conversion. The analysis of motion in a video sequence is embedded in many advanced video processing techniques, like advanced coding, noise reduction, and scan rate conversions such as de-interlacing and Frame Rate Conversion (FRC). In coding applications, motion information is used to minimize the prediction error and consequently the bit rate. Temporal noise filtering is very effective, but requires Motion Compensation (MC) if objects in the image move. De-interlacing, which is a prerequisite for high quality Motion Estimation (ME) and veπical scaling of interlaced video, requires information from multiple fields, which need therefore motion compensation. In case of scan rate conversion, motion information is needed for temporal interpolation that eliminates jerkiness of the motion portrayal and motion blur. Different applications may demand different types of motion estimators. In case of coding, motion estimation is based on the minimizing the difference between image parts, and coding the resulting error. The resulting motion vectors do not necessarily reflect to true motion within the video sequence. The motion estimation can be optimized to find the smallest error, resulting in the most efficient coding. For scan rate conversion applications, temporal interpolation is realized in the direction of the motion. Consequently, it is important that the motion vectors describe the true motion vectors within the video sequence. It is therefore not sufficient to demand for the smallest difference between image parts in the motion estimation. The motion behavior in the environment is also an important factor. Motion estimations designed for scan rate conversion can be used for coding purposes, but not immediately vice versa. The motion estimation described in this paper is therefore not restricted to scan rate conversion but designed to find the true-motion vectors. Most video data is available in a so-called interlaced format, i.e. a format in which the odd scan lines in odd fields and even lines in even fields together constitute a frame that describes an image, but the odd and even fields are NOT describing the image at the same temporal instance. Motion in video data is preferably measured on the shortest available time interval, i.e. the field interval in case of interlaced data. Due to vertical detail in the picture (causing alias in the sub-sampled field-grid), however, it may be impossible to correctly find the vertical displacement between two consecutive fields. The alias pattern and the original detail may move differently. Consider a stationary horizontal white line of two scanning lines width available in an interlaced video format. Odd and even fields will both show a single white line. Considering the information in two successive fields, it is ambiguous whether we deal with a one scanning line wide white line moving with a vertical velocity of 1 sample/field-period, or a stationary horizontal white line of two scanning lines width. A common way to deal with the mentioned problem, is to first up-convert at least one of the two fields into a progressive format, and perform the motion estimation between two frames or between a frame and a field [2] . An alternative exists in estimating the motion applying data from three successive fields [3]. Finally, it is possible to hope that the low-frequency content of the image is dominant, correct the phase for those frequencies and estimate motion more or less neglecting the problem [1]. A characteristic that [2] and [3] share, is that they double the access to the previous image data (from just the previous field, to the previously de-interlaced field, or the previous and the pre-previous field, respectively). Also the solution proposed in [3] introduces a new constraint in the motion estimator, in that the motion over a two-field-period has to be assumed constant. The option of [1] does not double the memory access (and capacity!) and introduces no constant motion constraints, but cannot solve the ambiguity either, as experiments on critical picture material show. It is, ter alia, an object of the invention to reduce ambiguity in the motion estimation in a simple manner. To this end, a first aspect of the invention provides a method and a device as defined in claims 1 and 5. A second aspect of the invention is defined in claim 6. Advantageous embodiments are defined in the dependent claims. In a method of estimating motion vectors from subsampled video data in accordance with a primary aspect of the invention, first vectors are estimated between an image with a first sub-sampling phase and an earlier image sub-sampled with a second sub-sampling phase, second vectors are estimated between an image with a second sub- sampling phase and an earlier image sub-sampled with a first subsampling phase, and the first and second vectors are combined to obtain output motion vectors. These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter. The drawing illustrates a possible implementation of the invention. As mentioned in the introduction, two successive fields cannot discriminate between stationarity and a vertical velocity of 1 sample/field for a line with a width of 2 pixels. Consequently, two successive fields do not contain enough information to solve the ambiguity problem. Block matching, as a possible basis for motion estimation, tries to find the best correlation between two image parts in the subsequent fields. As a consequence, it is very likely that the motion estimation detects the above mentioned ambiguity problem as an object with a vertical velocity of 1 sample/field. If the true velocity is 1 sample/field, it will continuously make the correct decision, but if the line is just stationary, the decision will toggle between a positive and negative velocity of 1 sample/field. The motion estimation that uses previous results as a first estimate of motion in the next time instance disturbs the convergence process in this situation. The new motion estimation exploits this principle, by making use of two separate prediction memories. The 'even' -prediction memory PM2 is used if the current field is even and the 'odd '-prediction memory PM1 is used if the current field is odd. Consequently, each prediction memory is updated with half the temporal frequency. Both prediction memories PM1, PM2 can converge to a stable situation, that does not necessarily reflect the true motion. However, combining the results stored in both prediction memories may provide a better clue concerning the true-motion. Fig. 1 illustrates the new algorithm, called the Twin motion estimation. An input field I is applied to a first input of a motion estimator ME, and to a field memory FM1 for furnishing a delayed field to another input of the motion estimator ME. An output of the motion estimator ME is applied to a demultiplexer DEMUX, whose outputs are connected to inputs of an odd prediction memory PM1 and an even prediction memory PM2. Outputs of the prediction memories PM1, PM2 are applied to a multiplexer MUX whose output is applied to the motion estimator ME. Outputs of the prediction memories PM1, PM2 are also applied to a combination device CD which furnishes output motion vectors MV. A field type indication signal FT is applied to the multiplexer MUX and the demultiplexer DEMUX. The input field I is also applied to a motion-compensated interpolator MCI, and a second field memory FM2 for furnishing a delayed field to another input of the motion-compensated interpolator MCI. The output motion vectors MV from the combination device CD are also applied to the motion-compensated interpolator MCI, whose output O is, for example, a signal having a doubled field frequency or a signal without interlace which is displayed on the display device D. Preferably, the same field memory device is used for both FM1 and FM2. The odd motion vector estimation is defined by d° (x, n) = ME ( f x, n-l ) , f 0 (x, n) ) , with f f and f 0 representing the even and odd input field respectively, and the even motion vector estimate as d' {x, n) - ME { f 0 [x, n-l ) , f . (x, n) ) , with ME(f,,f 2 ) indicating motion estimation between image f, and f,. As a consequence of the two prediction memories, the twin motion estimation uses two candidate sets (CS 0 and CS e ). So, d° {x, n) is also defined bv: d° {x, n ) _ { n ) ≤ f ( , ( x, otherwi se with d (x, n) the motion vector (d x ,d y )' at position (x,y) for field n, C a motion vector candidate, selected from a certain set of motion vectors or candidate set (CS) and ζ ( ) the error function. A similar expression can be written for d e {x, n) The combination device CD indicated in Fig. 1 generates the output motion vector d {x, n) according to: d (x, n) = φ ( d" (x, n) , d e , n) ) (4) In the text below d° {x, n) and d e (x, n) will be abbreviated as d° and d e respectively It seems reasonable just to average d° and d e , since the above mentioned problem will in case of stationaπty result in opposite vertical motion vectors So, ψ ( ) could be defined by ^ ( d°, de < ) = _ ≤ However, since every estimate is constructed out of different field pairs and images generally contain noise, the sign might be different, but the absolute magnitude of the vertical component is not necessarily equal Consequently, a straight forward average is generally not sufficient to solve the ambiguity problem Better results are expected with a non-linear function for ψ ( ) The horizontal direction does not suffer from the ambiguity problem, therefore, averaging for the horizontal direction is a reasonable solution In the text below, it is assumed that ( ) obeys the definition in equation 5, unless otherwise noted Many non-linear functions can be thought of Since toggling of the signs of the vertical motion component is very unrealistic in natural sequences, a possible non-linear function for i^ ( ) is defined by fe m ( ( ( - 2m) ( d v f -2m) ) < 0 l m e Z Ψ, 2 (d°, d e ) = dXd *!> h (6) , otherwise The ambiguity problem not only holds for the distinction between zero vertical motion and a motion of 1 pixel/field, but for all even velocities 2m with m e Z The argument for a non-vertical motion component can even be set stronger: Although the ambiguity problem occurs in the vertical direction only, the horizontal component might converge to an undesired situation, caused by the inherent bias of the algorithm in the vertical direction. Consequently, constraints as mentioned in the expressions above can be extended into the horizontal direction also: |2_n ( ( { d ' -2m) { d' -2m) ) < 0 ) , m e Z x A d°, d < d ,o + d j r . (8) , otherwise Note that ψ ( ) does not influence the content of the prediction memories, since d {x, n) is not stored, allowing both prediction memories to converge individually. A possible drawback of the twin motion estimation is the lower update rate of the prediction memory compared to a one prediction memory architecture, that might result in a slower convergence. On the other hand, eliminating the ambiguity in both prediction memories might even result in a faster convergence of the individual prediction memories. Experiments, in which resulting motion vectors were used for de-interlacing purposes, indicate indeed that a good quality motion vector results. The table below lists average MSEs (Mean Square Errors) calculated over a sequence of 30 fields of various image sequences, using the proposed combined motion vectors against the methods proposed in [1, 2]. It can be seen that it offers a quality level close to that of the best methods, at a price (memory capacity and bandwidth) more comparable to that of the simplest method. A primary aspect of the invention can be summarized as follows. It is an object of this primary aspect of the current invention to reduce ambiguity in the motion estimation, without increasing the access to, and the capacity of, the memory and without introducing constant motion constraints in the estimator. To this end, we first recognize that when estimating motion with an even vector y-component between a previous odd field and a current even field, the estimated motion vector may be wrong in vertical direction (y-component error) and the direction of the error is opposite (for the same object position relative to the interlaced sampling grid) when estimating motion with an even vector y-component between a previous even field and a current odd field. For an even vertical velocity, the object position (relative to the interlaced sampling grid) is identical in all fields, and combination of a first estimate resulting from a first field pair and a second resulting from the previous (or next) field pair may provide a better clue concerning the real velocity of the object. Having these recognitions in mind, the following method to estimate motion vectors from interlaced video data is disclosed. The invention reduces the ambiguity of the vertical component of the estimated motion vector due to the sub-sampling, by combining motion estimates from two consecutive field pairs. This solution reduces the memory bandwidth (factor of two) for the estimator, compared to the alternatives known from the literature, and does not introduce constant motion constraints as some of the mentioned alternatives do. The following salient features of the invention are noteworthy. A method, and apparatus realizing this method, for estimating motion vectors from subsampled video data, characterized in that it comprises a combination device, combining: first vectors estimated between an image with a first sub-sampling phase and an earlier image sub-sampled with a second sub-sampling phase with . second vectors estimated between an image with a second sub-sampling phase and an earlier image sub-sampled with a first subsampling phase. Preferably, the sub-sampled video data is so-called interlaced video data, and said image with a first sub-sampling phase is an odd field, and said image sub-sampled with a second sub-sampling phase is an even field. Preferably, the output of the combination device is used as the output vector of the estimator. Advantageously, the combination exists in setting the output vector y-component to zero in case the sign of the y-component of said first and said second vectors differs. Advantageously, the combination exists in setting the output vector y-component to an even value in case the values of the y-components of said first and said second vectors lie close to, but on either side of, said even value. Preferably, the 3D Recursive Search Block Matcher of [4] is used, and first vectors are written in a first prediction memory and the second vectors in a second prediction memory, whereby a first sub-sampling phase uses the first prediction memory only, and the second sub-sampling phase uses the second prediction memory only. Preferably, the vectors estimated on earlier video data are stored in a reduced form (subsampled prediction memory, or other data compression format). It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims. In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim. The word "comprising" does not exclude the presence of other elements or steps than those listed in a claim. The invention can be implemented by means of hardware comprising several distinct elements, and by means of a suitably programmed computer. In the device claim enumerating several means, several of these means can be embodied by one and the same item of hardware. References [1] M. Burl and P. Layton 'Video Image Processing', International Application WO 93/05616. [2] F.M. Wang, D. Anastassiou, and A.N. Netravali, 'Time-Recursive De- interlacing for IDTV and Pyramid Coding', in Signal processing: Image Communications 2, Elsevier, 1990, pp. 365-374. [3] L. Vanderdorpe, L. Cuvelier, B. Maison, P. Quelez, and P. Delogne, 'Motion-compensated conversion from interlaced to progressive formats,' in Signal Processing: Image Communication 6. 1994, pp. 193-211, Elsevier. [4] G. de Haan, P.W.A.C. Biezen, H. Huijgen and O.A. Ojo, "True-Motion Estimation with 3-D Recursive Search Block Matching, "IEEE transactions on Circuits and Systems for Videotechnology, Vol. 3, No. 5, October 1993, pp. 368-379. [5] J.L.Yen, "On Non uniform Sampling of Bandwidth Limited Signals, "IRE Tr. on Circuit Theory, vol. CT-3, pp. 251-257, December 1956. [6] E.B. Bellers and G.de Haan, "Advanced motion estimation and motion compensated de- interlacing, "Proc. of the Int. Workshop on HDTV'96, October 1996, Los Angeles, USA. [7] E.B. Bellers and G. de Haan, "Advanced de- interlacing techniques, "Proc. of the ProRlSC/IEEE Workshop on Circuits, Systems and Signal Processing, Mierlo, the Netherlands, November 27-28, 1996, pp. 7-17.

Claims: CLAIMS: 1. A method of estimating motion vectors from subsampled video data, the method comprising: estimating (ME, PM2) first vectors between an image with a first subsampling phase and an earlier image sub-sampled with a second sub-sampling phase; estimating (ME, PM1) second vectors between an image with a second sub-sampling phase and an earlier image sub-sampled with a first subsampling phase; and combining (CD) said first and second vectors. 2. A method as claimed in Claim 1, wherein said sub-sampled video data is interlaced video data, and said image with a first sub-sampling phase is an odd field, and said image sub-sampled with a second sub-sampling phase is an even field. 3. A method as claimed in Claim 1, wherein said combining step includes setting an output vector y-component to zero in case the signs of respective y-components of said first and said second vectors differ. 4. A method as claimed in Claim 1, wherein said combining step includes setting an output vector y-component to an even value in case values of respective y-components of said first and said second vectors lie close to, but on mutually different sides of said even value. 5. A device for estimating motion vectors from subsampled video data (I), the device comprising: means (ME, PM1, PM2) for estimating first vectors between an image with a first sub-sampling phase and an earlier image sub-sampled with a second sub-sampling phase, and for estimating second vectors between an image with a second sub-sampling phase and an earlier image sub-sampled with a first subsampling phase; and means (CD) for combining said first and second vectors. 6. A image signal display apparatus, comprising: a motion vector estimating device (ME, PMl, PM2, CD) as claimed in claim 6 for furnishing motion vectors (MV); a motion compensated interpolator (MCI) for generating an output image signal (O); a display device (D) for displaying the output image signal (O).

Inventors: BELLERS ERWIN BEN, DE HAAN GERARD, BELLERS, ERWIN, BEN, DE HAAN, GERARD
Applicants: KONINKL PHILIPS ELECTRONICS NV, PHILIPS SVENSKA AB, KONINKLIJKE PHILIPS ELECTRONICS N.V., PHILIPS AB
Application year: 1998
Main classification: H04N7
IPC codes: H04N7