VIDEO SIGNAL SCAN CONVERSION

Abstract: A scan conversion method of generating an output video signal from an interlaced input signal, comprising the steps of: furnishing (1) a first signal that comprises: first lines corresponding to original lines of the interlaced input signal; second lines corresponding to interpolated lines; providing (5, 11, 7, 13) a second signal, which is delayed or advanced with respect to the first signal, and which comprises: first processed lines corresponding to the first lines; second processed lines corresponding to the second lines; obtaining output lines in an output video signal in dependence upon: a first difference (3) between the first processed lines and the first lines; a second difference (9) between the second processed lines and the second lines.

Description: Video signal scan conversion. The invention relates to a method and apparatus for video signal scan conversion, and to a video signal display apparatus comprising such a conversion apparatus. WO-A-95/27362 (Attorney's reference PHN 14,794) discloses a method of converting an interlaced video signal into an ouφut sequentially scanned video signal. A first sequentially scanned video signal is furnished in dependence upon the interlaced video signal. Subsequently, a motion-compensated sequentially scanned signal is furnished in dependence upon the first sequentially scanned video signal. All video lines of the output sequentially scanned video signal are generated in dependence upon both the first sequentially scanned video signal and the motion-compensated sequentially scanned signal to prevent inaccuracies in motion vectors used in the motion-compensated processing operation rom resulting in visible distortions. The first sequentially scanned video signal and the motion- compensated sequentially scanned signal are combined to furnish the ouφut sequentially scanned video signal by means of a first mixer for first lines positionally corresponding to original lines of a present field of the interlaced video signal, a second mixer for second lines positionally corresponding to interpolated lines between the original lines, and a multiplexer for line-alternately selecting an ouφut of the first mixer or an ouφut of the second mixer. It is, inter alia, an object of the invention to provide an improved video signal scan conversion. To that end, a first aspect of the invention provides a method as defined in claim 1. A second aspect of the invention provides a scan conversion apparatus as defined in claim 8. A third aspect of the invention provides a video signal display apparatus as defined in claim 13. Advantageous embodiments are defined in the dependent claims. Accordingly, a first aspect of the invention provides a method of generating an ouφut video signal from an interlaced input signal, comprising the steps of: furnishing a first signal having first lines (F 4 (x - (0, l) τ , n)) corresponding to original lines of the interlaced input signal, and second lines (Fj(x, n)) in addition to the first lines (F.(x - (0, 1)\ n)); and providing a second signal which is delayed or advanced with respect to the first signal, the second signal having first processed lines (F^Cx - (0, l) τ - D, n-1)) corresponding to the original lines, and second processed lines Fg - D, n-1)) corresponding to the second lines (Fj(x, n)); in which ouφut lines (F^Gc, n)) of the ouφut video signal are obtained in dependence upon (see equations 21 and 27 in the detailed description of the preferred embodiments): a first difference between the first processed lines (F c - (0, l) τ - D, n-1)) and the first lines (F,(x - (0, l) τ , n)), and a second difference between the second processed lines (F^ - D, n-1)) and the second lines (F|(x,, n)). Preferably, the ouφut video signal is a sequentially scanned video signal, and the method comprises the further step of combining the first signal and said second signal to obtain the ouφut sequentially scanned video signal having first ouφut lines (F out tø - (0, l) τ , n)) corresponding to the original lines, and the ouφut lines (F^tø, n)) between the first ouφut lines (F^Q - (0, l) τ , n)). Advantageously, the ouφut lines (F^tø, n)) of the ouφut video signal are obtained further in dependence upon a relation between the first lines (F,(x - (0, l) τ , n)) and the first ouφut lines (F ix - (0, l) τ , n)). A second aspect of the invention provides that the ouφut lines are obtained in dependence upon the second difference and a third difference between the first ouφut lines (F ∞i (x - (0, l) τ , n)) and the first processed lines (F^tø - (0, l) τ - D, n-1)), in which the first difference as well as the relation between the first lines and the first ouφut lines are replaced by a third difference which is equivalent thereto in a preferred embodiment of the invention. Reference is made to equation 20 in the detailed description of the preferred embodiments. Another aspect of the invention provides that the combining step is controlled such that a difference between the ouφut lines (F-^x, n)) and the second processed lines (F^Qc - D, n-1)) corresponds to a difference between the first ouφut lines (F ∞t (x - (0, l) τ , n)) and the first processed lines (F ∞ ,(x - (0, l) τ - D, n-1)), see equation 18 in the detailed description of the preferred embodiments. The above-mentioned aspects of the invention improve the consistency along the motion trajectory or, put in other words, a smoother picture is obtained for moving objects. These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter. In the drawings: Fig. 1 illustrates an embodiment of a television receiver comprising a sequential scan converter in accordance with the present invention; Fig. 2 shows an embodiment of a device for calculating mixer coefficients kl and k2 for use in the embodiment of Fig. 1; and Fig. 3 shows an embodiment of a device for calculating the mixer coefficient kl for use in the embodiment of Fig. 1. 1. Introduction De-interlacing is a basic operation required for most video scanning format conversions. Vertical and temporal inteφolation of image data cause practical and fundamental difficulties, as the conditions of the sampling theorem are generally not met in video signals. Linear methods, based on sampling rate conversion theory, therefore negatively influence the resolution and/or the motion portrayal. The more advanced algorithms can be characterized by their common attempt to interpolate the 3-D image data in the direction with the highest correlation. To this end, they either have an explicit or implicit detector to find this direction. In case of 1-D temporal inteφolation the explicit detector is usually called a motion detector, for 2-D spatial inteφolation it is called an edge detector, while the most advanced device estimating the optimal spatio-temporal 3-D inteφolation direction is a motion estimator. The inteφolation filter can either be recursive or transversal, but the number of taps in the temporal direction is preferably small. Recently, some papers have been published proposing a recursive scheme for motion compensated sequential scan conversion [1,2]. Experiments indicate that the recursiveness yields a significant improvement of the motion-compensated median [3], on which these algorithms are based, if good quality motion vectors are available. Furthermore, recursiveness is expected to allow an improved performance on critical velocities, compared to recently published methods applying generalized sampling theory [4], if the velocities are accelerating. In this application an improved recursive sequential scan conversion algorithm is introduced that suppresses remaining artifacts of the prior art further. The invention provides the algorithms and a novel evaluation method that shows the improvement in an objective score. For high quality, sub-pixel accurate motion estimation the algorithm of [5] is used. 2. De-interlacing techniques In general the samples required for the motion compensated de-interlacing do not exist in the time discrete input signal, e.g. due to non-integer velocities. In the horizontal domain this problem can be solved with linear Sampling Rate Conversion (SRC) theory, see e.g. [6], but not in the vertical domain, as the constraints of the sampling theorem are not met. Three different solutions for this problem have been proposed recently in the literature: 1) A straight extension of the motion vector into earlier pictures until it points (almost) to an existing pixel [7]. 2) The application of a generalized sampling theory (GST) [4]. 3) Recursive de-interlacing of the signal [1,2]. Solution 1) is valid only if we assume the velocity constant over a larger temporal instance. This is a rather severe limitation which makes the method practically useless. The implication of GST is that it is possible to perfectly reconstruct a signal sampled at \ln times the Nyquist rate with n independent sets of samples that describe the signal. For the de-interlacing problem n •= 2, and the required two sets are the current field and the motion compensated previous field. If the two do not coincide, i.e. the object does not have an odd vertical motion vector component, the independency constraint is fulfilled, and the problem can theoretically be solved. Practical problems are: a) The velocity can have an odd vertical component. b) A perfect reconstruction requires the use of pixels from many lines, for which the velocity needs not be constant. c) For velocities near the vertical odds, noise may be enhanced. Solution 3) is based on the assumption that it is possible at some time to have a perfectly de- interlaced picture in a memory. Once this is true, the picture is used to de- interlace the next input field. With motion compensation this solution can be perfect as the de-interlaced picture in the memory allows the use of SRC-theory also in the vertical domain. If this new de-interlaced field is written in the memory it can be used to de-interlace the next incoming field etc... Limitations of this method are: I) Propagation of errors due to motion vector inaccuracy and inteφolation defects II) Even a perfectly de-interlaced picture can contain alias in the vertical frequency domain, assuming the common case of a camera without optical pre-filter. In practice problem I) is the more serious, particularly for nearly odd vertical velocities and/or noisy input signals. We concluded that recursive de-interlacing and de-interlacing based on GST are the best methods presently known. However, even these best methods are imperfect. It is our target to present an improvement that can be applied in combination with both methods to suppress the remaining artifacts in the de-interlaced ouφut signal. In fact our proposal can be used to improve any de-interlacing algorithm. 3. Description of the applied algorithms In [1] a time-recursive de- interlacing algorithm is proposed in which the lines that need to be inteφolated are found by motion compensating the previously found de-interlaced ouφut frame: U, π), ( original lines ) F 0 »,(& ») = Ψ na te - D. , n), n - l), ( inteφolated lines ) where F(x, t) is the interlaced input signal, F ^ x, n) the sequential ouφut, n the field number, and x is the spatial position. To prevent errors from propagating, in [1] several additional measures are described to protect the inteφolated lines. Particularly, the median filter is proposed to realize this protection: Although further alternatives are suggested in [1], we will use this algorithm as the basis for our comparison. As we expect the quality of the resulting algorithm to depend heavily on the performance of the motion estimator, we applied the motion estimation method of [5]. This high quality algorithm, yields a quarter pixel accuracy, and a close to true-motion vector field which is considered very important for scan rate conversion. Rather than calculating all possible candidate vectors, this recursive search block-matcher takes spatial and/or temporal "prediction vectors" from a 3-D neighborhood, and a single updated prediction vector. This implicitly assumes spatial and/or temporal consistency. The updating process involves update vectors added to either of the spatial prediction vectors. We applied a candidate set CS(X, ή), from which the block- matcher selects its result vector, defined by: CS( where the update vectors U a Q n) and u (X, n) are alternatingly (on block basis) available, and are taken from a limited fixed integer update set, in our case: -1 -3 US, = { (4) P • (lj' (-lj » j' i-2 , 0 0 . 0 J To realize sub-pixel accuracy, the update set of equation (4) is extended with fractional update values. We realized a quarter pixel resolution by adding die following fractional update vectors to the update set: 0 0.25 -0.25 rø / " { 10.25 -0.25 I o ) 0 } (5) Because of the small number of candidate vectors, d e method is very efficient and realizes, due to die inherent smoothness constraint, very coherent and close to true-motion vector fields, most suitable for scanning format conversion. 4. Recursive de-interlacing algorithm The main imperfection of the recursive de-interlacing algorithm is remaining alias in the ouφut signal. Although this imperfection is usually worse for alternative methods, further improvement seems attractive. Difficulty with this defect is, that it is hardly visible in single images but mainly in moving sequences. This makes it difficult to illustrate, while also quantitative measures to show the improvement seem to lack. A common method for evaluating the de-interlacing quality, is comparing an original sequentially scanned image with a de-interlaced result using a, what we will call here, MSE ^ - criterion: MSE a (n) = 1/N ∑ ( ^U, «) - FJA n)f (6 ) I € field This criterion is not exclusively sensitive for remaining alias, as it sums all differences without discriminating for different backgrounds, e.g. due to resolution losses, noise, vector errors, etcetera. An additional inconvenience of this criterion is that it cannot be applied to check d e performance of the algorithm on original interlaced source signals. In a perfectly de-interlaced picture (widiout residual alias), a characteristic is ύiat the sequence is stationary along the motion trajectory, in picture parts for which the motion model is valid. Based upon this characteristic, an alternative was suggested in [5], were we measured how well the current interlaced input field n was predicted by the motion compensated previously de-interlaced field: MSE i (n) = l/N £ ( (χ, n) - F Lz - B(χ, n), n - l) f (7) l e field This memod has the advantage diat it can be applied to judge the performance in absence of an original sequentially scanned sequence. However, die measure has limited value in case of critical velocities, as in that case the quality of the inteφolated lines is not reflected in the figure. In an attempt to improve on mis aspect, it is possible to measure a "Motion Trajectory Inconsistency" (MTI(n)) for all ouφut lines in field n, which we will define as: o 77(n) = 1/N ∑ (FJLx. n) - F Lt - fi n), n - l) (8) A problem with this measure is that a good score on this criterion is a necessary but not sufficient constraint in die processing. It is possible, e.g. applying a strong temporal filtering, to force this MTI to very low values, while obviously the picture quality is degraded. However, a lower score on die MTI criterion coupled to a hardly varying MSE score is a strong indication for quality improvement. There is a clear analogy with the motion vector smoothness constraint [8], where motion estimation techniques have been improved by adding a smoothness term in the match criterion which yields a significant consistency improvement accepting a slight MSE degradation. Quite similarly it is possible here to introduce a de-interlacing cost figure defined as: Cost(ή) = l/N[ MSEJ(n) + aMΩ(n)] = 1/ιV ∑ {Fix, n) - F L& - Ω. , n), n - l) (9) l €/ Id + *ffl Σ ( α« ( ϊ. O " F^ ~ fiU, « ) , n - 1)) 2 I e field The parameter a allows tuning of the cost function to match the subjective impression. After quantifying "remaining alias" applying equation (9), an improvement of the MTI figure could be realized by suppressing non-stationarities along the motion trajectory. As long as this not seriously degrades the MSE-score, it brings an improved Cost-score and, with a tuned correctly, also an improved subjective performance. As non-stationarities can reside on inteφolated lines as well as on original lines, the implication would be mat both have to be temporally filtered. That filtering of original lines, which is somewhat contra-intuitive, helps to suppress remaining alias can also be understood from me vertical frequency spectrum of the signal on original and inteφolated lines respectively. In the original recursive de-interlacing algoridim, the lines existing in the input field are always directly transferred to the ouφut and never modified. As the first repeat spectrum of the inteφolated lines will almost always suffer from inaccuracies in the motion vector estimates and me protection features, this spectrum cannot fully compensate for the (anti-phase) repeat spectrum resulting from the original lines. As a consequence of the above, the recursive de-interlacing method mat we propose inteφolates not only me new lines, but, in an attempt to maximize the motion trajectory consistency on boϋh type of lines equally strong, also the lines existing in me interlaced signal: F 0Kt , n) - k{RMFJLι -ZKϊ, n), M - 1 ) + (1 - *(*)). (F.U, n)) (10) where k(R) is a control parameter that reflects me reliability R of the motion vectors. F t (x, n) can be calculated according to equation (2) but also, with little disadvantage as we will show later, using a simpler intra-field inteφolation: In me literature sometimes the match error, or the match error corrected for die local picture contrast is used. When applying the 3-D Recursive Search block-matcher described in [5], anomer indication for motion vector reliability is available. This recursive estimator, due to me use of spatial and temporal predictions, implicitly assumes consistent motion vector fields. If the ouφut vector field is not smooth than the implicit assumption may be false, and therefore die motion vectors unreliable. For this smoothness any sum of absolute (or squared) differences of me current vector with its spatial and temporal neighbors can be applied, but using the 3-D RS block-matcher we obtained good results using the following definition for the vector smoothness S: ∑ (I DJz -a) - D X (R) I + I DJSL - &) - DJZ) I) (12) where £= (x, y, t) τ is the spatio-temporal position, a is a constant selected experimentally, while the neighborhood N in which the neighboring vectors are found defined as: In this definition X and Y denote me horizontal and vertical block dimensions as used in the motion estimator respectively. 5. Refinements of the algorithm The severest drawbacks that were found widi the above-described algoritiim were: 1) Large homogeneously moving picture parts occasionally break up into several areas with slighϋy different velocities (earthquakes). 2) In areas where large displacement discontinuities occurred, blocking artifacts were sometimes visible. These are due to me reliability indicator being available on block base only. The first of Uiese difficulties is direcdy related to ώe fundament of the proposed memod and therefore most difficult to cure: by recursively filtering both the original and die inteφolated lines, me ouφut becomes more and more "isolated from the input" if the reliability of the motion vectors is considered to be good. In mis section, tiierefore, we introduce a solution to prevent die ouφut pictures to drift too far from the input material. The basis for the refinement is to detect direcdy how far me filtered information on an original line differs from the unfiltered information and to reduce me filtering in case the deviation becomes to large. As this adaptation of the filtering can easily be realised on pixel base it further provides a means to cope with the drawback mentioned under number 2). The same difference is used to reduce die filtering of the inteφolated lines, albeit iat much larger differences turned out to be tolerable. In the refined algoridim (modification of equation (2) me ouφut luminance is found as: FJΛ ») = (14) f (1 - k XF ix, -HU, «), n - 1) + ^ .(F j U, n)) .inteφolated lines ( l - A 2 ). (F wιr -J2 , n), n - 1) + ^.(F.U /»)) .original lines and using a limiter function "clip" defined as: clip(L, R, ά) (15) & 2 is calculated as: fc j = l/lό.clipfO, 16, 10 - C 2 .Ji\IS + Δ ).Diff) ( 16) Consequendy, fc-. is a function of me vector smoothness S, defined in equation (4), and of me (pixel-) difference, Diff, in me motion compensated recursive loop measured on original lines of the interlaced grid only: Diffix, n) = ab ∑ {FJLi -£U n), « - 1) - (F.(i, «)) (17) where is a window containing the six pixels nearest to the currently inteφolated pixel on the lines directly above and below die inteφolated pixel. As Diff is related to me MSE t (n) of equation (7) it is possible by appropriately choosing C,, to tune the relative importance of MSE,(n) and MTI(n). Although this tuning seems rather straightforward for d e original lines, it is more complicated for me inteφolated lines. For diese lines an MTI(n) figure can be calculated, but a significant MSE t (n) cannot be found as die quality of the input lines to this temporal filter depends on me quality of me initial sequential scan conversion algorithm. To escape from this fundamental problem we propose here to tune fc* such that the contribution of original lines and inteφolated lines to me MTI(n) figure is identical per pixel, disregarding me energy over die temporal filter for me inteφolated lines. The assumption leads to: while at the same time: F LX., n) = k F t iχ, n) + ( 1 - *, )F aa - β, « - 1 ) (19) where Fjfe, n) is the ouφut of the initial de-interlacing algorithm, e.g. using equation (2) or (11). Combination of equations (18) and (19), results in a calculation of fc* according to: Combining this result with me lower part of equation (14) results in: As an implication the temporal recursive filtering on the inteφolated lines depends on d e quality of d e initial de- interlacing method. A simple line averaging algoridim will cause stronger temporal filtering, of the inteφolated lines, dian e.g. a motion compensated median filter. Experimentally we could show that the difference in resulting de-interlacing performance was small. The assumption of equation (18) leads to an adaptation of temporal recursive filtering on me inteφolated line applying differences measured on me upper neighboring original line. Rather than using the upper original line as a reference, me lower neighboring line can be used equally well. For symmetry considerations the average effect on the two neighboring original lines seems advantageous. Elaborating this results in a symmetrical alternative for equation (20): k , ,) . (22) with: Rather than averaging the absolute differences, it is possible to take the absolute value of the average, die maximum of the two averages, or apply (2-D) spatial filters before and/or after die rectifier (abs). The choices resemble those commonly applied in motion detectors. It is furthermore possible to apply the above control of the temporal filter on the inteφolated lines, while fixing the value of ^ to no filtering on the original lines ik 2 = 1). Generally this will lead to somewhat lower MSE { (n) figures, but a significantly higher MTI(n) score. In an alternative embodiment, k^ is calculated as: * 2 = clip(θ, 1, C 2 . fblf (24) with: DiJ i n) = abs F out iχ - ii, n), n - 1) - (F,(& n)) (25) Consequendy, k^ is a function of the prediction error, Diff, in the motion compensated recursive loop. As Diff is related to die MSE(n) of equation (7) it is possible by appropriately choosing C 2 , to tune the relative importance of MSE f (n) and MTI(n). As the quality of me inteφolated pixels, as resulting from die initial de- interlacing method, is obviously less man d at of the original pixels, the filtering of these pixels should be stronger. As, furthermore, Diff(x, n), at the position of the inteφolated lines, has little value to determine the quality of the motion compensated prediction we propose to control k x using: *, = where C, is smaller than C 2 , and die control of die recursive filter for inteφolated pixels is derived from the quality of the motion compensated prediction at d e vertically neighboring existing pixels. There is a risk in filtering alternate lines differendy, as it potentially introduces visible line structures. Although die advantage of high quality de-interlacing will be apparent in areas wim vertical detail, diere is no advantage in regions that lack such high vertical frequency components. Therefore, in order to prevent the introduction of line structure in image parts that never profit from individual filtering, k is made equal to ^ if: (27) >] DiMi ~ (JJ, n) + Diffix + j j, n) ≥ jl.Dm.* * ») To evaluate the proposals resulting from the previous section, we selected a set of 4 critical sequences. The sequences contain vertical detail, and motion wi various sub-pixel values in many directions. Using the algorithm of [1], in the version described in section 2 as a reference, we calculated MSEs and MTIs for another two algorithms, illustrating the proposals of mis paper. The first algorithm is the one illustrated in Fig. 1, in which the control of the recursive loop is according to equations (24,26). The second algorid m has fc. fixed at 1 , i.e. no recursive filtering of the original pixels, and is further identical. Compared to the reference algorithm bom proposed new algoridims yield a slightly improved MSE t figure, which is expected to be mainly due to me elimination of median defects in the high spatial frequencies. The MTI figures, however, have been improved dramatically, particularly for the first algoridim, i.e. die algorithm that performs recursive filtering even on the original lines. MSE ( figures of both algorithm differ only little. The additional recursive filtering of die original pixels mainly improves the MTI figure (with little or no disadvantage for the MSE t figure). Fig. 1 shows the resulting architecture of the proposed de- interlacing algorithm. In Fig. 1 , an interlaced input signal is applied from an input I to an initial sequential scan converter 1 which supplies original lines at its lower ouφut and inteφolated lines at its upper ouφut. The original lines are applied to a motion compensation stage 7 thru a mixer 3 and a field memory 5. Similarly, the inteφolated lines are applied to die motion compensation stage 7 thru a mixer 9 and a field memory 11. Motion vectors D for the motion compensation stage 7 are determined by a motion estimator 13 on the basis of the original lines supplied by the initial sequential scan converter 1 (or on the basis of the lines supplied by the mixer 3), and shifted lines supplied by d e motion compensation stage 7. Motion compensated shifted lines are supplied by die motion compensation stage 7 to the mixers 3 and 9. The mixer 3 mixes the original lines and die motion compensated shifted lines in me ratio k2:(l-k2). The mixer 9 mixes the inteφolated lines from the initial sequential scan converter 1 and die motion compensated shifted lines in the ratio kl :(l-kl). Ouφut signals from the mixers 3 and 9 are applied to a compress and multiplex stage 15 to generate a de-interlaced ouφut signal, which is displayed on a display unit 16. While this embodiment largely corresponds to diat described in WO-A-95/27362 (Attorney's reference PHN 14,794), the present invention is mainly concerned wid providing optimal values for kl (and k2). To this end, Fig. 2 shows an embodiment of a device for calculating the mixer coefficients kl and k2 for use in the embodiment of Fig. 1. The mixer coefficients calculating device of Fig. 2 contains a first calculation circuit 17 for calculating a vector smoothness S in response to me motion vectors P_ in accordance widi equations 12, 13, and a second calculation circuit 19 for calculating the mixing factors kl and k2 in dependence upon me four signals applied to die mixers 3 and 9 in accordance widi equations 14 di 23. Fig. 3 shows an embodiment of a device for calculating the mixer coefficient kl for use in the embodiment of Fig. 1. The mixer coefficient k2 has a fixed value of 1. The mixer coefficient kl is calculated in accordance widi equation 20 by a calculating device 20. 6. Conclusion A generally applicable improvement to existing de-interlacing algorithms and a new evaluation measure for such algoridims have been proposed in d is application. A motion- compensated temporal recursive filtering is used. A typical feature of the proposal is that this filtering is not limited to die inteφolated pixels only, but is extended to d e filtering of the pixels existing in the interlaced input signal. This somewhat contra-intuitive action followed from the assumption that in order to have die repeat spectra of original lines compensated by d at of the inteφolated ones, it is essential diat die two have identical frequency response. As it cannot be prevented diat of die inteφolated pixels may be distorted by the limited accuracy of the applied motion vectors, a similar distortion can best be applied to mat of me original pixels in order to maximally suppress alias. The application of the improvement to time- recursive de-interlacing was elaborated and the improved performance was verified. The verification has to be understood in a sense mat the classical MSE t performance measure had not suffered (in fact, even showed some improvement), whereas die new proposed "consistency along the motion trajectory" (MTI) had greatly improved. In a preferred embodiment, the invention provides a method of sequential scan conversion which uses a recursive temporal filtering of at least the inteφolated lines, in which me filter is controlled by locally giving a resulting consistency along the motion trajectory a fixed relation (e.g. 1) to die values determined for adjacent lines. It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and diat Λose skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims. While absolute differences are described, in some embodiments normal differences are used to preserve the sign of d e difference. Fig. 1 shows a recursive embodiment, in which die ouφuts of the mixers 3, 9 are applied to die motion-compensation stage 7 thru die field memories 5, 11; of course, a non-recursive embodiment in which the inputs of the field memories 5, 11 are directly connected to die ouφuts of die initial sequential scan convenor 1 is also possible. In Fig. 1, the inteφolated lines supplied by die initial sequential scan convenor 1 may be just white, black or grey lines; if an inteφolation algorithm is used to obtain the inteφolated lines, any algoridim will do. In d e claims, any reference signs placed between parentheses shall not be construed as limiting the claim. The invention can be implemented by means of hardware comprising several distinct elements, and by means of a suitably programmed computer. Accordingly, a preferred embodiment of the invention provides a method of generating a sequentially scanned video signal from an interlaced input signal, comprising the steps of: furnishing a first signal having first lines (F s (x - (0, l) τ , n)) conesponding to original lines of the interlaced input signal, and second lines (Fj(x, n)) in addition to die first lines (Fj(x - (0, l) τ , n)); and providing a second signal having first motion-compensated lines (F^tø - (0, l) τ - D, n-1)) corresponding to d e original lines, and second motion-compensated lines ( ^ x - D, n-1)) corresponding to die second lines (Fj(x, n)); combining the first signal and die motion-compensated signal to obtain an ouφut sequentially scanned video signal having first ouφut lines (F^ c - (0, l) τ , n)) corresponding to d e original lines, and second ouφut lines (F^tø, n)) between die first ouφut lines (F^Q - (0, l) τ , n)); wherein the second ouφut lines (F^tø, n)) are obtained in dependence upon (see equations 21 and 27): a first difference between the first motion-compensated lines (F o x - (0, l) τ - D, n-1)) and the first lines (F-(x - (0, l) τ , n)), a second difference between the second motion-compensated lines (F^Gc - D, n-1)) and die second lines (Fj(x, n)), and a relation (k 2 ) between the first lines (F-(x - (0, l) τ , n)) and die first ouφut lines (F^ - (0, l) τ , n)). However, it is not necessary that the ouφut video signal is a sequentially scanned video signal, as die invention can advantageously be used to derive an ouφut interlaced signal having its ouφut lines vertically positioned between the lines of the input interlaced signal. Such an interlaced-to-interlaced conversion is needed in 100 Hz conversion, where the lines one of the input interlaced fields need to be vertically shifted towards die positions of die lines of die otiier input interlaced field. The combining step is advantageous in a sequential scan conversion, but not necessary in otiier scan conversions. Also, whilst the second signal is preferably obtained by means of a motion-compensated inteφolation, satisfactory results can already be obtained when a less complex method is used to obtain a second signal which is delayed or advanced widi respect to the first signal, like one which uses a median filter. Finally, the dependence on die relation (k 2 ) between the first lines and the first ouφut lines appeared to contribute to an improved result, while good results were already obtained when the ouφut lines were dependent only on die first and second differences. The invention can advantageously be applied in a video signal display apparatus like a television receiver or a personal computer. 7. References [1] F.M. Wang, D. Anastassiou, and A.N. Netravali, 'Time-Recursive Deinterlacing for IDTV and Pyramid Coding', Signal processing: Image Communication 2, Elsevier 1990, pp. 365-374. [2] Kwon, Seo, Kim, and Kim, 'A Motion Adaptive De-Interlacing Metiiod', IEEE Transactions on Consumer Electronics, Vol. 38, No.3, 1992. [3] G. de Haan and G.F.M. De Poortere, 'Method and apparatus for processing a picture signal', EP-A-0,474,287. [4] P. Delogne, L. Cuverlier, B. Maison, B. Van Caillie, and L. Vandendoφe, "Improved Inteφolation, Motion Estimation, and Compensation for Interlaced Pictures", IEEE Transactions on Image Processing, Vol. 3, No. 5, September 1994. [5] G. de Haan, and P.W.A.C. Biezen, 'Sub-pixel motion estimation with 3-D recursive search block-matching', Signal Processing: Image Communication 6 (1994), pp. 229-239. [6] A.W.M. van den Enden and N.A.M. Verhoeckx, Discrete-time signal processing, Prentice Hall (ISBN 0-13-216763-8), pp. 233-. [7] J.W. Woods and S.C. Han, "Hierarchical Motion Compensated De-interlacing", Proc. SPIE Visual Communication and Image Processing VI, Boston, November 1991. [8] 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 & Systems for Video Technology, Vol. 3, October 1993, pp. 368-388.

Claims: Claims: 1. A scan conversion method of generating an ouφut video signal from an interlaced input signal, comprising die steps of: furnishing (1) a first signal having first lines (F { ( - (0, l) τ , n)) corresponding to original lines of said interlaced input signal, and second lines (F j (x., n)) in addition to said first lines (Fj(x - (0, l) τ , n)); and providing (5, 11, 7, 13) a second signal which is delayed or advanced with respect to said first signal, said second signal having first processed lines (F g x - (0, l) τ - D, n-1)) conesponding to said original lines, and second processed lines (F^ix - D, n- 1)) corresponding to said second lines (Fj(x, n)); characterized in that ouφut lines (F^ c, n)) of said ouφut video signal are obtained in dependence upon a first difference between said first processed lines (F^ c - (0, l) τ - D, n- 1)) and said first lines (Fj(x - (0, l) τ , n)), and a second difference between said second processed lines (F^ - D, n-1)) and said second lines (Fj(x, n)). 2. A method as claimed in claim 1 , wherein said ouφut video signal is a sequentially scanned video signal, and wherein the memod comprises the further step of combining (3, 9, 15) said first signal and said second signal to obtain an ouφut sequentially scanned video signal having first ouφut lines (F^Cx - (0, l) τ , n)) corresponding to said original lines, and said ouφut lines (F^ , n)) between said first ouφut lines (F^tø - (0, l) τ , n)). 3. A method as claimed in claim 2, wherein said ouφut lines (F^ c, n)) of said ouφut video signal are obtained further in dependence upon a relation (k 2 between said first lines (Fj(x - (0, l) τ , n)) and said first ouφut lines (F^tø - (0, l) τ , n)). 4. A method as claimed in claim 1, wherein said first processed lines (F ∞l (x - (0, l) τ - D, n-1)) and said second processed lines (F ∞l (x - D, n-1)) are obtained by means of a motion-compensated inteφolation. 5. A method as claimed in claim 2, wherein said ouφut lines are obtained in dependence upon said second difference and a third difference between said first ouφut lines (F^tø - (0, l) τ , n)) and said first processed lines (F, x - (0, l) τ - D, n-1)). 6. A method as claimed in claim 2, wherein said combining step is controlled such tiiat a difference between said ouφut lines (F OTt (x, n)) and said second processed lines (F^ - D, n-1)) conesponds to a difference between said first ouφut lines (F ort - (0, l) τ , n)) and said first processed lines (F & - (0, l) τ - D, n-1)). 7. A method as claimed in claim 1 , wherein said first and second differences are absolute differences. 8. A scan conversion apparatus for generating an ouφut video signal from an interlaced input signal, comprising: means for furnishing (1) a first signal having first lines (Fj(x - (0, l) τ , n)) conesponding to original lines of said interlaced input signal, and second lines (Fj(x, n)) in addition to said first lines (Fj(x - (0, l) τ , n)); and means for providing (5, 11, 7, 13) a second signal which is delayed or advanced with respect to said first signal, said second signal having first processed lines (F ∞t (x - (0, l) τ - D, n-1)) corresponding to said original lines, and second processed lines (F O * Λ (X. - 12, n-1)) conesponding to said second lines (Fj(x, n)); characterized by means (19, 9) for generating ouφut lines (F^ x, n)) of said ouφut video signal in dependence upon a first difference between said first processed lines (F ^ ^x - (0, l) τ - D, n- 1)) and said first lines (F ( (x - (0, l) τ , n)), and a second difference between said second processed lines (F^ix - D, n-1)) and said second lines (Fj(x, n)). 9. An apparatus as claimed in claim 8, wherein said ouφut video signal is a sequentially scanned video signal, and wherein die apparatus further comprises means for combining (3, 9, 15) said first signal and said second signal to obtain an ouφut sequentially scanned video signal having first ouφut lines (F o x - (0, l) τ , n)) corresponding to said original lines, and said ouφut lines (F^ c, n)) between said first ouφut lines (F^tø - (0, l) τ , n))* 10. An apparatus as claimed in claim 9, wherein said ouφut lines n)) of said ouφut video signal are obtained further in dependence upon a relation (k 2 ) between said first lines (F j (x - (0, l) τ , n)) and said first ouφut lines (F OT .(x - (0, l) τ , n)). 11. An apparatus as claimed in claim 9, wherein said ouφut lines are obtained (20, 9) in dependence upon said second difference and a tiiird difference between said first ouφut lines (F^ix - (0, l) τ , n)) and said first processed lines (F^tø - (0, l) τ - D, n-1)). 12. An apparatus as claimed in claim 9, wherein said combining means are controlled such diat a difference between said ouφut lines n)) and said second processed lines (F ∞t (x - D, n-1)) conesponds to a difference between said first ouφut lines (F oo c - (0, l) τ , n)) and said first processed lines (F^Oc - (0, l) τ - D, n-1)). 13. A video signal display apparatus comprising: means (I) for supplying an interlaced input signal; a scan conversion apparatus for generating an ouφut video signal from said interlaced input signal, as defined in any of the preceding claims 8 thru 12; and means for displaying (16) said ouφut video signal.

Inventors: BIEZEN PAUL WILLEM ALBERT CORN, DE HAAN GERARD, BIEZEN, PAUL, WILLEM, ALBERT, CORNELIS, DE HAAN, GERARD
Applicants: PHILIPS ELECTRONICS NV, PHILIPS NORDEN AB, PHILIPS ELECTRONICS N.V.
Application year: 1996
Main classification: H04N5
IPC codes: H04N5, H04N7