MOTION-COMPENSATED FIELD RATE CONVERSION

Abstract: In a method of processing input fields (I) of a picture signal to obtain interpolated fields (O) located temporally between the input fields (I), motion vectors (MV) are provided (30) between input fields (I), whereafter a quality (Err) of the motion vectors (MV) is assessed (40). In dependence upon the assessed quality (Err), the motion vectors (MV) are adaptated (50, 60) such that both decreased and increased motion vector lengths occur in respective interpolated fields (O). The interpolated fields (O) are obtained (20) from the input fields (I) in dependence upon the thus adapted motion vectors (k*MV).

Description: Motion-compensated field rate conversion. The invention relates to a motion-compensated field rate conversion, in which interpolated fields are obtained from input fields in dependence upon motion vectors. EP-A-0,475,499 discloses a system for field rate conversion in which the interpolated pictures result as shifted input fields. It is mentioned that, depending on the reliability of the motion vectors, the compensation can be more or less complete. In other words: if motion vectors are unreliable their effect is reduced by multiplying them with a factor smaller than one. In the described case of a field rate doubler, the result is some 50 Hz motion judder which is not very objectionable and can be interpreted as a velocity depending blurring. In field rate convenors with a higher or a non-integer ratio between the input and d e output field rate, the effect of reduced motion compensation is the breakthrough of a much lower frequency in the motion judder. This frequency equals the largest common factor of the output and the input field rate. For example, an up-conversion from 24 Hz film material to 60 Hz video material, introduces a 12 Hz component in the motion judder. Apart from this long repetition period of the judder, the motion portrayal often is very irregular. It is, inter alia, an object of the invention to provide a motion- compensated interpolation in which this disturbance is reduced. To this end, a first aspect of the invention provides a method as defined in claim 1. A second aspect of the invention provides an apparatus as defined in claim 4. A third aspect of the invention provides a television signal receiver as defined in claim 5. Advantageous embodiments are defined in the dependent claims. In a method of processing input fields of a picture signal to obtain interpolated fields located temporally between the input fields, motion vectors are provided between successive input fields, whereafter a quality of the motion vectors is assessed. In accordance with the present invention, the motion vectors are adapted in dependence upon the assessed quality, whereby both decreased and increased motion vector lengths occur in respective interpolated fields. The interpolated fields are obtained from the input fields in dependence upon the thus adapted motion vectors. Thus, if the estimated motion vectors are not believed to be sufficiently reliable for use in a straightforward motion-compensated interpolation, there is provided a (partial) fall-back at a regular motion judder, preferably of the highest possible frequency, i.e. half the output field frequency. This judder, as any other, decreases die visibility of artifacts in me picture due to unreliable motion vectors, but as such is much less annoying than the irregular and more low-frequent judder resulting from a straightforward down- scaling of die motion vectors as in the prior art. The present invention is applicable to field rate conversions between any input and output rate, and has die somewhat surprising property that die applied vector shifts are enlarged for some of me output fields. When input video fields are motion compensated to realize video data at a certain output field frequency, die common procedure is to take information from the temporally nearest input fields (or from me two fields on either side of die required temporal instance) and to shift mis data such mat objects are positioned at the motion trajectory, i.e. the imaginary line through die time-position graph of the object. Depending on the temporal distance between die available field and the required field, the length of the motion vectors as calculated between successive input fields has to be divided by a factor. In the proposed global fall-back algoridim, this factor is adapted such at the position of moving objects is field alternatingly above or below the motion trajectory. Consequently, motion vectors are in some fields reduced, but in o er fields enlarged. Preferably, mis adaptation of the motion vector lengdis is not simply field alternating, but depends on the ratio between input and output field rate and on the choice of me input field(s) that is used after shifting over the estimated motion vector to provide me output field. These and other aspects of the invention will be apparent from and elucidated widi reference to the embodiments described hereinafter. In die drawings: Fig. 1 illustrates a global fall-back according to the invention in me case of a 25 Hz film to 100 Hz video conversion; Fig. 2 illustrates the global fall-back mode according to me present invention for the case of a 24 Hz (film) to 60 Hz (video) conversion; Fig. 3 illustrates an implementation of the global fall-back measure for a 60 Hz to 50 Hz field rate conversion; Fig. 4 illustrates a first embodiment of a motion-compensated interpolation circuit in accordance widi the present invention; Fig. 5 illustrates a second embodiment of a motion-compensated interpolation circuit in accordance wim me present invention; and Fig. 6 shows an embodiment of a television receiver in accordance with the present invention. Fig. 1 an example is provided for the case of 25 Hz film to 100 Hz video up-conversion applying shifts to one nearby input field only. The horizontal axis indicates die field number, while the vertical axis indicates die position. Assuming a straight non¬ accelerated movement, an object would move along the motion trajectory MT, which is to be understood as a straight line drawn such that the deviation of the actual positions of the object in successive fields from this straight line is minimal on average; of course, this line may be shifted in time which shift corresponds to an overall processing delay. The small black circles indicate die position of an object in the 25 Hz input fields. The arrows indicate die motion vectors which are supposed to shift me object to the motion trajectory. However, when d e motion vectors are not believed to be sufficiently reliable to be used widiout any further measures, the fall-back according to the present invention becomes active. As shown in Fig. 1, in the fall-back case, die motion vectors are twice enlarged (+), than twice shrunk (-), etc, so as to obtain positions of die moving object which are field-alternatingly above or below the motion trajectory MT. As indicated by the dius adapted arrows in the figure, exacdy 50 movement phases per second result, i.e. die objects position in two successive fields is identical. This is just a choice, smaller and also larger deviations from the exact 25 to 100 Hz compensation are also possible. The choice may depend on die reliability of the motion vectors: me worse die (global) reliability, the larger the deviation from me ideal compensation in which all arrows would end on die motion trajectory MT. Fig. 2 shows a second example, where the current invention is elaborated for me case of 24 Hz film to 60 Hz video conversion. The large circles indicate die positions before compensation, while the small circles indicate die positions obtained after a partial motion-compensated interpolation according to the present invention. The large circles are formed by a standard 2-3 pull down conversion to obtain 60 output fields from 24 input fields. Again, in case of unreliable motion vectors, me motion vectors are adapted such that the positions of me moving object are field-alternatingly at one side or the other of d e motion trajectory MT. Fig. 3 shows how the same idea works out for a 60 Hz to 50 Hz video field rate conversion. The figure shows die original position of the (input) material in die first row (1), the fully compensated 60 Hz output in the last row (3), and between them, in row 2, d e "partially corrected" interpolation result according to die present invention. Row 3 is obtained with a straightforward motion-compensated interpolation which can be used when die estimated motion vectors are sufficiendy reliable. Row 2 shows positions which are alternatingly at the right-hand side and at die left-hand side of die corresponding positions in row 3. A position at die right-hand side of die corresponding position in row 3 is obtained by an enlarged motion vector, while a position at me left-hand side of me corresponding position in row 3 is obtained by a decreased motion vector. In one particular embodiment, adapted scaling of the motion vectors was made depending on die measured contrasts in the velocity plane. If the x-component of two horizontally neighboring displacement vectors or me y-component of two vertically neighboring displacement vectors differed more than a first threshold value, a slight deviation from the scaling as required for a perfect motion compensation was introduced. If the difference was more than a second threshold the scaling was fur er adapted and so on. Obviously, any number of thresholds can be used in die implementation. The background of this particular adaptation is that d e global reliability of motion vectors in a scene is expected to decrease if diere are objects moving fast in different directions close to each other (input material grows more difficult wid increasing velocities and particularly if the velocity field contains large contrasts). How rapidly die global vector reliability decreases depends on the applied estimation algorimm which therefore will determine die optimal thresholds to be used. Fig. 4 illustrates a first embodiment of a motion-compensated interpolation circuit in accordance widi the present invention, in which an interpolated output field O is obtained by shifting a single input field I along the motion trajectory. The input signal is applied to a motion vector controllable variable delay circuit 20 dim a time base corrector (TBC) 10. A motion estimator (ME) 30 furnishes motion vectors MV. A fall-back detection circuit 40 is coupled to die motion estimator 30 to receive an error signal Err indicative of the consistency in the motion vector field, die estimation match errors, or some omer information useful for determining me reliability of the estimated motion vectors. The fall-back detection circuit 40 determines whether the error signal Err deviates seriously enough from acceptable values and, if so, signals to a vector modifier 50 diat die motion vectors MV produced by the motion estimator 30 should be adapted such diat the visibility of motion estimation errors is reduced. The vector modifier 50 produces a vector lengdi modifying factor k which is applied to a vector multiplier 60 for multiplying the horizontal and vertical components of the motion vectors MV from die motion estimator 30 by mis factor k. It goes without saying mat if no adaptation is necessary, die vector length modifying factor k solely depends on the temporal position of the interpolated field with regard to die corresponding input field. The mus modified motion vectors k*MV are applied to the variable delay 20 for controlling the motion vector dependent shift. The variable delay 20 produces die output fields O. As mentioned above, if the motion vectors are to be adapted to reduce the visibility of motion vector estimation errors, they should be output- field-alternatingly adapted such mat the positions of moving objects are at alternating sides of die motion trajectory. To this end, die vector modifier 50 receives an odd/even field identifier signal o/e. A field may be interlaced or non-interlaced. Fig. 5 illustrates a second embodiment of a motion-compensated interpolation circuit in accordance wid die present invention, in which an interpolated output field O is obtained by shifting two input fields I along die motion trajectory, and averaging die dius shifted input fields. The embodiment of Fig. 5 is much alike mat of Fig. 4, so that a description of only die differences between diese embodiments will suffice. The output signal of the time base corrector 10 is also applied to a second variable delay 20' dirough a field delay 15. The motion estimator 30 of Fig. 4 is replaced by a motion estimator 30' which produces, in addition to me above-mentioned motion vectors MV, motion vectors MV for use wim me other input field. The vector modifier 50 of Fig. 4 is replaced by a vector modifier 50' which produces, in addition to d e above-mentioned factor k, a vector lengdi modifying factor 1-k for use with the odier input field. When two input fields are used in a motion-compensated interpolation, it will be clear diat if die motion vectors MV to be used widi one of these input fields are shortened (or made longer) by a factor k, die motion vectors MV' to be used widi die omer field are to be made longer (or shortened) by a corresponding factor 1-k. A vector multiplier 60' multiplies d e horizontal and vertical components of the motion vectors MV' from the motion estimator 30' by this factor 1-k. The dius modified motion vectors (l-k)*MV are applied to die variable delay 20' for controlling its motion vector dependent shift. An averager 25 receives respective motion-compensated shifted fields from die variable delays 20 and 20' for producing me output field O. Fig. 6 shows an embodiment of a television receiver in accordance widi me present invention. A tuner and IF circuit 100 receives antenna signals and furnishes demodulated video signals to a video processor 200. The video processor does all usual iings like brightness and contrast control, and furnishes processed video signals as an input picture signal I to an upconverter 300 which may be either of me embodiments of Figs. 4, or 5, to obtain interpolated output fields O. The upconverter 300 furnishes these interpolated output fields O to a display 400. A main aspect of the invention can be summarized as follows. In pictures resulting from motion-compensated field rate convenors, artifacts resulting from motion vector estimation errors can occur in scenes with complicated or very fast motion. If these artifacts degrade d e picture quality more than what is gained with motion compensation, it is good to gradually switch over to non-motion compensated interpolation. This disclosure describes an approach d at yields considerably better results dian a straightforward reduction of die motion vector length which is known from the prior art. In contrast with previous proposals, vector lengths are sometimes increased, so that both increased and decreased motion vector lengths occur in respective interpolated fields. Hereby, a motion judder is obtained which reduces me visibility of the motion vector estimation errors. It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and diat those skilled in die 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 invention can be implemented by means of hardware comprising several distinct elements, and by means of a suitably programmed computer. Clearly, sophistications in which the adaptation of die deviation is only allowed to change slowly in time are possible. These sophistications can be useful to prevent visible switching of the introduced global blur or motion error. In mis disclosure and in die claims, a field may be interlaced or non¬ interlaced.

Claims: Claims: 1. A method of processing input fields (I) of a picture signal to obtain interpolated fields (O) located temporally between said input fields (I), the memod comprising me steps of: providing (30) motion vectors (MV) between input fields (I); assessing (40) a quality (Err) of said motion vectors (MV); adapting (50, 60) said motion vectors (MV) in dependence upon said assessed quality (Err) to obtain adapted motion vectors (k*MV), wherein said motion vectors (MV) are adapted such that both decreased and increased motion vector lengths occur in respective interpolated fields (O); and obtaining (20) said interpolated fields (O) from said input fields (I) in dependence upon said adapted motion vectors (k*MV). 2. A method as claimed in claim 1, wherein said step (40) of assessing a quality (Err) of said motion vectors (MV) includes die step of measuring contrasts in a velocity plane formed by said motion vectors (MV). 3. A method as claimed in claim 2, wherein the step of measuring contrasts in a velocity plane formed by said motion vectors (MV) includes the step of comparing differences between horizontal and/or vertical components of motion vectors (MV) corresponding to neighboring image parts, to at least one mreshold value, whereby the motion vectors (MV) are adapted in dependence upon whether said at least one threshold value is exceeded. 4. An apparatus for processing input fields (I) of a picture signal to obtain interpolated fields (O) located temporally between said input fields (I), the apparatus comprising: means (30) for providing motion vectors (MV) between input fields (I); means (40) for assessing a quality (Err) of said motion vectors (MV); means (50, 60) for adapting said motion vectors (MV) in dependence upon said assessed quality (Err) to obtain adapted motion vectors (k*MV), wherein said motion vectors (MV) are adapted such mat both decreased and increased motion vector lengdis occur in respective interpolated fields (O); and means (20) for obtaining said interpolated fields (O) from said input fields (I) in dependence upon said adapted motion vectors (k*MV). 5. A television signal receiver, comprising: means (100, 200) coupled to receive a television signal for furnishing a picture signal; an apparams (300) as defined in claim 4, for processing input fields (I) of said picture signal, to obtain interpolated fields (O); and display means (400) coupled to said processing apparatus (300).

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: H04N
IPC codes: H04N5, H04N7