WO2005091625A1

WO2005091625A1 - De-interlacing

Info

Publication number: WO2005091625A1
Application number: PCT/IB2005/050868
Authority: WO
Inventors: Lambertus A. Van Eggelen; Marco K. Bosma; Robert H. Gelderblom
Original assignee: Koninklijke Philips Electronics NV
Current assignee: Koninklijke Philips NV
Priority date: 2004-03-17
Filing date: 2005-03-10
Publication date: 2005-09-29
Anticipated expiration: 2006-09-17
Also published as: TW200601851A

Abstract

A motion adaptive de-interlacing unit (200) is disclosed. The de-interlacing unit (200) comprises: match error computing means (202) for computing a match error on basis of comparing a first group of pixels (114) of a first video field (100) with a second group of pixels (112) of a second video field (106); motion threshold computing means (204) for computing a motion threshold; motion decision means (206) for computing a motion indicator which represents whether there is motion for a third group of pixels (108) by means of comparing the match error with the motion threshold; and interpolation means (208) for computing values of the third group of pixels (108), the interpolation means (208) being controlled. on basis of the motion detection means (206). The motion threshold computing means (204) is arranged to compute the motion threshold on basis of a further motion indicator which represents whether there is motion for a fourth group of pixels. (110) being located in a neighborhood of the third group of pixels (108).

Description

De-interlacing

The invention relates to a motion adaptive de-interlacing unit, comprising: match error computing means for computing a match error on basis of comparing a first group of pixels of a first video field with a second group of pixels of a second video field; - motion threshold computing means for computing a motion threshold; motion decision means for computing a motion indicator which represents whether there is motion for a third group of pixels by means of comparing the match error with the motion threshold; and interpolation means for computing values of the third group of pixels, the interpolation means being controlled on basis of the motion detection means. The invention further relates to an image processing apparatus comprising such a motion adaptive de-interlacing unit. The invention further relates to a computer program product to be loaded by a computer arrangement, comprising instructions for de-interlacing a sequence of video fields into a sequence of video frames, the computer arrangement comprising processing means and a memory, the computer program product, after being loaded, providing said processing means with the capability to carry out: computing a match error on basis of comparing a first group of pixels of a first one of the video fields with a second group of pixels of a second one of the video fields; - computing a motion threshold; computing a motion indicator which represents whether there is motion for a third group of pixels by means of comparing the match error with the motion threshold; and computing values of the third group of pixels on basis of the motion indicator. The invention further relates to a method of motion adaptive de-interlacing comprising: computing a match error on basis of comparing a first group of pixels of a first video field with a second group of pixels of a second video field; computing a motion threshold; computing a motion indicator which represents whether there is motion for a third group of pixels by means of comparing the match error with the motion threshold; and computing values of the third group of pixels on basis of the motion indicator.

An embodiment of the de-interlacing unit of the kind described in the opening paragraph is known from European patent application EP 0395263 A2. This patent application describes a motion adaptive progressive scan conversion. Progressive scan conversion can be carried out in a number of ways, such as by previous field replacement, median filtering in which three spatially consecutive lines are examined (temporally these three lines will come from two consecutive fields), or a motion compensated technique which utilizes multi gradient motion detection followed by multidirectional linear interpolation. In the cited patent application is disclosed to use inter-field interpretation in wholly static picture areas to retain as much vertical information as possible, and to use intra-field interpolation when significant motion is present. To detect whether there is motion, groups of pixels from consecutive images are compared in order to compute respective match errors. These match errors are compared with a motion threshold. The motion threshold is not constant but adapted to the local frequency content of the picture. If the frequency content is relatively high then also the motion threshold is relatively high. Unfortunately, false detections occur, i.e. motion is detected although there is no motion and vice versa. Consequently, an inappropriate interpolation is selected for the corresponding part of the picture resulting in a reduced quality.

It is an object of the invention to provide a de-interlacing unit of the kind described in the opening paragraph which provides output frames with a relatively high quality. This object of the invention is achieved in that: - the motion threshold computing means is arranged to compute the motion threshold on basis of a further motion indicator which represents whether there is motion for a fourth group of pixels being located in a neighborhood of the third group of pixels; or that the match error computing means is arranged to compute the match error on basis of a further motion indicator which represents whether there is motion for a fourth group of pixels being located in a neighborhood of the third group of pixels. The de-interlacing unit according to the invention comprises two alternative embodiments with an equivalent effect. One of the parameters to be compared in order to detect whether there is motion or not, i.e. the match error or the motion threshold is adapted on basis of a further motion indicator which represents whether there is motion for a fourth group of pixels. The fourth group of pixels is located in a spatial neighborhood of the third group of pixels or in a temporal neighborhood of the third group of pixels. In other words, the computation of the motion threshold or alternatively the computation of the match error is based on decisions about motion in the spatial and/or spatio-temporal environment of the third group of pixels. An advantage of the de-interlacing unit according to the invention is that the decision about motion or not is less sensitive to noise. Even with a poor local signal to noise ratio, the de-interlacing unit according to the invention is able to determine whether there is motion or not and consequently is able to select the type of interpolation. Preferably, the motion threshold computing means is arranged to compute the motion threshold on basis of multiple motion indicators corresponding to multiple groups of pixels being located in the neighborhood of the third group of pixels. Alternatively, the match error computing means is arranged to compute the match error on basis of multiple motion indicators corresponding to multiple groups of pixels being located in the neighborhood of the third group of pixels. It should be noted, that the multiple motion indicators corresponding to the multiple groups of pixels are on their turn also based on further motion indicators corresponding to further groups of pixels in the neighborhood of the multiple groups of pixels. That means that the computation of the motion threshold or the match error, and consequently of the motion indicator is recursive. In an embodiment according to the invention, the motion decision means outputs the motion indicator with a value corresponding to motion if the match error is larger than the motion threshold and the motion threshold is relatively low if a further value of the further motion indicator corresponds to motion. That means that detected motion in the neighborhood of the third group of pixels results in a "decrease" of the motion threshold which is used for the detection of motion for the third group of pixels. By decreasing the motion threshold, i.e. selecting a relatively low motion threshold, the probability that the match error is higher than the current motion threshold, is relatively high. Consequently, the probability that the motion indicator has a value corresponding to motion is also high for the third group of pixels. In this embodiment according to the invention, the motion threshold is relatively high if the further value of the further motion indicator corresponds to no motion. Consequently, the probability that the match error is higher than the current motion threshold, is relatively low and thus the probability that the motion indicator has a value corresponding to motion will be relatively low for the third group of pixels. In an embodiment according to the invention, the match error computing means is arranged to compute the match error by calculating a sum of differences between respective pixel values of the first group of pixels and the second group of pixels. For example, the match error might be the Sum of Absolute Difference (SAD). This match error is a relatively good measure for establishing a match between image parts and which does not require extensive computations. In an embodiment according to the invention, the interpolation means is arranged to perform: an intra-field interpolation if a first value of the motion indicator corresponds with motion for the third group of pixels; and an inter-field interpolation if a second value of the motion indicator corresponds with no motion for the third group of pixels. Alternatively, the interpolation means is arranged to perform an intra-field interpolation for the third group of pixels or an inter-field interpolation for the third group of pixels, the interpolation based on a final motion indicator which is based on the motion indicator and an other motion indicator which represents whether there is motion for another group of pixels. That means that not only the motion indicator for the third group of pixels is applied to decide between the two types of interpolation but also motion indicators being computed for further groups of pixels. Therefore the embodiment of the de-interlacing unit according to invention comprises a combination unit for combining the motion indicator with the other motion indicator into the final motion indicator. It should be noted that there is a difference between using motion indicators belonging to other groups of pixels for the computation of the current motion threshold and using motion indicators belonging to other groups of pixels for the decision between the type of interpolation to be performed. In the former case there is recursion while in the latter case there is no recursion. Because of the control of the motion threshold according to the invention, a robust motion detection is achieved. The motion indicator field, i.e. the two dimensional structure of motion indicators is relatively homogeneous or uniform. Typically a particular motion indicator will be equal to the motion indicators in its environment. Only if the corresponding match error has an extreme value, then the value of the particular motion indicator will be different from the motion indicators in its environment. The motion indicator field will in particular be determined on basis of the groups of pixels for which the presence or absence of motion can be determined with a relatively high robustness. Areas with relatively much noise and/or relatively small objects having a specific speed have only a limited influence on the motion indicator field. In an embodiment according to the invention, the motion threshold computing means is arranged to compute the motion threshold on basis of contrast within the first group of pixels. An advantage of this embodiment according to the invention is that the motion detection is adapted to the image content. Typically, the motion threshold is relatively high if the contrast within the first group of pixels is relatively high and the motion threshold is relatively low if the contrast within the first group of pixels is relatively low. With contrast is meant a value based on differences between values of pixels of the first group of pixels. In an embodiment according to the invention, the motion threshold computing means is arranged to compute the motion threshold on basis of a noise measurement. It is a further object of the invention to provide an image processing apparatus of the kind described in the opening paragraph which provides output frames with a relatively high quality. This object of the invention is achieved in that: - the motion threshold computing means of the de-interlacing unit is arranged to compute the motion threshold on basis of a further motion indicator which represents whether there is motion for a fourth group of pixels being located in a neighborhood of the third group of pixels; or that the match error computing means of the de-interlacing unit is arranged to compute the match error on basis of a further motion indicator which represents whether there is motion for a fourth group of pixels being located in a neighborhood of the third group of pixels. The image processing apparatus may comprise additional components, e.g. a display device for displaying the output images. The image processing unit might support one or more of the following types of image processing: Video compression, i.e. encoding or decoding, e.g. according to the MPEG standard. Image rate conversion: From a series of original input images a larger series of output images is calculated. Output images are temporally located between two original input images; and Temporal noise reduction. This can also involve spatial processing, resulting in spatial-temporal noise reduction. The image processing apparatus might e.g. be a TV, a set top box, a VCR (Video Cassette Recorder) player, a satellite tuner, a DVD (Digital Versatile Disk) player or recorder. It is a further object of the invention to provide a computer program product of the kind described in the opening paragraph which provides output frames with a relatively high quality. This object of the invention is achieved in that: computing the motion threshold is based on a further motion indicator which represents whether there is motion for a fourth group of pixels being located in a neighborhood of the third group of pixels; or that computing the match error is based on a further motion indicator which represents whether there is motion for a fourth group of pixels being located in a neighborhood of the third group of pixels. It is a further object of the invention to provide a method of the kind described in the opening paragraph which provides output frames with a relatively high quality. This object of the invention is achieved in that: computing the motion threshold is based on a further motion indicator which represents whether there is motion for a fourth group of pixels being located in a neighborhood of the third group of pixels; or that - computing the match error is based on a further motion indicator which represents whether there is motion for a fourth group of pixels being located in a neighborhood of the third group of pixels. Modifications of the de-interlacing unit and variations thereof may correspond to modifications and variations thereof of the image processing apparatus, the method and the computer program product, being described.

These and other aspects of the de-interlacing unit, of the image processing apparatus, of the method and of the computer program product, according to the invention will become apparent from and will be elucidated with respect to the implementations and embodiments described hereinafter and with reference to the accompanying drawings, wherein: Fig. 1 schematically shows portions of three consecutive video fields; Fig. 2 schematically shows an embodiment of the de-interlacing unit according to the invention; Fig. 3 schematically shows an other embodiment of the de-interlacing unit according to the invention; Fig. 4 schematically shows an other embodiment of the de-interlacing unit according to the invention, having a controllable match error computing unit; and Fig. 5 schematically shows an image processing apparatus according to the invention. Same reference numerals are used to denote similar parts throughout the figures.

Interlacing is the common video broadcast procedure for transmitting the odd or even numbered video frame lines alternately, i.e. field by field. De-interlacing attempts to restore the full vertical resolution, i.e. make odd and even lines available simultaneously for each video frame. Fig. 1 schematically shows portions of three consecutive video fields 100, 102 and 106. Fig. 1 also shows an output frame 104. The blocks, e.g. 108-114 which are depicted in Fig. 1 correspond to groups of pixels. Each group of pixels comprises e.g. 8 pixels. A group of pixels might also comprises only one pixel. The blocks which are depicted with solid lines correspond to pixel values which are actually present in the video signal as received, i.e. the input video fields. The blocks which are depicted with dashed lines correspond to pixel values which have to be computed on basis of the pixel values which are actually present in the video signal as received. The combination of actually received pixel values and computed pixel values together form the output video frame 104. The odd video lines y-3, y-1, y+1, y+3 of the output frame 104 are copies from the video lines of the current video field 102. The depicted portions of the even video lines y- 2, y, y+2, y +4 are based on corresponding video lines from one or more other video fields if no motion has been detected for these portions. This type of interpolation is called inter-field interpolation. However if motion has been detected, then also these even video lines y-2, y, y+2, y +4 are based on the video lines of the current video field 102. Preferably, the pixel values of the portions of the even video lines of the output frame 104 are computed by means of edge dependent, i.e. orientation dependent interpolation. This latter type of interpolation is called intra-field interpolation. In order to decide which type of interpolation, i.e. inter-field interpolation or intra field interpolation, has to be selected for a particular pixel or group of pixels, it is important to determine whether there is motion for the particular pixel or the group of pixels. Below the method according to the invention will be explained for a particular group of pixels 108. The particular group of pixels 108 is located at a particular spatial position x,y. The method according to the invention comprises the following steps: a match error E(x,y) is computed on basis of comparing a first group of pixels 114 of a first video field 100 with a second group of pixels 112 of a second video field 106. The spatial position of the first group of pixels 114, the spatial position of the second group of pixels 112 and the spatial position of the particular group of pixels 108 are mutually equal, i.e. x,y. The match error E(x,y) is computed by means of calculating a sum of absolute differences between respective pixel values of the first group of pixels 114 and pixel values of the second group of pixels 112. Alternatively, first a low pass filtering is performed; a motion threshold T(x,y) is computed for the particular group of pixels 108. Preferably, the motion threshold T(x,y) is based on a number of components: a predetermined constant, a noise dependent component, a contrast dependent component and an environment dependent component. Most of these components are optionally but the latter one is obligatory. The different optional components are explained in more detail in connection with Fig. 2. The environment dependent component is explained in more detail below; - a motion indicator M(x,y) which represents whether there is motion for the particular group of pixels 108 is computed by means of comparing the match error E(x,y) with the motion threshold T(x,y). If the match error E(x,y) is larger than the motion threshold T(x,y) then it is assumed that there is motion for the particular group of pixels 108. So, the motion indicator M(x,y) has a value corresponding with motion, e.g. M(x,y) =1. However, if the match error E(x,y) is lower than the motion threshold T(x,y), then it is assumed that there is no motion for the particular group of pixels 108. Consequently the motion indicator M(x,y) has a value corresponding with no motion, e.g. M(x,y)=0. The values of the pixels of the particular group of pixels are computed by means of interpolation. The type of interpolation depends on the computed value of the motion indicator M(x,y), as explained above. As said, the motion threshold T(x,y) comprises an environment dependent component. That means that the values of the motion indicators M(x,y) being computed for other groups of pixels, e.g. a fourth group of pixels 110, are taken into account to compute the current motion threshold T(x,y) belonging to the particular group of pixels 108. The basic rule is that the values of a predetermined number of motion indicators in the spatial and/or temporal environment of the particular group of pixels 108 are evaluated. For instance the values of the following motion indicators are evaluated: M(x-3, y), M(x-1, y), M(x, y-2), M(x+2, y-2), M(x, y+3). If the number of motion indicators in this set having a value corresponding to motion is relatively high, then the motion threshold T(x,y) for the particular group of pixels 108 is relatively low. If the number of motion indicators in this set having a value corresponding to no motion is relatively high, then the motion threshold T(x,y) for the particular group of pixels 108 is relatively high. It should be noted that, although only the motion indicators of a limited set are directly taken into account to determine the motion threshold T(x,y) for the particular group of pixels 108, the actual influence of previous motion indicators is much bigger. This is because of recursion, since the previous motion indicators are on their turn based on other motion indicators which have been determined before. Fig. 2 schematically shows an embodiment of the de-interlacing unit 200 according to the invention. The motion adaptive de-interlacing unit 200 comprises: a match error computing unit 202 for computing match errors on basis of comparing groups of pixels of different video fields; - a motion threshold computing unit 204 for computing motion thresholds; a motion decision unit 206 for computing motion indicators which represents whether there is motion for a group of pixels, for which the values have to be computed by means of interpolation. The motion decision unit 206 is provided with the match errors being computed by the match error computing unit 202 and is provided with the motion thresholds being computed by the motion threshold computing unit 204. The motion decision unit 206 is arranged to compute the motion indicators by means of comparing the match errors with the corresponding motion thresholds; and an interpolation unit 208 for computing pixel values. The interpolation unit 208 is controlled by the motion decision unit 206. The interpolation unit 208 is provided with input video fields and provides video frames at its output connector 216. The interpolation unit 208 is arranged to switch per pixel or group of pixels between inter-field interpolation and intra field interpolation on basis of the values of the provided motion indicators. The match error computing unit 202, the motion threshold computing unit 204, the motion decision unit 206 and the interpolation unit 208 may be implemented using one processor. Normally, these functions are performed under control of a software program product. During execution, normally the software program product is loaded into a memory, like a RAM, and executed from there. The program may be loaded from a background memory, like a ROM, hard disk, or magnetically and/or optical storage, or may be loaded via a network like Internet. Optionally an application specific integrated circuit provides the disclosed functionality. The de-interlacing unit 200 according to the invention is provided with consecutive video fields at its respective input connectors 210-214. The image processing apparatus in which the de-interlacing unit 200 is comprised, has a storage device for temporary storage of a number of video fields. This storage device is not depicted in Fig. 2. The match error computing unit 202 optionally comprises two horizontal low pass filters 218 and 220 for low pass filtering the input data. That means that low pass filtered pixel values of a first video field, typically the previous video field, are compared with low pass filtered pixel values of the second video field, typically the next video field. Preferably, comparing is based on computing differences. The reason for low pass filtering is to compensated for sync jitter. The motion threshold computing unit 204 comprises processing means 222- 226 for computing the different components of the motion thresholds. The motion threshold computing unit 204 comprises an environment evaluation unit 226 to evaluate the motion indicators in the environment, as described in connection with Fig. 1. Notice the feedback connection 228 by means of which values of motion indicators are provided from the motion decision unit 206 to the motion threshold computing unit, in particular the environment evaluation unit 226. The de-interlacing unit 200 comprises not depicted storage means for temporary storage of motion indicators. The motion threshold computing unit 204 preferably comprises a contrast computing unit 222 for computing local contrast. Preferably, mutual differences between pixel values within groups of pixels of multiple video fields are applied to compute the contrast values. Only a limited number of groups of pixels in the environment of the particular group of pixels under consideration are used for the calculation of the contrast value. If the contrast value is relatively high, then the corresponding motion threshold is also relatively high. If the contrast value is relatively low, then the corresponding motion threshold is also relatively low. The motion threshold computing unit 204 optionally comprises a noise computing unit 224 for computing a global noise value. Mutual differences between pixel values within groups of pixels are used to compute the global noise value. A limited number of groups of pixels being located at a number of locations within the video field are used. These groups of pixels are not necessarily located in a direct environment of the particular group of pixels. Preferably, the groups of pixels are located in regions where there is no motion. If the noise value is relatively high, then the motion thresholds are also relatively high. If the noise value is relatively low, then the motion thresholds are also relatively low. Fig. 3 schematically shows another embodiment of the de-interlacing unit 300 according to the invention. This embodiment of the de-interlacing unit 300 is almost equal to the de-interlacing unit 200 which is described in connection with Fig. 2. The only difference is the final decision unit 302. The control of the interpolation unit 208 is not only based on the current motion indicator M(x,y) for a particular group of pixels 108 being located at the spatial position (x,y) but is also based on other motion indicators in the environment. Preferably, the final motion indicator F(x,y) is based on motion indicators of adjacent groups of pixels, e.g. F(x,y) = M(x, y) * M(x-1, y) * M(x+1, y) * M(x, y-1) * M(x, y+1) where a "1" indicates motion and a "0" indicates no motion, i.e. still. Fig. 4 schematically shows another embodiment of the de-interlacing unit 400 according to the invention, having a controllable match error computing unit 202. This embodiment of the de-interlacing unit 400 is relatively similar to the de-interlacing unit 300 which is described in connection with Fig. 3. The major difference is the deployment and working of the environment evaluation unit 226. The match error computing unit 202 of the embodiment of the de-interlacing unit 400 comprises the environment evaluation unit 226. Notice the feedback connection by means of which values of motion indicators are provided from the motion decision unit 206 to the match error computing unit 202, in particular the environment evaluation unit 226. The de-interlacing unit 200 comprises not depicted storage means for temporary storage of motion indicators. The environment evaluation unit 226 is arranged to evaluate the motion indicators in the environment. However now the output of the environment interpolation unit 226 is opposite to the output as in the case of the de- interlacing unit 200 as described in connection with Fig. 1. The match error E(x,y) comprises a local component and an environment dependent component. The local component is based on pixel value differences of pixels being located at spatial position x,y. The environment dependent component is based on values of motion indicators being computed for other groups of pixels. That means that the values of the motion indicators being computed for other groups of pixels, e.g. a fourth group of pixels 110, are taken into account to compute the current match error E(x,y) belonging to the particular group of pixels 108. The basic rule is that the values of a predetermined number of motion indicators in the spatial and/or temporal environment of the particular group of pixels 108 are evaluated. For instance the values of the following motion indicators are evaluated: M(x-3, y), M(x-1, y), M(x, y-2), M(x+2, y-2), M(x, y+3). If the number of motion indicators in this set having a value corresponding to motion is relatively high, then the match error E(x,y) for the particular group of pixels 108 is relatively high. If the number of motion indicators in this set having a value corresponding to no motion is relatively high, then the match error E(x,y) for the particular group of pixels 108 is relatively low. It should be noted that, although only the motion indicators of a limited set are directly taken into account to determine the match error E(x,y) for the particular group of pixels 108, the actual influence of previous motion indicators is much bigger. This is because of recursion, since the previous motion indicators are on their turn based on other motion indicators which have been determined before. Fig. 5 schematically shows an image processing apparatus 500 according to the invention, comprising: receiving means 502 for receiving a signal representing input images; the de-interlacing unit 504 as described in connection with any of the Figs. 2 or 3 or 4; and - an optional display device 506. The signal may be a broadcast signal received via an antenna or cable but may also be a signal from a storage device like a VCR (Video Cassette Recorder) or Digital Versatile Disk (DVD). The signal is provided at the input connector 508. The image processing apparatus 500 might e.g. be a TV or a personal computer comprising a video signal receiving module. Alternatively the image processing apparatus 500 does not comprise the optional display device 506 but provides the output images to an apparatus that does comprise a display device 506. Then the image processing apparatus 500 might be e.g. a set top box, a satellite-tuner, a VCR player, a DVD player or recorder. Optionally the image processing apparatus 500 comprises storage means, like a hard-disk or means for storage on removable media, e.g. optical disks. The image processing apparatus 500 might also be a system being applied by a film-studio or broadcaster. 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 alternative embodiments without departing from the scope of the appended claims. In the claims, any reference signs placed between parentheses shall not be constructed as limiting the claim. The word 'comprising' does not exclude the presence of elements or steps not listed in a claim. The word "a" or "an" preceding an element does not exclude the presence of a plurality of such elements. The invention can be implemented by means of hardware comprising several distinct elements and by means of a suitable programmed computer. In the unit claims enumerating several means, several of these means can be embodied by one and the same item of hardware. The usage of the words first, second and third, etcetera do not indicate any ordering. These words are to be interpreted as names.

Claims

CLAIMS:

1. A motion adaptive de-interlacing unit (200) comprising: match error computing means (202) for computing a match error on basis of comparing a first group of pixels (114) of a first video field (100) with a second group of pixels (112) of a second video field (106); - motion threshold computing means (204) for computing a motion threshold; motion decision means (206) for computing a motion indicator which represents whether there is motion for a third group of pixels (108) by means of comparing the match error with the motion threshold; and interpolation means (208) for computing values of the third group of pixels (108), the interpolation means (208) being controlled on basis of the motion detection means (206), characterized in that: the motion threshold computing means (204) is arranged to compute the motion threshold on basis of a further motion indicator which represents whether there is motion for a fourth group of pixels (110) being located in a neighborhood of the third group of pixels (108); or that the match error computing means (202) is arranged to compute the match error on basis of a further motion indicator which represents whether there is motion for a fourth group of pixels (110) being located in a neighborhood of the third group of pixels (108).

2. A motion adaptive de-interlacing unit (200) as claimed in Claim 1, whereby the fourth group of pixels (110) is located in a spatial neighborhood of the third group of pixels (108).

3. A motion adaptive de-interlacing unit (200) as claimed in Claim 1, whereby the fourth group of pixels is located in a temporal neighborhood of the third group of pixels (108).

4. A motion adaptive de-interlacing unit (200) as claimed in Claim 1, whereby the motion decision means (206) outputs the motion indicator with a value corresponding to motion if the match error is larger than the motion threshold and whereby the motion threshold is relatively low if a further value of the further motion indicator corresponds to motion.

5. A motion adaptive de-interlacing unit (200) as claimed in Claim 1, whereby the match error computing means is arranged to compute the match error by calculating a sum of differences between respective pixel values of the first group of pixels (114) and the second group of pixels (112).

6. A motion adaptive de-interlacing unit (200) as claimed in Claim 1, whereby the interpolation means (208) is arranged to perform: an intra-field interpolation if a first value of the motion indicator corresponds with motion for the third group of pixels (108); and an inter-field interpolation if a second value of the motion indicator corresponds with no motion for the third group of pixels (108).

7. A motion adaptive de-interlacing unit (300) as claimed in Claim 1, whereby the interpolation means (208) is arranged to perfoπn an intra-field interpolation for the third group of pixels (108) or an inter-field interpolation for the third group of pixels (108), the interpolation based on a final motion indicator which is based on the motion indicator and an other motion indicator which represents whether there is motion for another group of pixels, the de-interlacing unit comprising a combination unit (302) for combining the motion indicator with the other motion indicator into the final motion indicator.

8. A motion adaptive de-interlacing unit (200) as claimed in Claim 1, whereby the motion threshold computing means (204) is arranged to compute the motion threshold on basis of contrast within the first group of pixels (114).

9. A motion adaptive de-interlacing unit (200) as claimed in Claim 1, whereby the motion threshold computing means (204) is arranged to compute the motion threshold on basis of a noise measurement.

10. An image processing apparatus (500) comprising: receiving means (502) for receiving a signal corresponding to a sequence of video fields; and a motion adaptive de-interlacing unit (200) for calculating a sequence of video frames on basis of the sequence of video fields, the motion adaptive de-interlacing unit (200) as claimed in Claim 1.

11. A computer program product to be loaded by a computer arrangement, comprising instructions for de-interlacing a sequence of video fields into a sequence of video frames, the computer arrangement comprising processing means and a memory, the computer program product, after being loaded, providing said processing means with the capability to carry out: computing a match error on basis of comparing a first group of pixels (114) of a first one of the video fields with a second group of pixels (112) of a second one of the video fields; computing a motion threshold; computing a motion indicator which represents whether there is motion for a third group of pixels (108) by means of comparing the match error with the motion threshold; and - computing values of the third group of pixels (108) on basis of the motion indicator, characterized in: computing the motion threshold on basis of a further motion indicator which represents whether there is motion for a fourth group of pixels (110) being located in a neighborhood of the third group of pixels (108); or in computing the match error on basis of a further motion indicator which represents whether there is motion for a fourth group of pixels (110) being located in a neighborhood of the third group of pixels (108).

12. A method of motion adaptive de-interlacing comprising: computing a match error on basis of comparing a first group of pixels (114) of a first video field (100) with a second group of pixels (112) of a second video field (106); computing a motion threshold; computing a motion indicator which represents whether there is motion for a third group of pixels by means of comparing the match error with the motion threshold; and computing values of the third group of pixels (108) on basis of the motion indicator, characterized in: - computing the motion threshold on basis of a further motion indicator which represents whether there is motion for a fourth group of pixels (110) being located in a neighborhood of the third group of pixels (108); or in computing the match error on basis of a further motion indicator which represents whether there is motion for a fourth group of pixels (110) being located in a neighborhood of the third group of pixels (108).