METHOD AND DEVICE FOR NOISE REDUCTION

Abstract: A method, and apparatus and a computer program product of reducing noise in a video signal which includes a plurality of video frames being composed of a plurality of pixels, the method comprising the steps of: comparing video information contained in a current video frame and a plurality of temporally adjacent video frames; selecting from said current video frame and said adjacent video frames the video information that according to a predetermined condition is likely to be correct for said current video frame; and finally assigning the selected video information to the current video frame to thereby produce a video frame wherein noise has been reduced.

Description: Method and Device for Noise Reduction Field of the Invention The present invention relates generally to noise reduction in digitized pictures and specifically to the reduction of noise in a video signal. Background of the Invention Noise reduction of a video signal is used on one hand to enhance the quality of images comprised in the video signal and on the other hand to prepare for an efficient compression of the video signal. Noise reduction is in fact of particular importance in connection with compression of image information, since noise in an image sequence may significantly reduce the effectiveness of compression schemes, such as frequency domain compression, e.g. MPEG-2. In image compression technology there is, however, a dualism between compression factor and image quality, and these aspects are not always easily reconcilable. Nevertheless, an effective noise reduction in connection with compression of a video signal may well serve both above mentioned purposes and produce enhanced images in addition to a well compressed video signal. The technical background of compression and different state of the art pre- processing and compression techniques are for example described in John Watkinson, Compression in Video & Audio, Focal Press 1995, ISBN 0 240 51394, which is hereby incorporated by reference. A previously known video signal noise reduction system is described, for example in US patent no. 5,361,105 to Siu-Leong Iu, the content of which is hereby incorporated by reference. In this system, image pixels are tracked across multiple frames and then averaged to produce respective noise reduced pixel values. In video signals representing a sequence of images that may change from frame to frame, e.g. in motion pictures and television, the mentioned prior art seeks to reduce noise by estimating the magnitude and direction of inter- frame motion in a sequence of image frames. The movement of an image is estimated by first calculating a trajectory vector for a block of picture elements (pixels) by comparing preceding and succeeding frames. The motion vector, i.e. the trajectory vector, is then used to reduce noise in the video signal by averaging each of a plurality of pixels corresponding to a moving block along an estimated trajectory. The described method, however, does not eliminate noise to a satisfactorily sufficient degree. An example of a drawback with the method described in said US 5,361,105 is that the trajectory estimation, which is a part of this noise reduction method, itself is sensitive to the noise. Another piece of prior art, viz. US patent no. 4,987,481 to Spears et al., which is hereby incorporated by reference, shows an apparatus for selectively reducing noise by non-recursively averaging video information contained in a sequence of video frames when the video information is found to be impaired by noise. A drawback with this method is the consequence of the averaging between video frames, namely that the video information of a noise reduced frame portion is calculated and original video information is lost even if it is correct and unimpaired by noise. Furthermore, with the above procedure there is a high probability for different values to appear in sequence, which in its turn leads to poorer resulting compression rate. Summary of the Invention The object of and the problem to be solved by the present invention is to improve the reduction of noise in a video signal representing a sequence of video frames, in particular a video signal representing moving pictures. An aspect of this problem is to achieve an enhanced compression factor for a compression scheme applied to the video signal, or in a different wording to reduce the required bit rate of the video signal. A further aspect is to achieve an enhanced perceived image quality, preferably combined with an enhanced or at least not reduced compression factor. The invention is based on the inventors realisation that there is a mutual relationship or a correlation between video information of frame portions within an observed video frame on one hand, i.e. a spatial correlation, and between video information of frame portions from a sequence of adjacent observed video frames, i.e. a correlation along the time axis of the frame sequence. The correlation is strong in sequences of frame portions where there is no local scene change or there is no movement in the image. Technically, a local scene change in this context means that the video information changes significantly between subsequent frame portions. In order to distinguish between a pixel that represents a local scene change, e.g. an edge of a moving object within the image, and a pixel constituting a randorn noise spike, the correlation between the current pixel under consideration and its surrounding pixels are analysed. Noise on the current pixel is then suppressed on the basis of the correlation analysis. If there is a weak correlation, a local scene change is assumed to take place between two compared frame portions, and no attempt is made to reduce noise on this pixel. If on the other hand a strong correlation is found between two compared frame portions, both frame portions qualify for participating in a following selecting step and a possible subsequent noise reduction. For a 2-dimensional (2D) video signal the correlation analysis is carried out in three dimensions, i.e. with respect to the surrounding pixels that are adjacent to the current pixel in the spatial domain and in the time domain, respectively. According to the invention the noise on a pixel is preferably replaced by a maximum likelihood signal based on the correlated pixels. Worded differently, from a subset of the values of the correlated pixels, the pixel that has a value which is most likely to have a similar predecessor or successor is selected to replace the current pixel. Each video frame is processed under consideration of a sequence of video frames temporally adjacent to the current video frame. The temporally adjacent frames may in different embodiments be ahead of, come after or surround the current video frame. In a preferred embodiment, the processing is made for each pixel of a current video frame by observing a frame portion consisting of the current pixel and a number of surrounding, spatially adjacent pixels. Such a portion is in this text also called a slice S. Each current slice, or more specifically the video information of the slice, is compared to equal sized spatially corresponding slices of the adjacent frames. Such a set of temporally consecutive slices is herein also called a tube T. The tube is analysed in order to find the number of slices adjacent the current slice that have a particularly strong correlation, i.e. the number of consecutive slices wherein there is no local scene change. Having selected a scene change free subset of temporally consecutive slices, the current pixel is compared to the spatially corresponding but temporally adjacent pixels of the selected subset of slices and the pixel that is most likely to have a preceding or subsequent pixel having the same value or video information content is selected as a new current pixel. In a different wording, an extreme pixel value is sorted out and is replaced by a pixel selected from a set of adjacent pixels and being judged to have a better correlation to the surrounding pixels. The selected pixel is then assigned to the current pixel. In one embodiment a new sequence of video frames is produced in the noise reduction process, whereas in another embodiment the current video frame is replaced by the new frame which thus may contribute in the analysis and processing of the next current video frame. When the invention is applied as a pre-processing for compression of an image signal, an object of the invention is to produce sequences of pixels having the same value. In advantageous cases, this leads to a number of subsequent similar or identical blocks of e.g. 8x8 pixels which, when encoded in a subsequent coding stage, are extensively compressed. In a further developed embodiment of the invention, account is also taken of the fact that the human eye has a different sensitivity to the different colours red, green and blue. Noise impaired pixels are then discriminated with regard to a dynamically computed noise threshold value, such that the precision in noise detection/scene change detection is adapted to the characteristics of the human vision. For example, detailed changes are hard to detect or are even undetectable in dark and in very bright areas. By taking the human eye sensitivity into consideration while computing the noise threshold value, the bit rate may be reduced further in regions of frequency spectra, or colour space where the human vision system is insensitive to small differences than in sensitive regions An image can thereby be achieved which is subjectively perceived by the eye as being enhanced with respect to noise despite the fact that video information actually is missing or noise actually is left in parts of the picture. In other embodiments a predetermined or fixed threshold value is selected and used for a selected range or for the whole frequency range of the picture. Further advantages and details of the invention will be seen from the following description of an embodiment of the invention with the aid of the accompanying drawings and in connection with the independent and dependent claims. Brief Description of the Drawings: Fig 1A shows a sequence of video frames demonstrating a prior art trajectory as well as the tube and slice concept of the invention; Fig IB and 1C show video frame storage means; Fig ID shows an example of a slice used in one embodiment of the invention; Fig 2 shows a general view of an embodiment of the inventive noise reduction apparatus; Fig 3 shows a general view of another embodiment of the inventive noise reduction apparatus; Fig 4 shows a general view of a third embodiment of the inventive noise reduction apparatus. Detailed Description of Embodiments Referring to Fig 1 A, a video signal or an image signal comprises a sequence of video frames F wherein image information is stored in a spatial domain. In moving images, the video information basically changes from frame to frame along a time axis t. In the inventive method, a video frame of digitized video information represented in a plurality of pixels P is processed portion by portion, where a frame portion typically would comprise at least one currently processed pixel and a number of adjacent pixels. For the sake of comparison, a prior art trajectory Traj of frame portions FP is shown in the upper part of Fig 1A. A trajectory is commonly known as a description of the path of a moving image object moving across the frames as a function of time. Such a trajectory is estimated in the previously mentioned prior art US 5,361,105, where the noise reduction is carried out by first tracking moving image pixels to estimate the trajectory and then producing a supposedly noise reduced pixel value for the tracked pixel, entailing i.a. the above mentioned drawbacks. Conversely, the invention preferably carries out the noise reduction before any motion estimation comprised in a following compression. According to the invention (Cf. also Fig 1A), a number of frame portions herein called slices S, preferably comprising a number of pixels P, having corresponding spatial positions in a sequence of temporally adjacent frames F are observed at the time. Such a set of slices is, as mentioned above, called a tube T and it is clear from Fig 1 A that a tube T is parallel to the time axis t. Each pixel is associated with some kind of colour data. e.g. for each of the primary colours red R, green G and blue B, also referred to as RGB. Naturally, other image data or colour representations are also possible within the inventive concept. For the purpose of the description of the invention, the following expressions and corresponding mathematical notation are used. The meaning of the expressions is also clarified by means of Fig 1A, and Fig ID . Frame: F(t) = {R(t), G(t), B(t)} is a frame signal matrix of RGB values at the t: th frame. Pixel: P(iJ,t)={R(iJ,t), G(i,j,t), B(i,j,t)} e F(t) is a picture element in frame F(t), with the position co-ordinates i,j in the t:th frame. Luminance: Y(i,j,t) is the luminance of the pixel P(i,j,t). Slice: S(i,j,t) is a collection of pixels forming a subset of the t:th frame. It forms a designed spatial window or 'slice'. In preferred embodiments, it includes the centre pixel P(i,j,t) and some surroundings pixels P(i',j',t). The notation i',j',t' means a position or time reference being adjacent to a currently processed pixel. Referring to FIG ID, it is shown an example of a slice in the form of a cross having a current pixel Pij in the centre position. Window: W(t)={F(τ ) I t-d < τ < t+d} is a set of consecutive frames, where d is a positive integer. F(t) is the centre frame and (2d+l) is the size of a sliding 'time-window' of consecutive frames. Tube: T(ij,t)= {S(i,j,τ) I t-d < τ < t+d} is a collection of slices, or a three dimensional 'time-tube' of pixels. It typically consists of (2d+l) consecutive 'slices', one from each of the consecutive frames belonging to the time-window W(t). In one embodiment, a tube typically includes the centre 'slice' S(i,j,t) and its adjacent in time 2d 'slices' S(i,j,t'). In the inventive method, each pixel is processed basically by: -determining a number of temporally adjacent pixels that have a strong correlation, i.e. that are not involved in a local scene change; -selecting from the set of strongly correlated temporally adjacent pixels, a pixel value that is most likely to have a predecessor or successor having an equal or similar value; and by -replacing the currently processed pixel with the selected pixel. The determining of the number of correlated pixels, i.e. the scene change analysis, is in a preferred embodiment carried out by observing a slice containing the current pixel and spatially adjacent pixels, i.e. a slice S(i,j,t), as well as a number of spatially corresponding but temporally adjacent slices S(i,j,t'). The slices may for example consist of a centre pixel P(i,j) and a selected number of the closest adjacent pixels P(i'j') forming e.g. a square, a cross or any other selected geometric shape of pixels. A more detailed exemplifying embodiment of the invention comprises the following features and steps of: -Receiving as an input a video signal which includes a plurality of video frames of digitized video information represented in a plurality of pixels. -Storing a current video frame and a plurality of temporally adjacent video frames. Typically an uneven number of video frames, e.g. 7 frames, are stored for the processing of a centrally positioned currently processed frame. -Selecting from the current video frame a current frame portion, i.e. a slice Si,j,t, comprising a current pixel Pi,j and at least one pixel spatially adjacent to the current pixel. In a preferred embodiment, cross shaped slices as shown in FIG ID are used assuming there is at least some degree of spatial correlation between the pixels of the slice. -Selecting from the temporally adjacent video frames temporally adjacent frame portions, i.e. slices, comprising a pixel or pixels that spatially corresponds to the pixels of the current frame portion. In other words, a collection of consecutive slices Si,j,t' making up a tube T. -Determining the luminance value of each pixel in said current frame portion and said adjacent frame portions, i.e. in all slices of the selected tube. -Determining the average luminance value of said current frame portion and said temporally adjacent frame portions, i.e. by averaging the luminance values of the pixels comprised in said frame portions. The foregoing steps can be seen as a preparation for making a following correlation analysis, and more specifically to determine whether a local scene change occurs between the slices of the tube. This is done by comparing said current frame portion and each of said adjacent frame portions by: -determining and comparing to a first threshold value the absolute value of the difference between the average luminance of said current frame portions and the average luminance of said adjacent frame portion; -if the absolute value of the luminance difference exceeds said first threshold value, considering there is a local scene change in the video information between said current frame portion and said adjacent frame portion. If latter procedure indicates that there is a large difference in luminance between two adjacent frames, it is thus considered that a local scene change takes place between the analysed current slice and the adjacent slice under consideration. If on the other hand, the luminance difference is small a further correlation test is made in the following steps: -determining and comparing to a second threshold value the absolute values of the difference between the values of a colour component of said current pixel and the corresponding pixel of said adjacent frame portion; -if the absolute value of the colour component value difference exceeds said second threshold value, considering there is a local scene change in the video information between said current frame portion and said adjacent frame portion. In fact this double test involving colour information is necessary to detect local scene changes where the luminance is more or less continuous despite a colour change, which in effect is a local scene change. If the two above threshold tests are passed it is decided or considered that no local scene change occur between said current slice and said adjacent slice, the next p k air of current and adjacent slices are analysed until the whole tube has been processed. Thereafter, it is from the tube selected an even number exceeding or equalling zero of adjacent frame portions considered to involve no local scene change in relation to said current frame portion. This could also be expressed as selecting the largest possible sub-tube or sub-set of input adjacent frames that do not involve a local scene change. For example, if the input tube comprises 7 slices, a subset consisting of 0, 3, 5 or 7 slices can be selected, when as in this case a number of slices symmetrically surrounding the current slice are analysed. Thereafter, the actual noise reducing steps takes place. This is preferably done by selecting the video information from a pixel that is probable to have equal successors for example by: -comparing the luminance values of said current pixel and the corresponding pixels in said selected adjacent frame portions and selecting the video information of the pixel having the median value of said luminance values; -assigning said selected video information to the current pixel to thereby produce a video frame wherein noise. The noise reduction is in different applications and embodiments carried to enhance image quality or to achieve a required compression rate in an encoded image signal. In advantageous cases the two purposes may even be combined. By selecting the median value of the set of temporally adjacent pixels, outliers such as noise spikes are filtered out and it is more likely that a current pixel taking on the median value of a sequence of pixels will have preceding or succeeding pixels having the same value. Fig IB shows a memory means 2, for example a shift register, which is devised to hold a number N+1 of first frames FtO-FtN of an input video signal. Assume that FtO is the currently processed frame and Ftl-FtN the N following frames, which are also called 'look-ahead' frames. Fig 1C shows a similar memory means 3 devised to hold a number N+1 of pixels PtO-PtN or a number of slices S(t- d) to S(t+d), d being a positive integer. In the same manner assume that PtO is the currently processed pixel and Ptl to PtN a number N look ahead pixels. Each pixel P contains colour information, e.g. RGB amplitude values. In another embodiment of the invention, the memory holds a number of precedent frames F(t-d) and subsequent frames F(t+d) in addition to the current frame F(tO). Another embodiment of the invention comprises the following steps, which are repeated and carried out for each frame in a stream of frames of an input video signal. a) The components of each pixel, in the exemplifying embodiment red, green and blue, of a currently processed frame are individually compared to preferably all of the N succeeding values of the corresponding pixel in the subsequent look- ahead frames. If the differences in amplitude between a currently processed pixel PtO and a number of, and preferably all of, its succeeding pixels are below a predetermined threshold level, the value of the next pixel Ptl is copied to PtO, otherwise PtO is left unchanged. Different threshold levels are specified and used for the different pixel components, thus taking account of differences in the sensitivity of the human eye to different wavelengths of the light. The fact that amplitude differences are compared to a minimum threshold value entails a discrimination between noise- impaired pixels and pixels belonging to a moving part of the image represented by the frame, whereby small changes in amplitude are reduced or eliminated. The choice of threshold level is dependent on the application of the invention, and it is within the scope of the invention to select or adapt threshold to other parameters than human eye sensitivity. b) When all pixels P in FtO have been processed according to a) above, frame FtO is transferred to an output. Using the exemplifying shift registers of Fig I and 1C, the other N frames Ftl to FtN are shifted to the left, so that Ftl is copied to FtO, Ft2 is copied to Ftl and so on. A new FtN is taken into the memory means 2 from an input frame stream of a video signal. If there are no more frames at the input, the new FtN inherits the values of the old FtN. Thereafter the next frame is processed according to a) above. A further embodiment of the inventive method described in a mathematical notation may comprise the following steps for producing a noise filtered output pixel Pout(i,j,t): Step 1: Calculating a luminance matrix{Yin (i\j',t') I V (i',j',t')eT} Step 2: Calculating the matrix of a indicate function I for the luminance differences between adjacent pixels, where {i(i',j',t') | v (i',j',t')eτ} and where fθ if |Yin (i,j,t) - Yin (i',j',t')| < E I (i',j\t') 1 otherwise and E is the value of a designed threshold function f thr of the selected parameter components, in this example the colour components, of the input pixel. E=f thr {Pin (i',j',t')e Win (t)} If I(i',j',t') = 1 the pixel is called an unequal pixel. Step 3: Calculating the number of unequal pixels in each slice S (i,j,τ) for τ = t-d, t-d+1,..., t+d separately μ i,j (τ)= ∑ I (i',j',τ) (i',j') e S (i,j,τ) Step 4: Calculating the number of unequal pixels in the three dimensional 'tube' T (i,j,t) t+d λ i,j = ∑ μ i,j(τ) τ=t-d Step 5: Determining the output pixel Pin (i,j,t) if i,j ≤ Λ, tube and V(// i.j < μ slice) Pout (i,j,t) Pin (i, j , t) otherwise where λ tube and μ slice are designed thresholds and P in (i,j,t) = Pin (i,j,f) for some t' at which Yin (i,j,f) is the median value of the set {Yin (ij,τ) I t-d < τ < t+d} When all the output pixels Pout (i,j,t) e F out (t) for different indexes (i,j) are obtained, a noise reduced frame Fout (t) is completed. Then, a new frame is input and processed according to the above described steps. Fig 2 shows a general view of noise reduction means 4 devised in accordance with the inventive method. An input frame Fin belonging to a set W of consecutive frames, i.e. Fin (τ) e Win (t), comprising input pixels Pin (i,j, τ)e Fin (τ) is input to the noise reduction means 4. An output frame Fout (t) is obtained and output from the noise reduction means 4, when all output pixels Pout (i,j,t) e Fout (t) have been obtained in the noise reducing process. Fig 3 shows an overview of a noise reduction means 4, comprising memory means 2, 4 and processing means 6, the noise reduction means being shown in the environment of a processing system for an analogue video signal. Thus, an analogue video input signal 8 is fed to a digital sampling means 10, commonly called a frame grabber, which in this embodiment is coupled to a converting means 12 for separating the red, green and blue (RGB) components of each pixel of a sampled frame. An RGB-separated frame is then input to the noise reducing means 4. After noise reduction, an output frame Fout is fed from the noise reducing means 4 to a digital format converting means 14. which in one embodiment is provided with an output for outputting a digital video output signal 16 and in another embodiment is provided with an output for outputting either a frame F or a digital video signal to a per se known digital-to-analogue video converting means 18. In a third embodiment, the digital format converting means 14 is provided with both mentioned outputs. The digital-to-analogue video converting means 18 produces and outputs an analogue video output signal 20. The inventive method can be realised both by a direct hardware implementation or by a software implementation executed on an information processing apparatus, such as a work station, personal computer, or signal processor or the like. In any case, the inventive apparatus comprises ^ means for performing the above mentioned steps of the different embodiments of the inventive method and an embodiment of such an apparatus is shown in Fig 4. Thus, an embodiment of a noise reduction means 4, according to the invention and as shown in Fig 4 comprises an input 22 for receiving an input frame of a set of input frames Fin and memory means 2 for storing one or more input frames Fin. Further, memory means 3 is provided for storing one or more pixels of a frame F to be processed. The noise reduction means 4 further comprises means 24 for calculating a luminance matrix Y for a pixel matrix stored in the memory means 4, means 26 for calculating a matrix I of a luminance difference indicate function dependent on a predetermined and stored threshold function 27, means 28 for calculating the number of unequal pixels in a slice, means 30 for calculating the number of unequal pixels in a tube T, means 32 for determining an output pixel Pout and an output 34 for outputting one or more noise reduced frames Fout. These functional means may be arranged in especially designed hardware, in a general information processing system or on a storing medium. Further Embodiments This section describes further embodiments of the invention. A method of reducing noise in a video signal which includes a plurality of video frames being composed of a plurality of pixels, the method comprising the steps of: - determining at least one pixel component with respect to at least one pre-selected parameter; and for each of the plurality of frames: - determining whether a pixel belongs to a moving part of a frame; - determining whether said pixel is impaired by noise, and if noise is detected, adjusting the pixel component value or values dependent on said pre-selected parameter of parameters. In one embodiment, the pixel components are determined with respect to different wavelengths of light, such as red, green and blue light. In one embodiment, each pixel component values is adjusted dependent on a, preferably individual, threshold level selected dependent on the human eye sensitivity to each pixel component. In one embodiment, it is determined whether a pixel belongs to a moving part of a frame or is impaired by noise by: - individually comparing the component values of each pixel of a currently processed frame with the succeeding values of the corresponding pixel of at least one, and preferably a plurality, of subsequent frames; - determining whether the differences between the component values of a currently processed pixel are below said threshold level; and - if said differences are below said threshold level, replacing the respective component value of the currently processed pixel with the values of the corresponding next succeeding pixel. A still further embodiment can be described as a method for reducing noise in a video signal which includes a plural number t of video frames F(t) being composed of a plurality of picture elements or pixels P(i,j,t) having position coordinates i,j within frame number t, the method comprising the steps of : filtering out noise from a pixel P(i,j,t) by - determining a tube T(i,j,t) of pixels, the tube T consisting of corresponding subsets of pixels, i.e. slices S, from a number of consecutive frames; - calculating a luminance matrix Y representing the luminance of the pixels of the said tube T; - calculating a luminance difference indication function I for determining luminance differences between corresponding pixels, wherein the values are set dependent on a pre-selected threshold function; - calculating the number μ of pixels in each slice S having a value below or equalling a threshold level set by said threshold function; - calculating the number λ of pixels in the tube T. having a value below or equalling said threshold level; and - determining a noise filtered pixel dependent on the relation of said numbers μ and λ to pixel number threshold values λ tube and μ slice. In an example of the latter embodiment, the threshold function is selected dependent on the human eye sensitivity to different wavelengths of light, such as red, green and blue. Preferably, said steps are carried out for each pixel component with r spect to different wavelengths of light, such as red, green and blue. The pixel number threshold values may be devised dependent on a pre-selected scheme for discriminating between noise impaired pixels and pixels belonging to a moving part or moving edge of a frame. An embodiment of the invention is realised as an apparatus for reducing noise in a video signal, said apparatus comprising: - an input for an analogue video signal (8), coupled to - digital frame sampling means (10), connected to - converting means (12) for separating components of picture elements or pixels and producing a frame having discrete pixel component values; said converting means being coupled to - noise reducing means (4), being devised to produce a sequence of noise filtered frames and to output said filtered frames to - digital format converting means (14) being devised to produce a digital video output and/or to transfer a stream of filtered frames to - digital-to-analogue video converting means (18) having an output for a filtered analogue video signal 20. In one embodiment of the apparatus, the converting means is devised to separate pixel components representing different wavelengths of light, such as red, green and blue. The last mentioned embodiments of the apparatus, are preferably adapted such that the noise reduction means is devised to perform the steps as described in the preceding paragraphs of this section. In one embodiment of the above described apparatus the noise reduction means comprises: - storing means 2 for storing a plurality of frames and storing means 3 for storing a plurality of pixels; - means (24) for calculating a luminance matrix Y for a pixel matrix stored in the memory means; - means (26) for calculating a matrix I of a luminance difference indication function dependent on a predetermined threshold function (27); - means (28) for calculation the number μ of pixels in a portion of a frame having a value below a threshold level set by said threshold function: - means (30) for calculating the number λ of pixels in a set of corresponding portions of a plurality of frames having a value below said threshold level; - means (32) for determining noise filtered output pixels dependent on the relation of said numbers μ and λ to pixel number threshold values. In one advantageous embodiment, the invention is realised as a computer program product, for use with a data processing and storage system, for reducing noise y in video signal, the computer program product comprising: - a recording medium - means, recorded on the recording medium, for directing the data processing and storage system to calculate a luminance matrix Y for a pixel matrix stored in the memory means; - means, recorded on the recording medium, for directing the data processing and storage system to calculate a matrix I of a luminance difference indication function dependent on a predetermined threshold function; - means, recorded on the recording medium, for directing the data processing and storage system to calculate the number μ of pixels in a portion of a frame having a value below a threshold level set by said threshold function; - means, recorded on the recording medium, for directing the data processing and storage system to calculate the number λ of pixels in a set of corresponding portions of a plurality of frames having a value below said threshold level; - means, recorded on the recording medium, for directing the data processing and storage system to determine noise filtered output pixels dependent on the relation of said numbers μ and λ to pixel number threshold values. The invention has above been described by means of exemplifying embodiments, and it should be appreciated that it may be adapted or modified to suit different specific applications within the scope of the appended claims.

Claims: Claims 1. A method for reducing noise in a video signal which includes a plurality of video frames, the method comprising the steps of: -comparing video information contained in a current video frame and a plurality of temporally adjacent video frames; -selecting from said current video frame and said adjacent video frames the video information that according to a predetermined condition is likely to be correct for said current video frame; -assigning the selected video information to the current video frame to thereby produce a video frame wherein noise has been reduced. 2. The method according to claim 1, wherein video information from a predetermined frame portions of the current video frame is compared to video information from spatially corresponding frame portions [slice] of said temporally adjacent video frames. 3. The method according to claim 2, wherein the video signal comprises a sequence of frames of digitized video information represented by a plurality of pixels; and wherein the predetermined frame portions consist of a predetermined number of spatially adjacent pixels. 4. The method according to claim 2, wherein said frame portion is adapted to be suitable for processing an interlaced video signal. 5. The method according to claim 2, comprising the further step of: -selecting, for a current frame portion of the current video frame, a number of spatially corresponding adjacent frame portions of a number of said temporally adjacent video frames wherein there is at least a predetermined degree of correlation between the video information of the current frame portion and the selected adjacent frame portions. 6. The method according to claim 5, wherein the degree of correlation between the video information of the current frame portion and a temporally adjacent frame portion is evaluated by comparing the difference between the average luminance of said current frame portion and said adjacent frame portion to a predetermined threshold value. 7. The method according to claim 5 or 6, wherein the degree of correlation between the video information of the current frame portion and a temporally adjacent frame portion is evaluated by comparing the difference between the values of a colour component of the current frame portion and said adjacent frame portion to a predetermined threshold value. 8. The method according to claim 1, wherein, for a predetermined part, e.g. a current pixel, of a current frame portion of the current video frame, is selected the video information of the predetermined part having the median value of the luminance of each of a set of spatially corresponding parts, e.g. temporally adjacent pixels, of said adjacent video frames. 9. The method according to claim 1, wherein the video information of the current video frame is replaced by the selected video information, to be included in the processing of other video frames of said video signal. 10. The method according to claim 1, wherein the selected video information is used to produce a new current video frame, to be excluded from the processing of other video frames of said video signal. 11. A method for reducing noise in a video signal which includes a plurality of video frames of digitized video information represented in a plurality of pixels, the method comprising the steps of: -storing a current video frame and a plurality of temporally adjacent video frames; -selecting from the current video frame a current frame portion comprising a current pixel and at least one pixel spatially adjacent to the current pixel; -selecting from the temporally adjacent video frames temporally adjacent frame portions comprising a pixel or pixels that spatially corresponds to the pixels of the current frame portion; -determining the luminance value of each pixel in said current frame portion and said adjacent frame portions; -determining the average luminance values of said current frame portion pixel and said spatially adjacent frame portions; determining whether a local scene change occurs between said current frame portion and each of said adjacent frame portions by: -determining and comparing to a first threshold value the absolute value of the difference between the average luminance of said current frame portions and the average luminance of said adjacent frame portion; -if the absolute value of the luminance difference exceeds said first threshold value, assuming there is a local scene change in the video information between said current frame portion and said adjacent frame portion; else -deterrnining and comparing to a second threshold value the absolute values of the difference between the values of a colour component of said current pixel and the corresponding pixel of said adjacent frame portion; -if the absolute value of the colour component value difference exceeds said second threshold value, assuming there is a local scene change in the video information between said current frame portion and said adjacent frame portion; else -assuming that there is no local scene change between said current frame portion and said adjacent frame portion; selecting an even number exceeding or equalling zero of adjacent frame portions considered to involve no local scene change in relation to said current frame portion; selecting the video information from a pixel that is probable to have equal predecessor and/or successors by: -comparing the luminance values of said current pixel and the corresponding pixels in said selected adjacent frame portions and selecting the video information of the pixel having the median value of said luminance values; -assigning said selected video information to the current pixel to thereby produce a video frame wherein noise has been reduced. 12. The method according to claim 11, wherein the video information of the current video frame is replaced by the selected video information. 13. The method according to claim 11 , wherein the selected video information is used to produce a new current video frame. 14. The method according to claim 11, wherein said plurality of temporally adjacent video frames are a predetermined number of video frames symmetrically preceding and/or succeeding a current video frame. 15. A processing method for use in an image signal coding system for coding an input image signal into a compressed output signal, the method comprising the steps of: -receiving an input image signal comprising digitized image data; -processing the image signal in accordance with any of the preceding claims 1-15; -coding the image signal into a resulting compressed image signal, e.g. according to an MPEG compression scheme. 16. An apparatus for reducing noise in a video signal which includes a plurality of video frames, comprising means for performing the method of any of the preceding claims 1-15. 17. A computer program product for use in a computer system adapted for reducing noise in a video signal which includes a plurality of video frames, comprising: -a recording medium; -means, recorded on the recording medium, for directing the computer system to perform the method of any of the preceding claims 1-16. 18. An apparatus for reducing noise in a video signal which includes a plurality of video frames, the apparatus comprising: -an input (22) for receiving an input frame of a set of input frames (Fin); -memory means (2) for storing one or more input frames (Fin); -memory means (3) for storing one or more pixels of a frame (F) to be processed; -means (24) for calculating a luminance matrix (Y) for a pixel matrix stored in the memory means (4); -means (26) for calculating a matrix (I ) of a luminance difference indicate function dependent on a predetermined and stored threshold function (27); -means (28) for calculating the number of unequal pixels in a slice; -means (30) for calculating the number of unequal pixels in a tube (T); -means (32) for determining an output pixel (Pout); and -an output (34) for outputting one or more noise reduced frames (Fout). 19. An apparatus for reducing noise in a video signal which includes a plurality of video frames, the apparatus having: -an input (22) for receiving an input frame of a set of input frames (Fin); -memory means (2) for storing one or more input frames (Fin); -memory means (3) for storing one or more pixels of a frame (F) to be processed; and -an output (34) for outputting one or more noise reduced frames (Fout); the apparatus comprising: -means for comparing video information contained in a current video frame and a plurality of temporally adjacent video frames; - means for selecting from said current video frame and said adjacent video frames the video information that according to a predetermined condition is likely to be correct for said current video frame; - means for assigning the selected video information to the current video frame to thereby produce a video frame wherein noise has been reduced. 20. The apparatus according to claim 19, further comprising means for comparing video information from a predetermined frame portions of the current video frame to video information from spatially corresponding frame portions [slice] of said temporally adjacent video frames. 21. The apparatus according to claim 20, wherein the video signal comprises a sequence of frames of digitized video information represented by a plurality of pixels; and wherein the predetermined frame portions consist of a predetermined number of spatially adjacent pixels. 22. The apparatus according to claim 20, wherein said frame portion is adapted to be suitable for processing an interlaced video signal. 23. The apparatus according to claim 20, further comprising: - means for selecting, for a current frame portion of the current video frame, a number of spatially corresponding adjacent frame portions of a number of said temporally adjacent video frames wherein there is at least a predetermined degree of correlation between the video information of the current frame portion and the selected adjacent frame portions. 24. The apparatus according to claim 23, comprising means for evaluating the degree of correlation between the video information of the current frame portion and a temporally adjacent frame portion by comparing the difference between the average luminance of said current frame portion and said adjacent frame portion to a predetermined threshold value. 25. The apparatus according to claim 23 or 24, comprising means for evaluating the degree of correlation between the video information of the current frame portion and a temporally adjacent frame portion by comparing the difference between the values of a colour component of the current frame portion and said adjacent frame portion to a predetermined threshold value. 26. The apparatus according to claim 19, comprising means for selecting, for a predetermined part, e.g. a current pixel, of a current frame portion of the current video frame, the video information of the predetermined part having the median value of the luminance of each of a set of spatially corresponding parts, e.g. temporally adjacent pixels, of said adjacent video frames. 27. The apparatus according to claim 19, comprising means for replacing the video information of the current video frame by the selected video information, to be included in the processing of other video frames of said video signal. 28. The apparatus according to claim 19, comprising means for producing from the selected video information a new current video frame, to be excluded from the processing of other video frames of said video signal. 29. An apparatus for reducing noise in a video signal which includes a plurality of video frames of digitized video information represented in a plurality of pixels, the apparatus comprising: -means for storing a current video frame and a plurality of temporally adjacent video frames; - means for selecting from the current video frame a current frame portion comprising a current pixel and at least one pixel spatially adjacent to the current pixel; - means for selecting from the temporally adjacent video frames temporally adjacent frame portions comprising a pixel or pixels that spatially corresponds to the pixels of the current frame portion; - means for determining the luminance value of each pixel in said current frame portion and said adjacent frame portions; - means for determining the average luminance values of said current frame portion pixel and said spatially adjacent frame portions; - means for determining whether a local scene change occurs between said current frame portion and each of said adjacent frame portions by: - means for determining and comparing to a first threshold value the absolute value of the difference between the average luminance of said current frame portions and the average luminance of said adjacent frame portion; - means for, if the absolute value of the luminance difference exceeds said first threshold value, assuming there is a local scene change in the video information between said current frame portion and said adjacent frame portion; else determining and comparing to a second threshold value the absolute values of the difference between the values of a colour component of said current pixel and the corresponding pixel of said adjacent frame portion; -means for, if the absolute value of the colour component value difference exceeds said second threshold value, assuming there is a local scene change in the video information between said current frame portion and said adjacent frame portion; else assuming that there is no local scene change between said current frame portion and said adjacent frame portion; - means for selecting an even number exceeding or equalling zero of adjacent frame portions considered to involve no local scene change in relation to said current frame portion; - means for selecting the video information from a pixel that is probable to have equal predecessor and/or successors by: comparing the luminance values of said current pixel and the corresponding pixels in said selected adjacent frame portions and selecting the video information of the pixel having the median value of said luminance values; - means for assigning said selected video information to the current pixel to thereby produce a video frame wherein noise has been reduced. 30. The apparatus according to claim 29, comprising means for replacing the video v information of the current video frame by the selected video information. 31. The apparatus according to claim 29, comprising means for producing a new current video frame from the selected video information. 32. The apparatus according to claim 29, wherein said plurality of temporally adjacent video frames are a predetermined number of video frames symmetrically preceding and/or succeeding a current video frame. 33. An apparatus for use in an image signal coding system for coding an input image signal into a compressed output signal, the apparatus comprising: -means for receiving an input image signal comprising digitized image data; -means for processing the image signal in accordance with any of the preceding claims 1-15; and/or an apparatus or means in accordance with any of the preceeding claims 19-32; -means for coding the image signal into a resulting compressed image signal, e.g. according to an MPEG compression scheme. 34. A computer program product for use in a computer system adapted for reducing noise in a video signal which includes a plurality of video frames, comprising: -a recording medium; -means, recorded on the recording medium, in accordance with the means of any the preceding claims 19-33.

Inventors: ANDERSSON ROGER, OLSSON KENT, XU YOUSHI, ZHANG TING TING, ANDERSSON, ROGER, OLSSON, KENT, XU, YOUSHI, ZHANG, TING, TING
Applicants: ANDERSSON ROGER, LIMT TECHNOLOGY AB, OLSSON KENT, XU YOUSHI, ZHANG TING TING, ANDERSSON, ROGER, LIMT LOCAL INSERTION MEDIA TECHNOLOGY AB, OLSSON, KENT, XU, YOUSHI, ZHANG, TING, TING
Application year: 1998
Main classification: H04N5
IPC codes: H04N5, H04N7