VIDEO SIGNAL NOISE LEVEL ESTIMATOR

Abstract: A method and apparatus for noise level estimation in a digital video signal, the apparatus including a pixel activity detector (102) which produces a mean pixel activity level for each picture in the video signal, a contrast analyser (106) which produces a contrast activity level for each picture and a noise level calculator (104) which is coupled to the activity detector and contrast analyser to produce an estimated noise level by offsetting the contrast activity level from accumulated output of the activity detector.

Description: Title of the Invention VIDEO SIGNAL NOISE LEVEL ESTIMATOR Background of the Invention The present invention relates to digital video processing and noise reduction More particularly, it relates to a method and apparatus for estimating noise level m video signals Description of Prior Art Noise reduction techniques have been applied to television or video signals to improve subjective viewing quality of the signal as well as to improve the compression efficiency in video encoding systems Such noise 1 eduction techniques usually include the use of various types of low-pass filters, median filters, adaptive spatial filters and temporal recursive filters Usually the noise level of input video is estimated m order to avoid under filtering of noise video or over filtermg of busy video sequences One example of noise estimation is disclosed in US Patent 5,844,627 This patent describes several ways of computing noise vanance signals for the estimation Basically, the noise variance is computed according to neighbourhood pixel variance of a smooth area in the video image The smooth image area may be selected by the user or automatically detected Another known technique for noise level detection is disclosed m the article Memor\ Integrated Noise Reduction IC for Television' b G de Haan, et al , IEEE Transactions on Consumer Electronics, May 1996, Volume 42, Number 2 In this article, the minimum absolute difference between two groups of neighbouring pixels on different positions in the same image field is used to estimate the noise level This is done assuming that a threshold exists between local noise estimates of plain areas and other areas in the image The techniques which require user interaction to define areas such as smooth or plain areas within the video sequence images are not really suitable for most consumer applications and real time operations Furthermore, algorithms tor automatic detection of such areas are usually difficult to implement due to the normally large dynamic range of video input source materials Accordingly, most existing methods of noise level estimation are video content dependent because they assume that at least some part of the video image is a smooth or plain area This assumption is not always correct because a smooth or plain area is not always present in each video field Accordingly, the sensitivity of such known noise level estimators is effected by video content and the results of the estimation are sometimes unreliable Disclosure of the Invention An object of the present invention is to provide a more robust method and apparatus for noise level estimation m digital video signals Generally speaking, the present invention recognises that within a picture the difference between a pixel value and its neighbour pixel value mean may be due to noise and/or actual image detail The intensity of such image detail affects the contrast or sharpness of the picture On the other hand, it is possible to determine a alue indicative of the contrast or sharpness of the picture Therefore, the noise level of a picture can be estimated b> removing this contrast indicative value from the average of the pixel difference More specifically, the mvention provides a method of noise level estimation m a digital video signal including the steps of determining, for each picture, mean pixel activity and contrast activity , and obtaining a noise le\ el estimation signal bv offsetting the contrast activity from the mean pixel activity , and wherein the mean pixel acti ity is determined by taking the sum of all pixel activity activities m a picture divided by the number of pixels in the picture, and the contrast activity is determined by assigning a value to contrast activity in accoi dance with the contrast in the picture The invention also provides apparatus for noise level estimation m a digital video signal, said apparams including a mean pixel activity detector means (102, 103) for receiving a digital video signal and producing a mean pixel activity level for each picture m the video signal, a contrast analyser (106) coupled to receive signals derived from the activity detector (102) and operable to produce a contrast activity level for each picture in the video signal, and a noise level calculator (104) coupled to receive signals derived from the activity detector and said contrast activitv level, said noise level calculator bemg operable to offset the contrast activity level from the signals derived from the activitv detector to generate output signals representative of noise levels m said video signal Brief Description of the Drawings Figure 1 is a block diagram of a system for estimating noise level in video signals, Figure 2 is a more detailed block diagram of part of the system shown in Figure 1 , Figure 3 is a histogram of pixel activity, and Figure 4 shows graphically parameters used to estimate contrast activity Detailed Description of the Preferred Embodiments Figure 1 shows in block diagram form a system 100 for digital video processing and noise reduction The major components of the system include a pixel activity detector 102, accumulator 103 noise level calculator 104, histogram generator 105 and contrast analyser 106 Typically , a digitised video signal is coupled to the input terminal 101 The video signal is applied to the input of the pixel activity detector 102 which calculates for each pixel m the input video signal a pixel activity value In the preferred embodiment of the invention, the pixel activ ltv \ alue is calculated as the absolute difference between the current pixel value and the mean value of its neighbouring pixels, I e pixel activity = abs [pixel - (pixel left + pixel + pixel right) /3] In the preferred embodiment, the neighbouring pixels are the two adjacent pixels in the line one either side of the pixel in question It would of course be possible to alternatively or additionally include pixels above and below the pixel m question or pixels which are two or three pixels distant to the left or right Techniques for selection of suitable neighbouring pixels is well known in the art Alternatively, a high pass filter with an absolute value output may be used as a pixel activity detector Many types of suitable high pass filters are known m the ait and one of these may be selected for use, the final selection largely being a question of quality, accuracy and cost A very simple example of a suitable high pass filter has the following coefficients [-0 25, 0 5, and -0 25] Output from the detector 102 is coupled to the input of the accumulator 103 which operates to sum all of the pixel activity values of a picture Output from the detector 102 is also inputted to the histogram generator 105 which generates a histogiam of the calculated pixel activity value The accumulator 103 is arranged to produce the mean pixel activity by summing all pixel activities from the detector 102 for picture and dividing by the total number of pixels m the picture It may be possible to simplify implementation of the circuit by reducing the size of the histogram table produced by the generator 105, tor example by scaling down or truncating pixel activity values Scaling down pixel activity value can simply be accomplished by deeming that the new pixel activity value equals the pixel activitv value divided by two Truncating pixel activity values can be accomplished by the following logical step it (pixel activity value < 32) new pixel activity value = pixel activity value else new pixel activity value = 31 Output from the histogram generator 105 is coupled to the input of the contrast analyser 106 At the end of a picture, the contrast analyser 106 examines the geneiated histogram and from that estimates a contrast activitv value Typically, it is possible to estimate how sharp or blurry a picture is by looking at the shape of the histogram In accordance with the invention, a method of estimating the contrast activity by reference to two parameters as follows The fast parameter is a maximum lectangular area size containing the histogram The second parameter is obtained from a look-up table which estimates contrast activity This method is diagrammaπcally illustrated m Figure 3 where the histogram 301 is shown with the horizontal axis being pixel activity and the vertical axis being the count of pixels The maximum rectangular area 302 is drawn from the points where the histogram meets the horizontal and vertical axes, as shown Figure 4 shows a look-up table in graphical form with the horizontal axis being rectangular area size and the vertical axis being contrast activity The look-up table determines the contrast activity as a function of the rectangular area size The curve 108 in the look-up table can be determined by experiments for translating the rectangular size to a contrast activity value for the picture The look-up table can be determined by experiments in which various images with known oi desired output noise levels are subjected to the system to generate corespondmg histogram rectangulai sizes and average pixel activity values, and from these collection of values a graph such as Figure 4 can be generated Accordingly, persons skilled in the art will be able to generate appropriate look-up tables Other methods such as linear interpolation or spline estimation may also be used instead of the look-up table These techniques are also well known m the art and generally relate to the field of estimation of a curve or tianster function From the accumulated pixel activ ity, an a erage pixel activity for the picture is calculated using the noise level calculator 104 which ieceives input from the accumulatoi 103 and contrast analyser 106 Basically, the noise level calculator 104 offsets or subti acts the accumulated pixel activity values by the contrast activity from the contiast analv sei 106 Output from the noise level accumulator 104 is coupled to an output terminal 107 me signals on which represent the estimated output noise level as required It is possible to limit the output noise lev els so as to leside withm predetermined minimum and maximum v alues according to requirements so as to make subsequent digital processing for efficacious Figure 2 illustrates in more detail an example of the pixel activity detectoi 102 Successive pixels of each line of video data are inputted at the terminal 101 The circuit calculates difference between a pixel and the mean value of the pixels at its left and right The circuit of Figuie 2 includes first and second delay elements 202 and 203, first and second adders 205 and 206 and hi st and second multiplier 204 and 207 Output from the second multipher is coupled to an absolute value circuit 208, the output of which is coupled to output terminal 209 which is the input to the histogram generator 105 and accumulator 103 The circuit components are operable to determine the difference between a pixel and the mean value of the pixel and its neighbouring left and right pixels because of the action of the delay elements 202 and 203 which have delays equal to a typical period of a pixel Other arrangements could be implemented for obtaining differences between the pixel and mean values of other adjacent pixels The absolute value of the difference constitutes the output pixel activity value appearing at the terminal 209 It is possible to implement the accumulator 103, noise level calculator 104, histogram generator 105 and contrast analyser 106 using techniques which are well known m the ait It is therefore not necessary to describe these components m detail It will be appreciated bv those skilled in the art that the present invention piovides an adaptive and robust method and system for noise level estimation in digital video signals The hardware implementation is comparatively simple and inexpensive The present invention can be beneficially used as an input m video noise reduction systems such as an input to determination the degiee of filtering to be used This improves the subjective quality of the video presentation The noise level estimation can also be applied to digital v ideo signals as a pre-processing step prior to video compression with the result that on decompression of the video signals an improved video display can be obtained Many modifications will be apparent to those skilled m the art without departing from the spirit and scope of the invention

Claims: CLAIMS 1 A method of noise level estimation m a digital video signal including the steps of determining, for each picture, mean pixel activity and contrast activity, and obtaimng a noise level estimation signal by offsetting the contrast activity from the mean pixel activity, and wherein the mean pixel activity is determined by taking the sum of all pixel activity activities in a picture divided by the number of pixels in the picture, and the contrast activity is determined by assigning a value to contrast activity m accordance with the contrast in the picture 2 A method as claimed in claim 1 wherein each pixel activity is determined by taking the absolute value of the differences between a pixel and the mean of the pixel and the pixels which are adjacent thereto in the same line of the video signal 3 A method as claimed in claim 2 wherein the step of obtaining a noise level estimation signal is obtained by subtracting the contrast activity from the mean pixel activity 4 A method as claimed m clami 2 or 3 wherein the contrast activity is determined by the steps of generatmg a histogiam of pixel activity values, analysing the shape of the histogram to determine contrast activity 5 A method as claimed m claim 4 wherein the step of analysing the shape of the histogram includes the steps of identifying points where there are zero and maximum pixel activities, utilising said points to define a rectangular area size parameter and determining contrast activity by reference to said area size parameter 6 A method as claimed in claim 5 wherein values of contrast activity are determined empirically and stored m a look-up table as a function of area size parameter and, for each picture, said area size parameter is determined and inputted to the look-up table to thereby determine the contrast activity tor the picture 7 Apparatus (100) for noise level estimation in a digital video signal, said apparatus including a mean pixel activity detector means (102,103) for receiving a digital video signal and producing a mean pixel activity level tor each picture m the video signal, a contrast analyser (106) coupled to receive signals derived from the activity detector (102) and operable to produce a contrast activity level for each picture m the video signal, and a noise level calculator (104) coupled to receive signals derived from the activity detector and said contrast activity level, said noise level calculator being operable to offset the contrast activity level from the signals derived from the activity detector to generate output signals representative or noise levels in said video signal 8 Apparatus as claimed m claim 7 wherein the mean pixel activity means includes a pixel activity detector (102) and an accumulator (103) and wherein the pixel activity detector is operable to generate a pixel activity level by taking the absolue valve of the difference between a pixel and the mean value of that pixel and selected neighbouring pixels 9 Apparatus as claimed m claim 8 wherein the pixel activity detector is operable to generate the pixel activity level by taking the absolute value of the difference between each pixel and the mean value of that pixel and its left and right neighbouring pixels in the same line of the picture 10 Apparatus as claimed m claim 8 or 9 wherein the accumulatoi (103) is operable to generate said mean pixel activity by summing all pixel activ ity lev els toi a pictuie and dividing bv the total number of pixels m the picture 11 Apparatus as claimed in claim 10 wherein the noise level calculatoi is opeiable to subtract the contrast activity level from the signals derived from the activity detectoi 12 Apparatus as claimed in claim 10 or 1 1 including a histogram generator ( 105) coupled between the pixel activity detectoi and the contrast analyser the histogram generator being operable to generate a histogram of pixel activity valves and wherein the contrast analyser analyses the shape of the histogram to determine said contrast activity level. 13. Apparatus as claimed in claim 12 wherein the contrast analyser is operable to identify points where there are zero and maximum pixel activities and to assign contrast activity levels by reference to said zero and maximum pixel activities. 14. Apparatus as claimed in claim 13 wherein the contrast analyser is operable to define an area size parameter which is derived from said zero and maximum pixel activities and the contrast activity levels are assigned by reference to a look-up table defining the relationship between values of area size parameters and contrast activity levels.

Inventors: HUI YAU WAI LUCAS, HUI, YAU, WAI, LUCAS
Applicants: HUI YAU WAI LUCAS, ST MICROELECTRONICS ASIA, HUI, YAU, WAI, LUCAS, STMICROELECTRONICS ASIA PACIFIC PTE LTD.
Application year: 1999
Main classification: H04N17
IPC codes: H04N17