Abstract:
A method of attenuating noise contained in an image signal according to the extent of the movement of the image signal based on the amount of noise, and an apparatus therefor are provided. This method includes measuring the amount of noise in an image signal by accumulating an offset between an input image signal and a delayed output image signal for a predetermined time, detecting the extent of the movement of the input signal based on the amount of noise measured, and forming an output image by controlling the specific weight of the input image signal and the delayed output image signal according to the detected movement.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to an apparatus for attenuating image noise, and more particularly, to a method of attenuating noise contained in an image signal according to the extent of the movement of a signal. The present application is based on Korean Application No. 2001-62670, filed Oct. 11, 2001, which is incorporated herein by reference. 
   2. Description of the Related Art 
   Advancements in the quality and variety of image appliances is causing demand for high-definition image appliances. In response to this demand, apparatuses for removing noise contained in image signals have been developed. In general, low-pass filters or recursive filters are used to filter out noise in a transmitted image signal. 
     FIG. 1  is a block diagram of an embodiment of a recursive filter according to a conventional apparatus for attenuating image noise. Referring to  FIG. 1 , an image signal containing noise and an image signal that is delayed for one frame by a delayer  140  are combined in a subtracter  110  to generate an offset signal. In this structure, the offset signal is limited to a lower level by a limiter  120 . At this time, the lower the level of the offset signal is limited by the limiter  120 , the more the noise can be reduced. However, this increases an artifact according to scene motion. Finally, an output of the offset signal, which is attenuated by the limiter  120 , is combined with an image signal input by an adder  130 . As a result, the noise contained in the image signal is canceled by the adder  130 . Then, the image signal whose noise was attenuated is fed back to the delayer  140  to be used in the next frame. 
   As described above, a recursive filter is designed to operate on the basis of the correlation between frames separated at two-frame intervals. Thus, noise-reducers such as a recursive filter tend to have inadequate effects since the correlation between frames separated at two-frame intervals is relatively low. 
   SUMMARY OF THE INVENTION 
   To solve the above problems, it is a first object of the present invention to provide a method of adaptively attenuating noise in an image signal according to the amount of noise and the extent of movement of the image signal. 
   It is a second object of the present invention to provide an apparatus for attenuating noise in an image signal using such a method. 
   To achieve the first object, there is provided a method of attenuating noise in an image signal, the method including measuring the amount of noise in an image signal by accumulating an offset between an input image signal and a delayed output image signal for a predetermined time, detecting the extent of the movement of the input signal based on the amount of noise measured, and forming an output image by controlling the specific weight of the input image signal and the delayed output image signal according to the detected movement. 
   To achieve the second object, there is provided an apparatus for attenuating noise in an image signal, the apparatus including a filter for filtering an output image signal that is delayed for a predetermined time, a subtracter for generating an offset signal between the filtered output image signal and an input image signal, a movement detector for accumulating the offset signal generated by the subtracter, and for detecting the extent of the movement of the image signal based on the accumulated value, a first multiplier for multiplying a signal output from the subtracter and the extent of the movement detected by the movement detector, and an adder for forming an output image signal by adding the signal output from the subtracter and the signal output from the filter. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above object and advantages of the present invention will become more apparent by describing in detail a preferred embodiment thereof with reference to the attached drawings in which: 
       FIG. 1  is a block diagram of an apparatus for attenuating a recursive image noise that is an example of a conventional apparatus for attenuating image noise; 
       FIG. 2  is a block diagram of an apparatus for attenuating image noise according to the present invention; 
       FIG. 3  is a view of the structure of a movement detector of  FIG. 2 ; and 
       FIGS. 4A and 4B  are views for explaining signals input to a median filter of  FIG. 2  according to the type of a field. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 2  is a block diagram of an apparatus, according to the present invention, for attenuating noise contained in an image signal. The apparatus of  FIG. 2  includes a movement detector  220  for measuring the movement of pixels of an image signal input; first and second field delayers  250  and  260  for delaying an image signal output; a line delayer  270  for delaying an image signal, which is delayed by one field, by +1 or −1 line according to the type of input current field, e.g., a top field or bottom field; a median filter  280  for outputting the mean value of image signals output from the first and second field delayers  250  and  260  and the line delayer  270 ; a subtracter  210  for generating an offset signal between a signal output from the median filter  280  and an image signal input; a multiplier  230  for multiplying a signal output from the subtracter  210  and a signal output from the movement detector  220 ; and an adder  240  for combining a signal output from the adder  230  and the signal output from the median filter  280 . 
   In the operations of an apparatus for attenuating image noise according to the present invention, the subtracter  210  subtracts an output image signal m, which is delayed by the media filter  280 , from the input image signal i containing noise, so as to form a difference signal d. 
   At this time, the median filter  280  calculates the mean value of (i) an image signal that is delayed by one field by the first field delayer  250 ; (ii) an image signal that is delayed by one field by the first field delayer  250 , and then delayed by +1 or −1 line by the line delayer  270  according to the type of current input field such as bottom field and top field; and (iii) an image signal that is delayed by one field by the first field delayer  250 , and then delayed by one field by the second field delayer  260 . Then, the median filter  280  outputs the mean value as a delayed image signal m. Here, the top field is a field including a first line from the top of two fields constituting a frame, and the bottom field is a field including a second line from the top of two fields constituting one field. 
   The movement detector  220  detects the movement of each pixel of an image signal from an offset signal d output from the subtracter  210 , and generates a movement parameter k. 
   In the movement parameter k detected by the movement detector  220  with reference to  FIG. 3 , the offset signal d is converted into an offset signal d 1  of a low-pass component via a low-pass filter  310 , and at the same time, is converted into an absolute value da via a first absolute value converter  370 . The offset signal d 1  having a low-pass component, and the absolute value da generate an offset signal of a high-pass component in a third adder  320 . The offset signal output from a third adder  320  is converted into an absolute value by a second absolute value converter  330 . The offset signal output from the second absolute value converter  330  is accumulated by an accumulator  340  for a predetermined time, e.g., one frame or one field. At this time, a value accumulated by the accumulator  340  is a value n indicating the amount of noise accumulated for a predetermined time. The value n accumulated in the accumulator  340  and the offset signal output from the third adder  320  are multiplied with each other by a second multiplier  350 . An output value of the second multiplier  350  is combined with the absolute value of the first absolute value converter  370  by the fourth adder  360 , which outputs the movement parameter k. The movement parameter k is calculated by the following equation:
 
 k=da+n*d 1+(1 −n )* da   (1)
 
wherein n is a number between 0 and 1.
 
   From the equation (1), it is noted that the greater the noise n is, the more the offset signal d 1  increases, and the less the noise n is, the more the absolute value da increases. 
   Next, referring to  FIG. 2 , a signal output from the subtracter  210  is multiplied by a signal output from the movement detector  220 , and is output as a signal that is weighted filtered. 
   Then, a signal output from the first multiplier  230  is combined with an image signal m, which is delayed by the median filter  280 , by the adder  240 , and then constructed as an image signal o. Here, the image signal o is calculated by the following equation:
 
 o=m+k* (1 −m )= k*i +(1 −k )* m   (2)
 
wherein k is a number between 0 and 1.
 
   From equation (2), it is noted that the greater the movement parameter k is, the more the image signal i increases, and the less the movement parameter k is, the more the delayed image signal m increases. That is, an increase in the movement of an image signal results in an increase in the image signal input, and a decrease in the movement of the image signal results in an increase in the delayed image signal m. 
     FIGS. 4A and 4B  are views for explaining signals input to the median filter  280  of  FIG. 2  according to the type of field. In detail,  FIG. 4A  is a view of signals input to the median filter  280  when an input field is a top field. More specifically, a signal that is delayed by one field, a signal that is delayed by one field and then is further delayed by −1 line, and a signal that is delayed by two fields, are input to the median filter  280 , and then, the median filter  280  outputs a mean pixel value of the above signals. 
   On the other hand,  FIG. 4B  is a view of signals input to the median filter  280  when an input field is a bottom field. That is, a signal that is delayed by one field, a signal that is delayed by one field and then is delayed by +1 line, and a signal that is delayed for two fields, are input to the median filter  280 , and then, the median filter  280  outputs a mean pixel value of the above signals. 
   While the present invention has been particularly shown and described with reference to a preferred embodiment thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims. 
   According to the present invention as described above, noise contained in moving images, as well as in still images, can be effectively attenuated according to the amount of noise in an image signal and movement of the image signal. Further, it is possible to reject impulse noise in an image signal while adaptively delaying still images and moving images.