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Describe the low pass filter.

Low Pass Filter : Considering the grey-scale values of a part of an image as below,

Here, we have taken 9 grey scale values. It is seen that the 5th value is very different than the other 8 values, that is a big change is occurring at that point due to the big change of the grey-scale value at that point than the other points.
            So to change the center value to a closer value to the neighboring values, low pass filter can be applied. This filter is also called averaging filter. Implementation of this filter is as follows,
(i)          First a mask of odd number such as 3×3, 5×5 etc has to be taken. Here 3×3 mask has been taken.
(ii)        Now, all the values will be added. Here all the 9 values will be added.
(iii)      In this case, the summation will be divided by 9 to get the average. In a 5×5 mask the summation will be divided by 25 etc. 
(iv)      Finally the center value of the mask will be replaced by the obtained average value.
      Places to Imply This Filter :
(i)          To reduce noise.
(ii)        To eradicate unwanted spots on images.
              Disadvantages :
(i)          It causes “Blur” effect for which the sharpness of the image will be reduced.

As for averaging it tends to turn an image to an image consisting almost similar grey-scale levels everywhere.

Describe the median pass filter. DIP