noise

What is noise?
Noise is any undesired information that contaminates an image. Noise
appears in image from a variety of source. The digital image a acquisition
process, which converts an optical image into a continuous electrical
signal that is then sampled is the primary process by which noise appears
in digital images. At every step in the process there are fluctuations caused by natural phenomena that add a random value to exact brightness value for a given pixel. In typical image the noise can be modeled with one of the following distribution:
1. Gaussian (“normal”) distribution.
2. Uniform distribution.
3. Salt _and _pepper distribution.




Spatial masks
Many image enhancement techniques are based on spatial operations performed on local neighborhoods of input pixels.
The image is usually convolved with a finite impulse response filter called spatial mask. The use of spatial masks on a digital image is called spatial filtering.
Suppose that we have an image of size and we define a neighbourhood around each pixel. For example let this neighbourhood to be a rectangular window of size



















If we replace each pixel by a weighted average of its neighbourhood pixels then the response of the linear mask for the pixel is . We may repeat the same process for the whole image.
A spatial mask operating on an image can produce:
(a) a smoothed version of the image (which contains the low frequencies) or
(b) it can enhance the edges and suppress essentially the constant background information.
The behavior is basically dictated by the signs of the elements of the mask.


Noise Removal using Spatial Filters:
Spatial filtering is typically done for:
1. Remove various types of noise in digital images.
2. Perform some type of image enhancement.
Note: These filters are called spatial filter to distinguish them from frequency domain filter.
The three types of filters are:
1. Mean filters
2. Median filters (order filter)
3. Enhancement filters
Mean and median filters are used primarily to conceal or remove noise, although they may also be