Ch5 Image RestorationCS446 Instructor: Nada ALZaben

Image restorationRestoration attempts to reconstruct or

recover an image that has been degraded by using a prior knowledge of the degradation phenomenon

Restoration techniques are oriented toward modeling the degradation and applying the inverse process in order to recover the original image

Some restoration techniques are best formulated in the spatial domain, while others are better suited for the frequency domain

A model of the Image Degradation/Restoration Process [1]

The degradation process: modeled as a degradation function H together with an additive noise term operates on an input image f(x,y) to produce a degraded image g(x,y)

Given knowledge about the additive noise term n(x,y), the objective of restoration is to obtain an estimate f’(x,y) of the original input image

A model of the Image Degradation/Restoration Process [2]

If H is a linear position-invariant process then the degraded image is given in the spatial domain by:g(x,y) = h(x,y) * f(x,y) + n(x,y) ◦ Where: h(x,y) is the spatial representation of the

degradation function and * indicates convolutionWe may also write the model in the frequency

domain representation:G(u,v) = H(u,v) F(u,v) + N(u,v)◦ Where the terms in capital letters are the Fourier

transform of the corresponding terms in the previous equation

We will assume that H is the identity operator and we deal only with degradation due to noise

Noise ModelThe principle sources of noise in digital

images arise during image acquisition (digitization) and/or transmission

Frequency properties refer to the frequency content of noise in the Fourier sense◦when the Fourier spectrum of noise is constant

the noise usually is called white noiseWe will assume that noise is independent of

spatial coordinates and that it is uncorrelated with respect to the image itself (i.e. no correlation between pixel values and the values of noise components)

Some Important Noise Probability Density Functions (PDF)

The spatial noise descriptor is the statistical behavior of the gray-level values in the noise component of the model

These may be characterized by a probability density function PDF

The most common PDFs in image processing:◦Gaussian noise◦Impulse noise (salt and pepper)◦Gamma noise … etc

Gaussian NoiseGaussian (also called normal) noise

models are frequently used in practice

The PDF of a Gaussian random variable z is given by:

◦Where: z represent gray level, µ is the mean of average value of z and σ is its standard deviation

Impulse (Salt and Pepper) Noise

The PDF of impulse (bipolar) noise given by:

◦If b > a: Gray level b will appear as a light dot in the image Level a will appear like a dark dot

◦If either Pa or Pb is zero, the impulse noise is called unipolar noise

◦If neither Pa or Pb is zero, and if they are approximately equal the impulse noise is called salt and pepper.

◦For an 8-bit image a = 0 (black) and b = 255 (white)

Mean FiltersArithmetic mean filter:

◦ let Sxy be the set of coordinates in a rectangualr subimage window of size mxn centered at point (x,y),

◦The arithmetic mean filtering process computes the average of the corrupted image g(x,y) in the area defined by Sxy:

◦The operation can be implemented using a convolution mask in which all coefficients = 1/mn

Mean FiltersGeometric mean filter:

◦is given by the expression:

◦Achieves smoothing comparable to the arithmetic mean filter but it tends to lose less image detail in the process

Mean FiltersHarmonic mean filter:

◦Works well for salt noise but fails for pepper noise◦ It does well also with other types of noise.

Contraharmonic mean filter:

◦Well suited for reducing the effect of salt-and-pepper noise

Median filter:

◦ Practically effective in the presence of both bipolar and unipolar impulse noise

Max and min filters:

◦ Max: useful for finding the brightest points in an image (pepper noise reduced)

◦ Min: useful for finding the darkest points (reduces salt) Midpoint filter:

◦ Computes the midpoint between the maximum and minimum values in the area encompassed by the filter

◦ Works best for randomly distributed noise

Order-Statistic Filters

Adaptive FiltersAdaptive filters are filters whose

behavior changes based on statistical characteristics of the image inside the filter region defined by the m × n rectangular window Sxy

They are capable of performance superior to that of the filters discussed thus far

Adaptive Median FilterThe median filter performs well as long as the

spatial density of the impulse noise is not large (as a rule of thump, Pa and Pb less than 0.2)

Adaptive median filter ◦ can handle impulse noise with larger probabilities ◦ And it seeks to preserve detail while smoothing

nonimpulse noiseThe adaptive median filter works in a

rectangular window area Sxy, but it changes the size of Sxy during filter operation◦ Keep in mind: The output of the filter is a single

value used to replace the value of the pixel at (x,y)

Adaptive Median FilterIt have three main purposes:

◦To remove salt and-pepper (impulse) noise◦Provide smoothing of other noise (may not be

impulsive)◦Reduce distortion such as excessive thinning or

thickening of object boundariesConsider the following notation:

◦zmin = minimum gray level value in Sxy

◦zmax = maximum gray level value in Sxy

◦zmed = median of gray levels in Sxy

◦zxy = gray level at coordinates (x,y)◦Smax = maximum allowed size of Sxy

Adaptive Median Filter The adaptive median filtering algorithm works in two levels,

denoted as level A and level B, as follows: Level A:

◦ A1 = zmed – zmin

◦ A2 = zmed – zmax

◦ If A1 > 0 AND A2 < 0 Go to level B

◦ Else increase the window size If window size ≤ Sxy

repeat level A Else output zxy

Level B:◦ B1 = zxy – zmin

◦ B2 = zxy – zmax

◦ If B1 > 0 AND B2 < 0 Output zxy

◦ Else output zmed

ResourceR.C. Gonzalez and R.E. Woods,

Digital Image Processing, 2rd Edition, Prentice Hall