IMAGE ENHANCEMENT

Jun 23 • Notes • 1601 Views • No Comments on IMAGE ENHANCEMENT

IMAGE ENHANCEMENT

The main objective of Image enhancement is to process an image so, that result is more accurate than original image for a specific application.

Image enhancement is done by using two method

  1. Spatial domain method
  2. Frequency domain method

SPATIAL DOMAIN METHOD

  • The term spatial domain means image plain itself and the method is based on direct manipulation of pixels in an image.
  • Spatial domain is represented by the expression

g(x,y) = T[f(x, y)] where,

g(x,y) =processed image

f(x,y) = input image

GRAY LEVEL TRANSFORMATION

This transformation is simplest among all enhancement technique

S = T(r) where,

S= pixel value after processing

r = pixel value before processing

T means transformation which map ‘r’ into ‘s’

  1. IMAGE NEGATIVE

With gray-level in the range [0, L-1] the negative of the image can be found by the help of negative transformation that is given by

S = L-1-r

  1. Log TRANSFORMATION

S = C log(1+r) where, C = constant

From the shape of the log curve this transformation maps low gray level value in the input image into wider range of output level. The opposite is also true for high gray level of input value.

With large variation in pixel value log function compresses the dynamic range of image.

  1. POWER LAW TRANSFORMATION

S = crˠ where, c and ˠ are positive constant.

By varying ɣ simply transformation curve can be obtained , when ɣ < 1 it have the opposite effect as generated when ɣ > 1  and hence it reduces to the identity transformation  and

C = ɣ = 1

GAMMA CORRECTION

Gamma is the exponent in power law  equation. Gamma correction is the procedure to correct the power law response.

On a computer screen inorder to display an image accurately gamma correction is important. Gamma correction changes brightness and also the ratio of red to green to blue

CONTRAST STRETCHING

It is a piecewise linear transformation .  main idea of contrast stretching is to increase the dynamic range of the gray level .

FREQUENCY DOMAIN METHOD

Image enhancement in the frequency domain is straightforward. We simply compute the Fourier transform of the image to be enhanced, multiply the result by a filter (rather than convolve in the spatial domain), and take the inverse transform to produce the enhanced image.This method is use to enhancement of an image by modifying the fourier transform of an image. filtering process is done by the frequency domain method.

Multiplication in frequency domain is equivalent to the convolution in spatial domain  because of this some computation is involved so,frequency domain filtering is better than spatial filtering.

Algorithm for frequency domain filtering is given as

  1. Let us consider an original image f(x, y) for which filtering is required then find the fourier transform of image and also the fourier spectrum. and to centre the transform multiply by (-1)xy .
  2. Frequency domain filter with transfer function h(x, y) is desgined. Depending upon the application  requirement the shape mask may be circle, rectangle or any custom. Then find fourier spectrum H(u, v) of h(x, y).
  3. And then multiply the fourier spectrum of image and filter we get

G(u,v) = H(u, v) F(u, v) where,

G(u, v) in the frequency domain it is filtered output.

  1. Then to retrieve the filtered image in spatial domain apply inverse fourier transform to filtered output G(u, v).
  2. Display the image.

Many frequency domain filter is implemented by this algorithm.

 

QUESTION & ANSWER

  1. What is filter??

ANSWER

Filter is a method by which certain frequency component can be selected or rejected. For example a high-pass filter chose only high frequency component and attenuates low frequency component.

2. How filtering is done ??

ANSWER

Filtering is done by two process and these are

  • Fereqency domain filtering
  • Spatial filtering

3. What is image enhancement?? Explain??

ANSWER

The main objective of the image is to process an image so, that the resulting image is more suitable than the original image.

Image enhancement is done by using two method

  1. Spatial domain method
  2. Frequency domain method                                                                                                               4. What do you mean by gamma correction??

ANSWER

Gamma correction is the process/way to rectify the power law response.

Value of gamma correction changes the brightness and also the ratio of red to green to blue.

Example-

GATE Syllabus-

1. Gate Syllabus for Electronics and Communication

IES Syllabus-

1. IES Syllabus for General Ability

2. IES Syllabus for Electronics and Telecomm

 

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