1. Digital Image Processing Contrast Enhancement: Part I

2. Contrast Enhancement
Contrast Stretching: improves the contrast in an image by stretching the range of intensity values to span a desired range of values.

3. Converts to black & white
Linear Part contributes to the contrast stretching
© 2002 R. C. Gonzalez & R. E. Woods

4. k: number of bits used to represent each pixel
Some basic grey-level transformation functions used for contrast enhancement
L=2k
k: number of bits used to represent each pixel
© 2002 R. C. Gonzalez & R. E. Woods

5. Image Negatives s= (L-1)-r
s is the pixel value of the output image and r is the pixel value of the input image.
(left) Original digital mammogram. (right) Negative image obtained using the negative transformation
© 2002 R. C. Gonzalez & R. E. Woods

6. Logarithmic Transformations
s= c log(1+r)
s is the pixel value of the output image and r is the pixel value of the input image.
(left) Fourier spectrum of Barbara’s image. (right) Result of applying the log transformation

7. Logarithmic Transformations
s= c rγ
s is the pixel value of the output image and r is the pixel value of the input image. (γ ≥ 0 and 0 ≤ r ≤ 1)
Plots for various values of γ (c=1)
© 2002 R. C. Gonzalez & R. E. Woods

8. Logarithmic Transformationsb c d
(a) original image. (b) γ = (c) γ = (d) γ = 0.7.

9. Piecewise-Linear Transformations
An example of piecewise linear transformation function
© 2002 R. C. Gonzalez & R. E. Woods

10. Piecewise-Linear Transformations Other pixel values are darkened
grey-level slicing
Pixel values between [A,B] are highlighted
Other pixel values are preserved
Other pixel values are darkened
© 2002 R. C. Gonzalez & R. E. Woods

11. Piecewise-Linear Transformations
Bit Plane slicing
An 8-bit fractal image
© 2002 R. C. Gonzalez & R. E. Woods

12. Piecewise-Linear Transformations
Bit Plane slicing
MSB
LSB
© 2002 R. C. Gonzalez & R. E. Woods

13. Histogram Processing
Histogram : is the discrete function h(rk)=nk , where rk is the kth gray level in the range of [0, L-1] and nk is the number of pixels having gray level rk.
Normalized histogram : is p(rk)=nk/n, for k=0,1,…,L-1 and p(rk) can be considered to give an estimate of the probability of occurrence of ray level rk.

14. Histogram of 4 basic grey-level characteristics
Dark image
Bright image
© 2002 R. C. Gonzalez & R. E. Woods

15. Histogram of 4 basic grey-level characteristics
Low contrast image
High contrast image
© 2002 R. C. Gonzalez & R. E. Woods

16. Summary We have looked at: What is contrast stretching?
What are different transformations?
What is histogram and histogram processing?
Next time we will talk about more on contrast stretching by using other techniques