1. CH2. Point Processes
Arithmetic Operation
Histogram Equalization
<Experiment 4>
Contrast Stretching
Intensity Transformation
<Experiment 5>

2. Arithmetic Operation
What happens if we add, subtract, multiply, divide the pixel value by a constant.
Let two images be X[i,j] & Y[i,j], and obtain
aX[i,j]+(1-a)Y[i,j], 0 < a < 1.
LUT(Look-Up Table) Operation: fastest pixel operation

3. Histogram Equalization
Image histogram: distribution of pixel values, probability density function (Photoshop: info. Histog.)
Histogram Equalization: Modify the histogram to be uniform -> image enhancement for dark or bright area
(Photoshop: image -> adjustment -> equalization)
Histogram Equalization for grey image
Count # of grey pixels & save in {int h[0]-h[255]}
Obtain the CDF of h[0]-h[255] & save in {int cdf[0]-cdf[255]}
cdf[i]=cdf[i]*255/cdf[255] : normalization
image x[i][j] = cdf[(int)x[i][j]] : LUT method
If [ slope of cdf = 1] no changed pixels,
If [ slope of cdf < 1] larger range of pixels occupy smaller range
If [ slope of cdf > 1] smaller range of pixels occupy larger range

4. <Experiment 4>
Open Lena.raw & Baboon.raw images, & obtain the images of aX[i,j]+(1-a)Y[i,j], for a=0.25, 0.5, 0.75.
Develop your own program for the histogram equalization.
Compare your results with the Photoshop’s

5. Contrast Stretching
Rule:
high, low: The maximum & the minimum pixel value in an image

6. Histogram Equalization vs. Contrast Stretching
Observe the histograms of Lena_dark.raw, Lena_bright.raw, Lena_dark_bright.raw.
Compare the performances of Histogram Equalization & Contrast Stretching

7. Intensity Transformation
Implement by using LUT Operation:
photoshop, image>adjustments>curves
Gamma Correction
& normalization
Compensate the nonlinearity between cameras & monitors
Ex) RGB monitor: gamma = 1.4 – 2.8

8. Intensity Transformation
Gamma Correction

9. Intensity Transformation
Posterizing: quantization
photoshop, image>adjustments>posterize

10. Intensity Transformation
Thresholding: binalization, two-tone imaging
photoshop, image>adjustments>threshold

11. Intensity Transformation
Bit clipping & contrast stretching

12. Intensity Transformation
Iso-intensity contouring & range highlighting:

13. Intensity Transformation
Solarizing:
photoshop, filter>stylize>solarize

14. Intensity Transformation
Parabola transformation:

15. <Experiment 5>
Develop your own program for Contrast Stretching & compare the results with those of Histogram Equalization
Implement all the LUT operations introduced in this lecture note.