1. IMAGE PROCESSING
AKSHAY P
S3 EC
ROLL NO. 9

2. What is an image?
We can think of an image as a function, f, from R2 to R:
f( x, y ) gives the intensity at position ( x, y )
Realistically, we expect the image only to be defined over a rectangle, with a finite range:
f: [a,b]x[c,d]  [0,1]
A color image is just three functions pasted together. We can write this as a “vector-valued” function:
As opposed to [0..255]

3. What is a digital image?
We usually operate on digital (discrete) images:
Sample the 2D space on a regular grid
Quantize each sample (round to nearest integer)
If our samples are D apart, we can write this as:
f[i ,j] = Quantize{ f(i D, j D) }
The image can now be represented as a matrix of integer values

4. Image processing
An image processing operation typically defines a new image g in terms of an existing image f.
We can transform either the range of f.
Or the domain of f:
Use photoshop to make something grayscale

5. Histogram based Enhancement
Histogram of an image represents the relative frequency
of occurrence of various gray levels in the image
MATLAB function >imhist(x)

6. Why Histogram?
It is a baby in the cradle!
Histogram information reveals that image is under-exposed

7. Histogram Equalization
Uniform
Quantization
Note: y
cumulative probability function L 1 x L
Image processing

8. Image Example
before
after

9. The Sobel Edge Detector
The Sobel operator, sometimes called the Sobel–Feldman operator or Sobel filter, is used in image processing and computer vision, particularly within edge detection algorithms where it creates an image emphasising edges. -1 -2 1 2 -1 1 -2 2 9

10. The Canny Method
Canny is a more effective method than sobel. Its input is output of sobel.
It produces finer edges than sobel
After finding gradient using sobel method,canny uses a method called Hysterisis thresholding to remove anomalities due to noise.

11. (each pixel has some chance of being switched to zero or one)
Image noise
Original image
White Gaussian noise
Salt and pepper noise
(each pixel has some chance of being switched to zero or one)

12. Median filtering
A median filter operates over a window by selecting the median intensity in the window
Better at salt’n’pepper noise
Not convolution: try a region with 1’s and a 2, and then 1’s and a 3

13. Median filter
What advantage does median filtering have over mean filtering?