Introduction to Artificial Intelligence
Worksheet #9
Image Processing and Edge Detection

1. Download the image file: fig1.pgm. (View HTML version)
   Copy the Lisp starter file: edgeDetect.lsp.
   
   Use xv to view fig1.pgm. You should see a picture of a camera tripod.
   
   
2. Perform the following edge detection routines:
   A. Horizontal differencing:

           Image2[i,j] = Image[i+1,j+1] - Image[i,j]
           Set the pixels in the last column equal to 0
      

   B. Vertical differencing:

           Image2[i,j] = abs(Image[i+1,j] - Image[i,j])
           Set the pixels in the last row equal to 0
          

   C. Robert's cross with sqrt:

           Image2[i,j] = sqrt (sqr (image1[i,j] - image1[i+1, j+1]) +
                               sqr (image1[i,j+1] - image1[i+1, j]))
   			   
      

   Use (floor x) to truncate the result
   Also set the pixels in the last row and last column to 0.
   
   Note - Here are two variations on the Robert's cross pattern:
   
   Variation 1: Robert's cross using absolute value:

           Image2[i,j] = (abs (image1[i,j] - image1[i+1, j+1])) +
                         (abs (image1[i,j+1] - image1[i+1, j]))        
       

   Variation 2: A slightly different version of the cross pattern:

   	Image2[i,j] = abs(image1[i,j-1] - image1[i, col+1]) +
                         abs(image1[i-1, j] - image1[i+1, j])
   	

   Set the first and last rows and cols to 0.
   
   D. Sobel operator:
   For this pattern around pixel e:

            a  b  c
            d  e  f
            g  h  i
   
           dx = c + 2f + i - a - 2d - g
           dy = a + 2b + c - g - 2h - i
           
           Image[i,j] = sqrt (sqr(dx) + sqr(dy))
       

   Use (floor x) to truncate the sqrt.
   If the value is > 255, reset it to 255 so that 255 is the max value
   Set the first and last rows and cols to 0
   
   
3. Save each of the previous edge detected images under different file names.
   • Fig1HDiff.pgm - Horizontal Differencing
   • Fig1VDiff.pgm - Vertical Differencing
   • Fig1Roberts.pgm - application of the Robert's cross pattern
   • Fig1Sobel.pgm - application of the Sobel operator pattern
   
   
   Compare the strength of each edge detection algorithm.