Image Processing

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					       Image Processing

Rule 1: Always save your primary data
Rule 2: Be able to describe
quantitatively what you have done
         3 types of operations
• Point operations: operate on a pixel-by-pixel
• Neighborhood operations: operate on a small
  group of adjacent pizels
• Reciprocal space operations: deal with image-
  wide patterns and chartacteristics
Point operations (Black-and-White)


    Gamma Curves
    Histogram Equalization
       Point Operations (Color)
• Color maps
  – RGB (More common in scientific imaging)
  – CMYK (Printers, etc)
• Color Balance
              Shading correction
• Two sources of shading variation in images
   – Dye binding to background: Image subtraction is
   – Camera/light source nonuniformity: Image division is
• Image subtraction
• Image division: Divide data image by blank image and
• If you don’t have a blank image: erode features and
  smooth to derive background (“flatten image” in Image
  Pro Plus)
         Geometric correction
• Geometric distortion: pincushion and barrel
• Geometric distortion: trapezoidal distortion
• Tiepoints: set of points with known geometric
  relationships to each other
• Set up a matrix of actual geometric positions
  in the image as a function of pixel coordinates
• Interpolate beween nonintegrap pixels
  positions to get square pixels.
     Neigborhood operations – allow
           feature extraction
• Convolution operator: a matrix that applies a
  kernel (say 3X3) to every point in the image
• Replaces the central point by the resultant of
  multiplying that 3X3 matrix by neighboring

 Molecular Expressions
 Web Site
                    Averaging kernel
• Replace central point with average of
     1/9                   1/9                  1/9

     1/9                   1/9                  1/9

     1/9                   1/9                  1/9

  Most software packages do the normalization automatically, so you can
  use “1”’s instead of “1/9”’s
              Smoothing kernel
• Gaussian (3X3)
          1            4         1
          4            12        4
          1            4         1

• (5x5)

     1        2    3        2        1
     2        7    11       7        2
     3        11   17       11       3
     2        7    11       7        2
     1        2    3        2        1
                Sharpening kernels
• Laplacian
     -1                    -1                   -1
     -1                    8                    -1
     -1                    -1                   -1

   Approximates a Laplacian operator, which replaces the central value
   with the differential in x and y
                 Directional kernels
• Vertical edge
        -1                     0             1
        -1                     0             1
        -1                     0             1

    Average in vertical direction
    Difference in horizontal direction

• Diagonal edge
    2                      1             0
    1                      0             -1
    0                      -1            -2
Complex neighborhood operations
• Median filter: replace central pixel with median
  of neighborhood
   – Very effective at removing “shot noise”
• Roberts cross: 2 perpendicular directional filter
• Sobel:
   – Calculate derivatives in 2 perpendicular directions
   – Replace central magnitude with
      √ ((δB/ δx)2 + (δB/ δy)2 )
• Kirsch: Apply each of 8 directional filters, and
  replace central value with maximum
Complex neighborhood operations
• Olympic filter: in each 5X5 neighborhood. Ignote the
  brightest and darkest 4. Replace the central value with
  the average of the remaining 17
• Top hat: replace values greater than the average of a
  neighborhood by the average for that neigborhood
• Gray scale opening
   – First pass: replace central pixel with brightest neighbor
   – Second pass: replace pixel with darkest neigbor
   – Net effect: dilation of dark features, and erosion of bright
 Hybrid – sharpening by difference of
      Gaussians (unsharp mask)
• Apply 2 different size Gaussians to same image
• Subtract smaller from larger Gaussian filtered
• Unsharp masking
  – Photographically:
     • Image in and out of focus
     • Invert out-of-focus
     • Mat reversed image with in-focus
  – Digitally: subtract blurred from unblurred
         Character recognition
• Instead of regular convolution masks, use
  masks that represent characters in the image
• You get a “hit”, or high match, whenever the
  mask matches the character!
• However, the characters must ba aligned,
  undistorted, etc.
Automatic number plate recognition

   Wikipedia: Automatic number plate recognition
                  Algorithms for ANPR
•   There are six primary algorithms that the software requires for identifying a
    licence plate:
•   Plate localisation – responsible for finding and isolating the plate on the picture
•   Plate orientation and sizing – compensates for the skew of the plate and adjusts
    the dimensions to the required size
•   Normalisation – adjusts the brightness and contrast of the image
•   Character segmentation – finds the individual characters on the plates
•   Optical character recognition
•   Syntactical/Geometrical analysis – check characters and positions against country
    specific rules
•   The complexity of each of these subsections of the program determines the
    accuracy of the system. During the third phase (normalisation) some systems use
    edge detection techniques to increase the picture difference between the letters
    and the plate backing. A median filter may also be used to reduce the visual
    "noise" on the image.

     General object recognition
• How do we recognize specific objects (such as
  tanks in aerial images) using machine vision
  – Problem of orientation: any orientation may
    present itself
     • First, scan image with circularly averaged structures
     • Then, scan again with specific orietations
     • Highly computationally expensive, and not terribly
• We can often do better with Fourier transform
  Fourier Transform Image Processing
• Any periodic object can be represented by a
  summation of a series of cosine waves
• The Operation of Fourier transformation of an
  image replaces the image (real space) be a series
  of amplitudes and frequencies of the cosine
  waves that make it up
• Fourier space is also referred to as frequency
• If there are repeats in the stucture at specific
  frequencies, these will appear as peaks in Fourier
  Fourier Transform Image Processing
• High- and low-pass filters
• By enhancing or supressing specific frequencies, we
  can enhance or suppress periodic structures within the

                                Molecular Expressions

                                Java simulation
• Nuclear pore complex
  – Markham rotation
  – Fourier transform
• Removal of halftone screen noise
   Dangers of Fourier transforms
• Can introduce periodicities where none are
• Edge effects