EDGE DETECTION

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EDGE DETECTION
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IMAGE HEIGHT SURFACE
Graylevel
I(x,y)
Y
IMAGE PLANE
X
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STEP EDGE
Gray_2
Gray_1
Sideview
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FIRST ORDER EDGE DETECTION
Prewitt Templates
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PROPERTIES OF A GOOD EDGE DETECTOR

• Isotropic
• Good Localization
• Good Signal-To-Noise Characteristics
• Accurate Determination of Edge Orientation

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ISOTROPIC EDGE DETECTION
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SIGNAL-TO-NOISE RATIO AND LOCALIZATION
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PATTERNS OF VARIOUS FREQUENCIES
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DERIVATIVES OF A STEP-EDGE
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SECOND ORDER EDGE OPERATOR
MARR-HILDRETH OPERATOR
Combine this with Gaussian smoothing.
Detect edge at Zero-Crossing looking for
positive and negative peaks on either side.
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GAUSSIAN CURVES IN 1-D AND 2-D
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MEXICAN HAT FILTER
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SECOND ORDER EDGE OPERATOR
CANNY OPERATOR
Take second derivative in direction of gradient
(Ix,Iy). Look for Zero-Crossing of second
derivative in this direction. Can also
threshold with respect to first order edge
strength. Direction of first order gradient
(Ix,Iy) gives edge orientation. Proved to have
optimal localization properties.
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CANNY EDGE OPERATOR

• Only consider pixel points with first order
  edge-strength above thresh_1.
• Look for zero-crossings of second derivative in
  direction of image gradient.
• Nonmaximum Supression Only consider those
  zero-crossings that constitute a maximum of the
  image intensity gradient.
• Edge Tracking Pick thresh_2ltthresh_1 and travel
  perpendicular to gradient selecting pixels with
  image intensity gradient gtthresh_2.

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IMAGE COMPRESSION

• Compression Ratio
• (Uncompressed File Size)/(Compressed File Size)
• Bits per Pixel
• (Number of bits)/(Number of pixels)

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OBJECTIVE METRICS FOR IMAGE COMPRESSION
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RUN LENGTH ENCODING(BINARY)
8, 4, 5, 2, 6, 5, 3,3,2,1,3,1,2,2,1,4,1,1,10
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RUN LENGTH ENCODING(GRAYLEVEL)
1
0
1
1
1
0
0
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BITPLANES
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LZW COMPRESSION

• Uses a code table to encode reoccurring strings
  of pixel values.
• Example Consider a grey-level image with 8 bits
  per pixel. Make a code table of 12-bit words
  (therefore 4096 entries) where first 256 entries
  are individual grey values and next 3840 entries
  encode strings of grey values.

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QUAD-TREE REPRESENTATION