Image Processing

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Image Processing 2Point Processing colors

• Topics
• Point processing
• Digital colors

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Fundamental Steps in Computer Vision

• Today
• Image acquisition
• Pre-processing
• Segmentation

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Point Processing

• What is point processing?
• Grey level mapping
• Histograms
• Segmentation using thresholding

But first ImageJ
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ImageJ

• SW to do Image Processing and Analysis
• Free http//rsb.info.nih.gov/ij/
• Stabile (Java)
• Extremely easy to learn and use
• Comes with a C-like programming language, but
  well only use the menus
• Show

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What is point processing?

• Only one pixel in the input has an effect on the
  output
• For example
• Changing the brightness, thresholding, histogram
  stretching

Input
Output
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Point processing

• Grey level enhancement
• Process one pixel at a time independent of all
  other pixels
• For example used to correct Brightness and
  Contrast (remote control)

Too low contrast
Too high contrast
Too high brightness
Too low brightness
Correct
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Brightness

• The brightness is the intensity
• Change brightness
• To each pixel is added the value b
• f(x,y) is the input image
• g(x,y) is the (enhanced) output image
• If bgt0 gt brighter image
• If blt0 gt less bright image

g(x,y) f(x,y) b
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Contrast

• The contrast describes the level of details we
  can see
• Change contrast
• Each pixel is multiplied by a
• f(x,y) is the input image
• g(x,y) is the (enhanced) output image
• If agt1 gt more contrast
• If alt1 gt less contrast

g(x,y) a f(x,y)
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Combining brightness and contrast
g(x,y) a f(x,y) b

• Both
• A straight line
• Greylevel mapping
• X-Axis Input Value
• Y-Axis Output Value
• This plot Identity
• Output equals Input a1 and b0
• Apply to each pixel!
• To save time the greylevel
• mapping can be written as a
• Lookup-Table

Output image g(x,y)
Input image f(x,y)
(Show bridge, signe)
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Histogram
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How to set the greylevel mapping
k

• Histogram processing a powerfull tool!

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Histogram Types
Dark Image
Bright Image
Low Contrast
High Contrast

• How can a histogram help? (show PET_scan,
  TEM_filter)

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Histogram processing

• Different types of mapping
• Piecewise linear

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Histogram processing

• Non-linear, e.g., Logarithmic
• Arbitrary

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Improving contrast

• Humans cannot tell the difference between
  greylevel values too close to each other
• So spread out the greylevel values
• This is called histogram stretching

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Histogram stretching
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Something really different
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Segmentation

• Until now Image processing (manipulation)
• Image analysis segmentation
• The task
• Information versus noise
• Foreground (object) versus background

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Segmentation

• Use greylevel mapping and the histogram
• When two peaks (modes) of a histogram correspond
  to object and noise (Show AuPbSn40, bridge)
• Find a THRESHOLD value, T, that separates the two
  peaks. This process is called THRESHOLDING
• Algorithm
• If f(x,y) gt T then g(x,y) 1, else g(x,y) 0
• ( or reverse )
• Result a binary image where
• object pixels 1 and noise 0
• (Show AuPbSn40, bridge, 2Dgel, blobs)

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Segmentation

• Often, obtaining a bi-modal histogram is the
  sole purpose of the image acquisition
• Lighting
• Setup
• Camera
• Lins

Background
Object
Ideal
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Segmentation

• How to define the Theshold?
• If we have a good setup gt the Threshold is
  static!
• Find it during training
• But the histogram is NEVER static!!
• (show Eyesweb prg)

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What to remember

• Point processing
• Pixel-wise operations
• Greylevel mapping
• Setting brightness and contrast
• Histogram processing
• Segmentation Thresholding. Bimodal histogram

Output image g(x,y)
Input image f(x,y)
Background
Object
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Exercises (1/3)

• Discuss the PE-questions
• Download ImageJ http//rsb.info.nih.gov/ij/
• Use ImageJ to play around with greylevel mapping,
  histograms and thresholding. Make sure you
  understand these concepts!

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Exercises (2/3)

• Download the following images from Moodle
  enhance_me, dots, papir
• Use ImageJ to improve the quality of the image
  enhance_me
• Use ImageJ to improve the image dots so that
  only 28 non-connected black dots remain and the
  rest is white
• Use ImageJ to improve the image papir so that
  the text is black and the rest is white.

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Exercises (3/3)

• Make an algorithm (on paper) that can do
  Histogram Stretching
• How will the Threshold-algorithm look like if two
  threshold values are used instead of just one?
• Is it wise to have a binary image consisting of
  0s and 1s?