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Recap of Lecture 16
β’ Mask processing
β’ Spatial filtering
β’ Averaging
β’ Differentiation
β’ Correlation
β’ Convolution
Outline of Lecture 17
β’ Image enhancement
β’ Dynamic range
β’ Point processing
β’ Contrast stretching
β’ Intensity level slicing
Introduction
β’ Improve the quality of an image as perceived by human being
β’ Can be performed at both spatial and frequency domain
β’ Different types of enhancement
β’ Noise suppression
β’ Increase contrast
β’ Sharpen or smoothen image feature
β’ Emphasise
β’ Display and analysis
Common techniques
Image enhancement
Point processing Spatial operation Transform operation
Pseudocoloring
β’ Contrast stretchingβ’ Noise clippingβ’ Window slicingβ’ Histogram modeling
β’ Noise smoothingβ’ Median filteringβ’ Unsharp maskingβ’ Filtering
β’ Linear filterβ’ Root filterβ’ Homomorphic filter
Dynamic Range
β’ The ratio between the maximum and minimum measurable light intensities
β’ In the real world, one never encounters true white or black
Image: cambridgeincolour
ILLUMINANCE & REFLECTIVITY
β’ Both contribute to the dynamic range of a scene
β’ Illuminance to specify only incident light
β’ Vast variation possible for incident light
Dynamic Range of Camera
Black Level(Limited by Noise)
White Level(Saturated)
Darker White Level(Low Capacity)
Cavity Array Light Cavities
Image: cambridgeincolour
Human eye
Weber ratio as a function of intensity
HDR Images, Posterization
Images: Gonzalez & Woods, 3rd edition
Intensity transformation
π (π₯ , π¦ )=π ( π (π₯ , π¦ ))
: Transformed image
: Input image
: Transformation function
Images: Gonzalez & Woods, 3rd edition
π =π (π )neighbourhood
Point processing
Point processing
Images: Gonzalez & Woods, 3rd edition
Contrast stretching function
0 255
255
Thresholding function
0 255
255
Intensity transformation: Negative
πΏβππ =ΒΏTransformation function
Images: Gonzalez & Woods, 3rd edition
Intensity transformation: Log-transform
π =ππππ (1+π ) ;π β₯0Transformation function
β’ Maps narrow low intensity to wider range of intensity
β’ Compresses the dynamic range of images
Images: Gonzalez & Woods, 3rd edition
Intensity transformation: Gamma
π =πππΎTransformation function
β’ Fractional value maps narrow low intensity to wider range of intensity
β’ Higher value maps narrow low intensity to wider range of intensity
Images: Gonzalez & Woods, 3rd edition
Gamma correction: Display
Images: Gonzalez & Woods, 3rd edition
Gamma corrected image as viewed in monitor
Gamma correction
Gamma corrected image
Original image Input image as viewed in monitor
Contrast stretching
Images: Gonzalez & Woods, 3rd edition
πΌ π½ πΎ
π ={ πΌπ ;0β€π β€π 1π½ (π βπ)+π 1 ;π1β€π β€π 2πΎ (πβπ)+π 2;πβ€π β€ πΏ
ππ ππ
Spans the full range of intensity level
π1=π 1 ,π 2=π 2 Linear
π1=π2 ,π 1=0 , π 2=πΏβ1Thresholding
ππ
ππ
Contrast stretching
Input image Contrast stretched image
Thresholded image
Images: Gonzalez & Woods, 3rd edition
Gray-level slicing
β’ Highlight a specific range of gray values
Without background
π ={ πΏ ; π΄β€π β€π΅0 ; hππ‘ πππ€ππ π
With background
π ={ πΏ; π΄β€π β€π΅π ; hππ‘ πππ€ππ π