# Gray-Scale Morphological Filtering

## Presentation on theme: "Gray-Scale Morphological Filtering"— Presentation transcript:

Gray-Scale Morphological Filtering
Generalization from binary to gray level Use f(x,y) and b(x,y) to denote an image and a structuring element Gray-scale dilation of f by b (f⊕b)(s,t)=max{f(s-x,t-y)+b(x,y)|(s-x), (t-y)Df and (x,y)Db} Df and Db are the domains of f and b respectively (f⊕b) chooses the maximum value of (f+ ) in the interval defined by , where is structuring element after rotation by 180 degree ( ) Similar to the definition of convolution with The max operation replacing the summation and Addition replacing the product b(x,y) functions as the mask in convolution It needs to be rotated by 180 degree first

Gray-Scale Dilation (f⊕b)(s)=max{f(s-x)+b(x)|(s-x)Df and xDb}
Illustrated in 1D (f⊕b)(s)=max{f(s-x)+b(x)|(s-x)Df and xDb} f(x) with slope 1 b(x) A x x a max{f(s1-x)+b(x)| )|(s-x)Df and x[-a/2,a/2]} =f(s1+a/2)+b(-a/2)= f(s1+a/2)+A=f(s1)+a/2+A {f(s1-x)+b(x)| )|(s-x)Df and x[-a/2,a/2]} f ⊕ b A A A+a/2 s s s1

Flat Gray Scale Dilation
In practice, gray-scale dilation is performed using flat structuring element b(x,y)=0 if (x,y)Db; otherwise, b(x,y) is not defined In this case, Db needs to be specified as a binary matrix with 1s being its domain (f⊕b)(x,y)=max{f(x-x’,y-y’), (x’,y’)Db} It is the same as the “max” filter in order statistic filtering with arbitrarily shaped domain Db can be obtained using strel function as in binary dilation case

Flat Gray-Scale Dilation
(f⊕b)(s)=max{f(s-x)| xDb} f(x) with slope 1 Db 1 A x x a f ⊕ b s

Effects of Gray-Scale Dilation
Depending on the structuring element adopted If all the values are non-negative (including flat gray scale dilation), the resulting image tends to be brighter Dark details are either reduced or eliminated Wrinkle removal

Gray-Scale Erosion Gray-scale erosion of f by b
(f⊖b)(s,t)=min{f(s+x,t+y)-b(x,y)|(s+x), (t+y)Df and (x,y)Db} Df and Db are the domains of f and b respectively (f⊖b) chooses the minimum value of (f-b) in the domain defined by the structuring element

Gray-Scale Erosion (f⊖b)(s)=min{f(s+x)-b(x)|(s+x)Df and xDb}
f(x) with slope 1 b(x) A x x a min{f(s1+x)-b(x)| )|(s+x)Df and x[-a/2,a/2]} =f(s1-a/2)-b(-a/2)= f(s1-a/2)-A= f(s1)-a/2-A f ⊖ b A+a/2 A {f(s1+x)-b(x)| )|(s+x)Df and x[-a/2,a/2]} s s s1

Flat Gray Scale Erosion
In practice, gray-scale erosion is performed using flat structuring element b(x,y)=0 if (x,y)Db; otherwise, b(x,y) is not defined In this case, Db needs to be specified as a binary matrix with 1s being its domain (f⊖b)(x,y)=min{f(x+x’,y+y’), (x’,y’)Db} It is the same as the “min” filter in order statistic filtering with arbitrarily shaped domain Db can be obtained using strel function as in binary case

Effects of Gray-Scale Erosion
Depending on the structuring element adopted If all the values are non-negative (including flat gray scale dilation), the resulting image tends to be darker Bright details are either reduced or eliminated

Examples Reduced Eliminated Eliminated Reduced

Dual Operations Gray-scale dilation and erosion are duals with respect to function complementation and reflection It means dilation of a bright object is equal to erosion of its dark background

Gray-Scale Opening and Closing
The definitions of gray-scale opening and closing are similar to that of binary case Both are defined in terms of dilation and erosion Opening (erosion followed by dilation) A◦b=(A⊖b)⊕b Closing (dilation followed by erosion) A•b=(A⊕b)⊖b Again, opening and closing are dual to each other with respect to complementation and reflection

Geometric Interpretation

Properties of Gray-Scale Opening and Closing
( f ◦ b )  f If f1  f2, then (f 1◦ b)  (f 2◦ b ) (f ◦ b )◦ b =f ◦ b Closing f  ( f • b ) If f1  f2, then (f 1 • b)  (f 2 • b ) (f • b • b) =f • b The notation e  r is used to indicate that the domain of e is a subset of r and e(x,y)r(x,y) The above properties can be justified using the the geometric interpretation of opening and closing shown previously

Example 9.31 (a) may be not correct

Applications of Gray-Scale Morphology
Morphological smoothing Opening (reduce bright details) followed by closing (reduce dark details) Alternating sequential filtering Repeat opening followed by closing with structuring elements of increasing sizes A◦b5 A◦b5•b5 A•b2◦b2•b3◦b3•b4◦b4•b5◦b5

Applications of Gray-Scale Morphology
Morphological gradient Effects of dilation (brighter) and erosion (darker) are manifested on edges of an image g = (f⊕b) - (f⊖b) can be used to bring out edges of an image

Applications of Gray-Scale Morphology
Top-hat transform Defined as h = f – (f◦b) Useful for enhancing details in the presence of shading

Another Application of Top-Hat Transform
Compensation for nonuniform background illumination