Images and Filters CSE 576 Ali Farhadi Many slides from Steve - - PowerPoint PPT Presentation

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Images and Filters CSE 576 Ali Farhadi Many slides from Steve - - PowerPoint PPT Presentation

Dec 12, 2022 431 likes •1.02k views

Images and Filters CSE 576 Ali Farhadi Many slides from Steve Seitz and Larry Zitnick Administrative Stuff See the setup instructions on the course web page Setup your environment Project Topic Team up (discussion board)

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SLIDE 1

Images and Filters

CSE 576 Ali Farhadi

Many slides from Steve Seitz and Larry Zitnick

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SLIDE 2

Administrative Stuff

  • See the setup instructions on the course web page
  • Setup your environment
  • Project

– Topic – Team up (discussion board) – The project proposal is due on 4/6

  • Use the dropbox link on the course webpage to upload
  • HW1

– Due on 4/8 – Use the dropbox link on the course webpage to upload

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SLIDE 3

What is an image?

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SLIDE 4

P = f (x, y) f : R2 ⇒ R

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SLIDE 5

P = f (x, y) f : R2 ⇒ R

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SLIDE 6

F( ) =

Image Operations

(functions of functions)

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SLIDE 7

F( ) =

Image Operations

(functions of functions)

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SLIDE 8

F( ) =

Image Operations

(functions of functions)

0.1 0.8 0.9 0.9 0.9 0.2 0.4 0.3 0.6 0.1 0.5 0.9 0.9 0.2 0.4 0.3 0.6 0.1 0.9 0.9 0.2 0.4 0.3 0.6

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SLIDE 9

F( , ) =

Image Operations

(functions of functions)

0.23

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SLIDE 10

Local image functions

F( ) =

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SLIDE 11

How can I get rid of the noise in this image?

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SLIDE 12

90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90

Credit: S. Seitz

] , [ ] , [ ] , [

,

l n k m f l k g n m h

l k

+ + = ∑

[.,.] h

[.,.] f

Image filtering

1 1 1 1 1 1 1 1 1

] , [ g ⋅ ⋅

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SLIDE 13

90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 10 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90

[.,.] h

[.,.] f

Image filtering

1 1 1 1 1 1 1 1 1

] , [ g ⋅ ⋅

Credit: S. Seitz

] , [ ] , [ ] , [

,

l n k m f l k g n m h

l k

+ + = ∑

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SLIDE 14

90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 10 20 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90

[.,.] h

[.,.] f

Image filtering

1 1 1 1 1 1 1 1 1

] , [ g ⋅ ⋅

Credit: S. Seitz

] , [ ] , [ ] , [

,

l n k m f l k g n m h

l k

+ + = ∑

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SLIDE 15

90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 10 20 30 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90

[.,.] h

[.,.] f

Image filtering

1 1 1 1 1 1 1 1 1

] , [ g ⋅ ⋅

Credit: S. Seitz

] , [ ] , [ ] , [

,

l n k m f l k g n m h

l k

+ + = ∑

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SLIDE 16

10 20 30 30 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90

[.,.] h

[.,.] f

Image filtering

1 1 1 1 1 1 1 1 1

] , [ g ⋅ ⋅

Credit: S. Seitz

] , [ ] , [ ] , [

,

l n k m f l k g n m h

l k

+ + = ∑

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SLIDE 17

10 20 30 30 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90

[.,.] h

[.,.] f

Image filtering

1 1 1 1 1 1 1 1 1

] , [ g ⋅ ⋅

Credit: S. Seitz

?

] , [ ] , [ ] , [

,

l n k m f l k g n m h

l k

+ + = ∑

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SLIDE 18

10 20 30 30 50 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90

[.,.] h

[.,.] f

Image filtering

1 1 1 1 1 1 1 1 1

] , [ g ⋅ ⋅

Credit: S. Seitz

?

] , [ ] , [ ] , [

,

l n k m f l k g n m h

l k

+ + = ∑

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SLIDE 19

90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 10 20 30 30 30 20 10 20 40 60 60 60 40 20 30 60 90 90 90 60 30 30 50 80 80 90 60 30 30 50 80 80 90 60 30 20 30 50 50 60 40 20 10 20 30 30 30 30 20 10 10 10 10

[.,.] h

[.,.] f

Image filtering

1 1 1 1 1 1 1 1 1

] , [ g ⋅ ⋅

Credit: S. Seitz

] , [ ] , [ ] , [

,

l n k m f l k g n m h

l k

+ + = ∑

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SLIDE 20

What does it do?

  • Replaces each pixel with

an average of its neighborhood

  • Achieve smoothing effect

(remove sharp features)

1 1 1 1 1 1 1 1 1

Slide credit: David Lowe (UBC)

] , [ g ⋅ ⋅

Box Filter

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SLIDE 21

Smoothing with box filter

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SLIDE 22

Practice with linear filters

1 Original

?

Source: D. Lowe

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SLIDE 23

Practice with linear filters

1 Original Filtered (no change)

Source: D. Lowe

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SLIDE 24

Practice with linear filters

1 Original

?

Source: D. Lowe

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SLIDE 25

Practice with linear filters

1 Original Shifted left By 1 pixel

Source: D. Lowe

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SLIDE 26

Practice with linear filters

Original 1 1 1 1 1 1 1 1 1 2

  • ?

(Note that filter sums to 1)

Source: D. Lowe

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SLIDE 27

Practice with linear filters

Original 1 1 1 1 1 1 1 1 1 2

  • Sharpening filter
  • Accentuates differences with local

average

Source: D. Lowe

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SLIDE 28

Sharpening

Source: D. Lowe

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SLIDE 29

Other filters

  • 1

1

  • 2

2

  • 1

1 Vertical Edge (absolute value)

Sobel

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SLIDE 30

Other filters

  • 1
  • 2
  • 1

1 2 1 Horizontal Edge (absolute value)

Sobel

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SLIDE 31

Basic gradient filters

1

  • 1
  • 1

1 1

  • 1
  • r

Horizontal Gradient Vertical Gradient

1

  • 1
  • r
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SLIDE 32

Gaussian filter

* =

Input image f Filter h Output image g Compute empirically

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SLIDE 33

Gaussian vs. mean filters

What does real blur look like?

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SLIDE 34
  • Spatially-weighted average

0.003 0.013 0.022 0.013 0.003 0.013 0.059 0.097 0.059 0.013 0.022 0.097 0.159 0.097 0.022 0.013 0.059 0.097 0.059 0.013 0.003 0.013 0.022 0.013 0.003

5 x 5, σ = 1

Slide credit: Christopher Rasmussen

Important filter: Gaussian

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SLIDE 35

Smoothing with Gaussian filter

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SLIDE 36

Smoothing with box filter

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SLIDE 37

Gaussian filters

  • What parameters matter here?
  • Variance of Gaussian: determines extent of

smoothing

Source: K. Grauman

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SLIDE 38

Smoothing with a Gaussian …

Parameter σ is the “scale” / “width” / “spread” of the Gaussian kernel, and controls the amount of smoothing.

Source: K. Grauman

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SLIDE 39

First and second derivatives

* =

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SLIDE 40

First and second derivatives

Original First Derivative x Second Derivative x, y What are these good for?

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SLIDE 41

Subtracting filters

Original Second Derivative Sharpened

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SLIDE 42

for some Combining filters

* *

  • 1

1

  • 1
  • 1

4

  • 1
  • 1

= =

It’s also true:

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SLIDE 43

Combining Gaussian filters

More blur than either individually (but less than )

* =

?

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SLIDE 44

Separable filters

* =

Compute Gaussian in horizontal direction, followed by the vertical direction. Not all filters are

  • separable. Freeman and Adelson, 1991

Much faster!

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SLIDE 45

Sums of rectangular regions

243

239 240 225 206 185 188 218 211 206 216 225 242 239 218 110 67 31 34 152 213 206 208 221 243 242 123 58 94 82 132 77 108 208 208 215 235 217 115 212 243 236 247 139 91 209 208 211 233 208 131 222 219 226 196 114 74 208 213 214 232 217 131 116 77 150 69 56 52 201 228 223 232 232 182 186 184 179 159 123 93 232 235 235 232 236 201 154 216 133 129 81 175 252 241 240 235 238 230 128 172 138 65 63 234 249 241 245 237 236 247 143 59 78 10 94 255 248 247 251 234 237 245 193 55 33 115 144 213 255 253 251 248 245 161 128 149 109 138 65 47 156 239 255 190 107 39 102 94 73 114 58 17 7 51 137 23 32 33 148 168 203 179 43 27 17 12 8 17 26 12 160 255 255 109 22 26 19 35 24

How do we compute the sum

  • f the pixels in the red box?

After some pre-computation, this can be done in constant time for any box.

This “trick” is commonly used for computing Haar wavelets (a fundemental building block of many object recognition approaches.)

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SLIDE 46

Sums of rectangular regions

The trick is to compute an “integral image.” Every pixel is the sum of its neighbors to the upper left. Sequentially compute using:

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SLIDE 47

Sums of rectangular regions

A B C D

Solution is found using:

A + D – B - C

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SLIDE 48

* * *

input

  • utput

Same Gaussian kernel everywhere.

Slides courtesy of Sylvian Paris

Constant blur

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SLIDE 49

space weight range weight

I

normalization
 factor

Bilateral Filter Definition:
 an Additional Edge Term ( )

( )

∑

∈

− − =

S

I I I G G W I BF

q q q p p p

q p | | || || 1 ] [

r s

σ σ

Same idea: weighted average of pixels.

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SLIDE 50

Illustration a 1D Image

  • 1D image = line of pixels
  • Better visualized as a plot

pixel
 intensity pixel position

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SLIDE 51

space

Gaussian Blur and Bilateral Filter

space range normalization

Gaussian blur

( )

( )

∑

∈

− − =

S

I I I G G W I BF

q q q p p p

q p | | || || 1 ] [

r s

σ σ

Bilateral filter


[Aurich 95, Smith 97, Tomasi 98] space space range p p q q

( )

∑

∈

− =

S

I G I GB

q q p

q p || || ] [

σ

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SLIDE 52

q p

Bilateral Filter on a Height Field

  • utput

input

( )

( )

∑

∈

− − =

S

I I I G G W I BF

q q q p p p

q p | | || || 1 ] [

r s

σ σ

p

reproduced
 from [Durand 02]

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SLIDE 53

Space and Range Parameters

  • space σs : spatial extent of the kernel, size
  • f the considered neighborhood.
  • range σr : “minimum” amplitude of an edge

( )

( )

∑

∈

− − =

S

I I I G G W I BF

q q q p p p

q p | | || || 1 ] [

r s

σ σ

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SLIDE 54

Influence of Pixels

p

Only pixels close in space and in range are considered.

space range

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SLIDE 55

* * *

input

  • utput

Same Gaussian kernel everywhere.

Slides courtesy of Sylvian Paris

Constant blur

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SLIDE 56

* * *

input

  • utput

The kernel shape depends on the image content.

Slides courtesy of Sylvian Paris

Bilateral filter

Maintains edges when blurring!

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SLIDE 57

Borders

What to do about image borders:

black fixed periodic reflected