sejong univ. ch3. area processes convolutions blurring sharpening averaging vs. median filtering

Sejong Univ.Sejong Univ.

CH3. Area Processes

• Convolutions

• Blurring

• Sharpening

• Averaging vs. Median Filtering

• <Experiment 6>

Sejong Univ.Sejong Univ.- 2 -

Area process: Weighted sum of neighboring pixels Weight is called as mask, filter, kernel, or window

i.e. Area process = convolution with the mask If mask is separable, faster processing is possible

Ex) per pixel (Multiplication & addition 9 times) -> (3*2 = 6 times)

= &

Convolutions

1 2 1

0 0 0

-1 -2 -1

1

0

-1

1 2 1


Ex) Embossing: p71 Fig.3.5, Photoshop: filter>stylize>emboss Convolving with the following directional mask & +128

Convolutions

-1 0 0

0 0 0

0 0 1

0 0 1

0 0 0

-1 0 0

1 0 0

0 0 0

0 0 -1


What about processing the boundary pixels?

1.Zero padding: traditional method, no good2.Symmetric expansion is better.

Convolutions


Blurring

Remove noises, low-pass filter Averaging filter: p75 Fig. 3.7

Photoshop: filter>blur>average

1/9 1/9 1/9

1/9 1/9 1/9

1/9 1/9 1/9

1/25 1/25 1/25 1/25 1/25

1/25 1/25 1/25 1/25 1/25

1/25 1/25 1/25 1/25 1/25

1/25 1/25 1/25 1/25 1/25

1/25 1/25 1/25 1/25 1/25


Blurring

Gaussian smoothing filter: Improves contrast Photoshop: filter>blur>Gaussian

1/16 1/8 1/16

1/8 1/4 1/8

1/16 1/8 1/16


Inverse process of blurring, sharpening the image

p.77 Fig. 3.9, Photoshop: filter>sharpen

Sharpening

0 -1 0

-1 5 -1

0 -1 0

1 -2 1

-2 5 -2

1 -2 1

-1 -1 -1

-1 9 -1

-1 -1 -1


High-pass filterp.78 Fig. 3.19, Photoshop: filter>etc>highpass

High-boost filter(Output) = α(Input) – Low-pass filtered valueEx) When low-pass filter is averaging filter, w = 9α -1

Sharpening

-1/9 -1/9 -1/9

-1/9 8/9 -1/9

-1/9 -1/9 -1/9

-1/9 -1/9 -1/9

-1/9 w/9 -1/9

-1/9 -1/9 -1/9


Averaging vs. Median Filtering

Median value in the windowPhotoshop: filter>noise>median

Compare the performance of median and averaging filter for scratch noise & Gaussian noise, respectively.


<Experiment 6>

1.Implement 1) Embossing, 2) Averaging (3*3 mask), 3) Gaussian Smoothing, 4) Median, 5) Sharpening, 6) High-pass filter by using the given masks.

2. Compare Averaging Filter & Median Filter for lena-scratch-noise.raw & lena-gaussian-noise.raw images.


< HW & QZ #2 on 04/10 Fri. >

You need to complete 5 computer programs.1.One point per a program is given.2.All the programs are very similar to the programs

given in experiment 4-6.3.In order to get the point, you need to show the

result within the given time for each program.4.It is highly recommended to bring your own source

codes for experiment 4-6.

sejong univ. ch3. area processes convolutions blurring sharpening averaging vs. median filtering

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