# 9_20Image_20Processing_20I by dvinkster

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ENGG 167
MEDICAL IMAGING
Lecture 9: Thursday Oct. 12

Image Processing I
Image Types & Linear Transforms

Reference: Gonzalez, Woods, Eddins
Digital Image Processing using MATLAB, Prentice Hall, (2004)   1

Preparation -

Review Imaging Processing Toolbox Help Manual

(from the web… just do a Google search)

Most of the next two lectures are adapted from
Textbook by Gonzalez, Woods, Eddins
Digital Image Processing using MATLAB, Prentice Hall, (2004)

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Human light perception is sensitive but not linear

Gray scale perception is not linear
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Ref: Gonzalez et al, Text

This leads to ‘optical illusion’ in certain cases

Gray scale perception is not linear

Contrast, length, orientation are not
entirely linear
4
Ref: Gonzalez et al, Text

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Image sensors – quantization of light from images

5
Ref: Gonzalez et al, Text

Grey scale values… humans can discriminate
about 128 levels (7 bit digitization needed)

256      128          16                8

64        32           4                2

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Ref: Gonzalez et al, Text

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Enhancing images – Intensity transforms

Negative transform
Contrast Stretching
Logarithm transform – gamma factor
Thresholding (upper and lower)

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Ref: Gonzalez et al, Text

Negative transform of image f
g = imadjust(f, [low_in high_in], [low_out high_out], gamma);

g = imadjust(f, [0 1], [1 0]); or g = imcomplement(f);

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Ref: Gonzalez et al, Text

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Enhancing images – contrast stretching
high I
Image out
Low I

Low I         high I
Image in
high I
Image out
Low I

Low I         high I
Image in
high I
Image out
Low I

Low I         high I
Image in                               9
Ref: Gonzalez et al, Text

Enhancing images – gamma factor
high I
Image out

gamma = 1
Low I

Low I         high I
Image in
high I

gamma > 1
Image out
Low I

Low I         high I

Image in
high I
Image out

gamma < 1
Low I

Low I         high I                     10
Image in             Ref: Gonzalez et al, Text

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Enhancing images – Histogram transforms –in
paint shop pro…

Histogram stretch

midtone compression

midtone expansion

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Ref: visible human project

Enhancing images – Histogram transform
(histogram equalization, stretching)
h(rk) = nk the histogram, h, is the number of pixels, n, at each k intensity
level, with r being the new scale level ordinate
# of pixels

0   Grey scale values         255
# of pixels

0   Grey scale values         255   12
Ref: visible human project

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Enhancing images – Histogram processing
& resulting transform curves

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Ref: Gonzalez et al, Text

Enhancing images – Histogram processing
& resulting transform curves
Repeating again on
this edge corrects
this locally

Scanned image ^
(text on either side of
page is showing
through!)                 Removing this
large peak at the
top of the intensity
range eliminates all
this ‘crosstalk’

14
Ref: Gonzalez et al, Text

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Enhancing images – Histogram processing
global vs local processing

15
Ref: Gonzalez et al, Text

Enhancing images – Spatial Filtering &
Processing

1) Linear spatial filtering

2) Nonlinear spatial filtering

3) Geometric transformations

16
Ref: Gonzalez et al, Text

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Spatial Filtering with a filter mask to map the
initial image onto a new calculated image

17
Ref: Gonzalez et al, Text

image.

18
Ref: Gonzalez et al, Text

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Edge enhancement filter masks - Laplacian

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Ref: Gonzalez et al, Text

Edge enhancement filter masks – other derivatives

20
Ref: Gonzalez et al, Text

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Image enhancement via multistage processing

21
Ref: Gonzalez et al, Text

Assignments this week

Journal Club Presentations: Monday, Oct. 16.
Image processing paper

Image Processing with MATLAB: Due Wed. Oct. 18.
Choose one Function in MATLAB Image processing toolbox and describe and
demonstrate the function, with an example.
Options :
Dilation and Erosion
Image Deblurring
Color space transform
Linear Filter design
Segmentation
Something else…?
1)   Use an image and show the effect that can be achieved with the MATLAB function.
2)   Show when the effect is dominant and when it is not dominant (what are the main
control parameters).
3)   Include all MATLAB programs in your handed in work, as well as all images       22

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