lecture 03 - chap2 point processes revised.ppt [?? ??]fit.mta.edu.vn/files/danhsach/chap2_point...
TRANSCRIPT
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CHAPTER 2CHAPTER 2
Digital Image Processing 1
POINT PROCESSESPOINT PROCESSES
Slides courtesy of Prof. Soo-Jin Lee, Paichai Univ.
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Digital Image Processing
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Digital Image ProcessingSlides from CS232, Stanford.
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Digital Image ProcessingSlides from CS232, Stanford.
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Digital Image Processing 5Slides from CS232, Stanford.
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IntroductionIntroductionIntroductionIntroduction
� Point Processes vs. Area Processes
� Point processes: operate on a pixel based solely on that pixel’s value.
� Area processes: use the input pixel as well as the pixels around it to generate a new pixel.
Point processes are easily implemented as “look-up tables”.
Digital Image Processing 6
� Point processes are easily implemented as “look-up tables”.
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Arithmetic OperationsArithmetic OperationsArithmetic OperationsArithmetic Operations
� Adding, subtracting, dividing, and multiplying pixels by a constant value.
Digital Image Processing 7original image
addition (+40) subtraction (-40)
multiplication (x1.2) division (/1.2)
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Arithmetic OperationsArithmetic OperationsArithmetic OperationsArithmetic Operations
� Problems
� Can create negative values and values greater than the maximum possible values.
� Clamping
� Set negative values to 0.
� Set values greater than 255 to 255.
Digital Image Processing 8
� Set values greater than 255 to 255.
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Arithmetic OperationsArithmetic OperationsArithmetic OperationsArithmetic Operations
� Image Subtraction and Change Detection
� medical imaging application : display blood-flow paths
� automated inspection of printed circuits
� security monitoring
Digital Image Processing 9
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Arithmetic OperationsArithmetic OperationsArithmetic OperationsArithmetic Operations
Digital Image Processing 10
add operation
subtract operation
Original image
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XOR OperationsXOR OperationsXOR OperationsXOR Operations
� XOR (Exclusive OR): X’Y+XY’
X Y X’Y+XY’
0 0 0
0 1 1
1 0 1
The XOR function can be used to find all pixels ofa certain value. Every pixel that is the specifiedvalue will be set to black. All other pixels will be
Digital Image Processing 11
1 1 0
original image XOR 48 XOR 128 XOR 255
value will be set to black. All other pixels will benon-black.
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XOR OperationsXOR OperationsXOR OperationsXOR Operations
� The XOR function is frequently used on graphics system to generate a cursor for the mouse.
� By XORing the cursor mask with the existing pixels, the colors are changed but you can still see the shape of the image below the cursor.
� The beauty of the XOR function when superimposing is
Digital Image Processing 12
� The beauty of the XOR function when superimposing is that you need not know the value of the background to create a high contrast.
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LookLook--Up TablesUp TablesLookLook--Up TablesUp Tables
7 7 5 4 2
7 6 4 3 0
7 6 4 3 1
0
0
1
• Use the current pixel value as the array index.• New value is the array element pointed by this index.
5 5 3 2 10
1
2
Digital Image Processing 13
7 6 4 3 1
6 6 4 2 0
5 5 3 1 0
1
1
2
3
4
5
2
3
4
5
6
7
Operation of a 3-bit look-up table
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HistogramsHistogramsHistogramsHistograms
� A bar graph of pixel intensities.
� The pixel intensities are plotted along the x-axis and the number of occurrences (frequency) for each intensity represents the y-axis.
5
6
Digital Image Processing 14
4 4 3 3
4 4 3 3
4 1 2 3
0 1 2 3image
1
2
3
4
1 2 3 4 5 6
Pixel intensity
Fre
quency
0
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HistogramsHistogramsHistogramsHistograms
Digital Image Processing 15
original imageAddition (+40) Subtraction (-40)
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HistogramsHistogramsHistogramsHistograms
Digital Image Processing 16
original imagemultiplication (x1.2) division (/1.2)
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HistogramsHistogramsHistogramsHistograms
Digital Image Processing 17
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HistogramsHistogramsHistogramsHistograms
� RGB Color vs. Gray Images
Digital Image Processing 18
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HistogramsHistogramsHistogramsHistograms
� Compression, Expansion, and Shift
(1)
Digital Image Processing 19
(a) (b)
(c) (d)
(2)
(3)
(4)
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Histogram EqualizationHistogram EqualizationHistogram EqualizationHistogram Equalization
� Goal: To obtain a uniform histogram.
� Histogram equalization will NOT “flatten” a histogram.
� It redistributes intensity distributions.
� “Spreading” is a better term than “flattening” to describe histogram equalization.
Digital Image Processing 20
� Three Steps for Histogram Equalization
� Compute histogram
� Calculate normalized sum of histogram
� Transform input to output image
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Digital Image Processing
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Digital Image Processing
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Histogram EqualizationHistogram EqualizationHistogram EqualizationHistogram Equalization
original histogram normalized sum of histogram resulting uniform histogram
Digital Image Processing 23
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Histogram Equalization (Spreading)Histogram Equalization (Spreading)Histogram Equalization (Spreading)Histogram Equalization (Spreading)
Digital Image Processing 24
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Histogram Equalization (HE)Histogram Equalization (HE)Histogram Equalization (HE)Histogram Equalization (HE)
� Effects of HE
� HE stretches contrast (expand the range of gray levels) for gray levels near histogram maxima.
� Compresses contrast in areas with gray levels near histogram minima.
� Contrast is expanded for the most of the image pixels.
Digital Image Processing 25
� Contrast is expanded for the most of the image pixels.
� HE usually improves detectability of many image features.
� Similar effect of enhancement could be achieved by manual contrast stretching approach, but the advantage of HE is fully automatic.
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Histogram EqualizationHistogram EqualizationHistogram EqualizationHistogram Equalization
Digital Image Processing 26
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Histogram EqualizationHistogram EqualizationHistogram EqualizationHistogram Equalization
nrow=512; ncol=512; npixels = nrow*ncol;maxlevel = 255;void hist_equalize(int im[][], nrow, ncol) {
int hist[maxlevel+1], sum[maxlevel+1];// initialize : hist[]=0;
for (int k=0; k<=maxlevel; k++) hist[k]=0;// calc histogram
for (int i =0; i<nrow;i++) for (int j=0; j<ncol; j++)
hist[ im[i][j] ]++;
Digital Image Processing 27
hist[ im[i][j] ]++;// calc sum
sum[0] = hist[0];for (int k=1; k<=maxlevel; k++) sum[k]=sum[k-1]+hist[k];
// calc normalized sum : for (k=0; k<=maxlevel; k++)
sum[k] = (int) ((float) (sum[k]*maxlevel) / (float)npixels + 0.5);// transform : sum[]을 LUT로 이용
for (int i =0; i<nrow;i++) for (int j=0; j<ncol; j++)
im[i][j] = sum[ im[i][j] ];}
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Color Histogram EqualizationColor Histogram EqualizationColor Histogram EqualizationColor Histogram Equalization
Digital Image Processing 28Original Image Equalized Image
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Digital Image Processing
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Digital Image Processing
STOP HERE 2014/01/21
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Digital Image Processing 31
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Digital Image Processing 32
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Histogram SpecificationHistogram SpecificationHistogram SpecificationHistogram Specification
� Make a histogram you wish to have.
� Step 1: Histogram equalize the original image
� Step 2: Perform an inverse histogram equalization on the equalized image
Digital Image Processing 33
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Histogram SpecificationHistogram SpecificationHistogram SpecificationHistogram Specification
0 0.0 0
1 0.0 0
2 0.0 5
3 0.0 5
4 0.0 6
5 2.6 6
Normalizedsum
Inversevalue
3
4
5
6
7
desi
red f
requency
3
4
5
6
7
norm
aliz
ed s
um
Histogram Equalized Input Pixel
Digital Image Processing 34
5 2.6 6
6 4.8 7
7 7.0 71 2 3 4 5 6 7
1
2
3
Pixel intensity
desi
red f
requency
1 2 3 4 5 6 7
1
2
3
Pixel intensity
norm
aliz
ed s
um
Desired Histogram Normalized Sum ofDesired Histogram
the index closest to 2
(2에가장가까운 index)
67 25
6 5 5
57 2.187
6 5 5
2.6
2.6
4.825 2.187 12
4.812 7.2.187 6.99 09
× =+ +
× =+ +
+ =
+ = ≈
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Histogram SpecificationHistogram SpecificationHistogram SpecificationHistogram Specification
Digital Image Processing 35Original Desired Transformed Results
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ContrastContrastContrastContrast
� Separation of signal (image) features from background
� Contrast describes relative brightness of a feature
( )1 11
2
: 2 2S b
C CS b
−= ⇒ −
+K
: 1S b
C C−
= ⇒ − ∞K
( )12
S b+
Digital Image Processing 36
3 3 : 0S b
C CS
−= ⇒ ∞K
2 2 : 1S b
C Cb
−= ⇒ − ∞K
Background, b Signal, S
0
S b−
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30%
36%
36%
42%
C1 =0.18 C1 =0.15
Contrast ValuesContrast ValuesContrast ValuesContrast Values
Digital Image Processing 37
30% 36%
42%
48%
48%
54%
C1 =0.13 C1 =0.12
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Contrast StretchingContrast StretchingContrast StretchingContrast Stretching
� Contrast stretching is applied to an image to stretch a histogram to fill the full dynamic range of the image.
� It works best with images that have a Gaussian or near-Gaussian distribution.
Digital Image Processing 38Low and high contrast histograms
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Contrast StretchingContrast StretchingContrast StretchingContrast Stretching
� Basic Contrast Stretch
� Expands the image histogram to cover all ranges (0~255) of pixels.
255
new
pix
el
( )1 1y y a x x− = −
Digital Image Processing 39� Works best on images that have all pixels concentrated in one
part of the histogram, the middle.
oldpixel
low high0
( )1 1y y a x x− = −
( )2 5 5
n e w p ix e l o ld p ix e l lo wh ig h lo w
= −−
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Contrast StretchingContrast StretchingContrast StretchingContrast Stretching
� End-In-Search
� A certain percentage of the pixels must be saturated to full white or full black.
0 for x low≤
255
y
Digital Image Processing 40� Works well for images that have pixels of all possible intensities
but have a pixel concentration in one part of the histogram.
( )255 for low x high
255 for high x
x lowy
high low
−= × ≤ ≤
− ≤
low high0
x
255
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Intensity TransformationsIntensity TransformationsIntensity TransformationsIntensity Transformations
� Convert an old pixel into a new pixel based on some predefined function.
� Easily implemented with simple look-up tables.(ex: image negative)
255
255
Digital Image Processing 41
old pixel0
new
pix
el
255 old pixel0
new
pix
el
255
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Intensity TransformationsIntensity TransformationsIntensity TransformationsIntensity Transformations
� Gamma CorrectionOften used in image processing to compensate for nonlinear responses in imaging sensors, displays and film.
1 /Y X
γ=
XGamma
Correction
255
255
255
Digital Image Processing 42
old pixel0
new
pix
el
255 old pixel0
new
pix
el
255 old pixel0
new
pix
el
255
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Intensity TransformationsIntensity TransformationsIntensity TransformationsIntensity Transformations
Digital Image Processing 43Fig. 2.16 (a) Gamma correction transformation with γ=0.45;(b) gamma corrected image; (c) gamma correction transformation with γ=2.2; (d) gamma corrected image.
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Intensity TransformationsIntensity TransformationsIntensity TransformationsIntensity Transformations
� Contrast Stretch/Compression Transformations
old pixel0
255
new
pix
el
255 old pixel0
255
new
pix
el
255 old pixel0
255
new
pix
el
255
Digital Image Processing 44
old pixel0 255 old pixel0 255 old pixel0 255
contrast stretchedoriginal contrast compressed
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Intensity TransformationsIntensity TransformationsIntensity TransformationsIntensity Transformations
Digital Image Processing 45Fig. 2.17 (a) Contrast stretch transformation; (b) contrast stretched image; (c) Contrast compression transformation; (d) contrast compressed image.
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Intensity TransformationsIntensity TransformationsIntensity TransformationsIntensity Transformations
� Posterize TransformationReduces the number of gray levels in an image
Digital Image Processing 46Fig. 2.18 (a) 8-Level posterize transformation;(b) posterized image; (c) threshold transformation;(d) threshold image; (e) bounded threshold;(f) bounded threshold image.
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Intensity TransformationsIntensity TransformationsIntensity TransformationsIntensity Transformations
� Bit-Clipping
� Sets a certain number of the most significant bits of a pixel to 0.
� Effect of breaking up an image into several subregions with the same intensity cycles.
Setting 1 MSB to zero Setting 2 MSB’s to zero
Digital Image Processing 47
000 000 0
001 001 1
010 010 2
011 011 3
100 000 0
101 001 1
110 010 2
111 011 3
Fig. 2.19 (a) 2-bit bit-clipping transformation;(b) resulting image.
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Fig. 2.20 Bit clipped imagecontrast stretched.
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Intensity TransformationsIntensity TransformationsIntensity TransformationsIntensity Transformations
� Iso-Intensity Transformation
� Sets particular input intensity values to black or white.
� Can be used to create contours on an image at specific intervals.
Digital Image Processing 48Fig. 2.21 (a) Iso-intensity contouring transformation; (b) contoured image.
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Intensity TransformationsIntensity TransformationsIntensity TransformationsIntensity Transformations
� Range-Highlighting Transformation
Fig. 2.22 (a) Range-highlighting transformation; (b) resulting image.
Digital Image Processing 49
� Solarize Transformation
transformation; (b) resulting image.
Fig. 2.23 (a) Solarize transformation using a threshold of 150; (b) solarized image.
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Intensity TransformationsIntensity TransformationsIntensity TransformationsIntensity Transformations
� Parabola Transformations
Digital Image Processing 50Fig. 2.24 (a) First parabola transformation; (b) transformed image; (c) second parabola transformation; (d) second transformed image.